Programster's Blog

Tutorials focusing on Linux, programming, and open-source

PostgreSQL Cheatsheet

cheatsheet PostgreSQL

PostgreSQL - The RETURNING Clause

Basics
1. Installation
2. Login As Master/Root User
3. Login As Normal User
4. List / Show Databases
5. Switch Database
6. Show / List Tables
7. Describe Table
8. Show / List Sequences
9. Exit / Logout
10. Quotation Marks
Output Formatting
1. Flip Rows And Columns
2. Scrolling Results - Turn Off Paging
3. No Wrap
4. Output Results To File
5. Clear Screen
User Management
1. Create User
2. Create Role
3. Delete Role
4. Change User Password
5. Allow User To Login
6. List Roles / List Users / Check Privileges
7. Rename User / Role
8. Grant User Superuser Privileges
9. Remove User Superuser Privileges
10. Grant User Full Access To Database
11. Grant User Readonly Access To Database
12. Grant Ability To Create Databases
13. Grant Access To Public Schema
14. Add User To Role
15. Remove User From Role
16. Change Database Owner
17. Change Table Owner
18. Mass Change Ownership
19. Change Table Owner
20. Delete User / Role
General Administration
1. Show Running Queries
2. List User Connections
3. Show Settings / Variables
4. Kill User Connection
5. Backup Database
6. Import / Restore Database
Extensions
1. List Extensions
2. Install Extension
Databases
1. Create Database
2. Drop Database
3. Change Database Name
Create Table
1. Basic Create Table Example
2. Create Table With Foreign Keys
3. Create Table With Enums
4. Create Table With Unique Column
5. Create Table With Combined Unique Key
Editing Tables
1. Rename Table
2. Add Column
3. Edit / Alter / Modify Column
  1. Drop Default
  2. Set Not Null / Nullable
4. Rename Column
5. Drop Column
Indexes / Constraints / Checks
1. Create Index
2. Add Combined Unique Key Constraint
3. Add Foreign Key Constraint
4. Add Check Constraint
5. Create Table With Check Constraint
6. Remove / Drop Constraint
Enums / Types
1. List Types
2. Describe Type / Enum
3. Create Type (Enum)
4. Drop Type (Enum)
Selecting Data
1. Select Distinct Values
2. Select Distinct Combinations
3. Select First Row Of Distinct Value
Inserting Data
Updating Data
Deleting Data
Schemas
1. Create Schema
2. Show Current Schema
3. Switch / Set Schema
4. Rename Schema
5. Delete / Drop Schema
6. Set Search Path
  1. Setting Search Path Changes Current Schema
7. Show Search Path
Working With JSON
Working With Time
1. Convert Unix Timestamp to Timestamp
2. Convert Unix Timestamp to Date
3. Convert Timestamp to Unix Timestamp
4. Selecting Between Dates
5. Grouping Dates By Month
Working With Network Types
1. Network Types
  1. cidr
  2. inet
  3. macaddr
  4. macaddr8
2. Operations
Comments
1. Table Comments
2. Column Comments
3. SQL Comments
PostGIS
1. Latitude and Longitude Order
2. Spatial Reference Identifier (SRID)
3. Inserting Geometries
Misc
1. Passwordless Login
2. Quotation Marks
3. Output of Nulls
4. Get Table Stats
References

Basics

Installation

If you need to connect to a remote server, Ubuntu 16.04 users can install the PostgreSQL client with:

sudo apt install postgresql-client-common postgresql-client-9.5
Copy to clipboard

postgres is the master user, (and comes with their own database by default).

sudo -u postgres psql
Copy to clipboard

psql \
  --user $USER \
  --password \
  -d $DATABASE \
  --host $HOST
Copy to clipboard

If the user has their own database and you wish to connect to that, you can skip -D $DATABASE as you would automatically connect to that.
if you wish to connect to localhost, you can skip --host
If the user doesnt need a password, you can skip --password
If your current BASH user is the same name as the user in psql you wish to connect as, you can skip --user.

If you are having difficulty connecting to the local database (localhost), you might not have configured your PostgreSQL database for local connections like you probably want to.

List / Show Databases

\list
Copy to clipboard

Switch Database

Technically this command does not "switch" database, you are just closing one connection and opening another.

\connect DBNAME
Copy to clipboard

Show / List Tables

If you are using the psql cli tool:

\dt
Copy to clipboard

This shows all the tables in the schemas in your search path (usually set to just public). The default schema is called public, but there may be others. To show all tables across all schemas, execute the following:

\dt *.*
Copy to clipboard

If you are connecting using something like PHP, then you need to do:

SELECT tablename FROM pg_catalog.pg_tables
Copy to clipboard

Describe Table

If you want to show a table, use:

\d+ tablename
Copy to clipboard

Example output:

                                            Table "public.users"
  Column  |          Type          | Collation | Nullable | Default | Storage  | Stats target | Description 
----------+------------------------+-----------+----------+---------+----------+--------------+-------------
 name     | character varying(255) |           | not null |         | extended |              | 
 username | character varying(255) |           | not null |         | extended |              | 
Indexes:
    "users_username_key" UNIQUE CONSTRAINT, btree (username)
Copy to clipboard

... or you can use the following to get just the basic information:

\d tablename
Copy to clipboard

                        Table "public.users"
  Column  |          Type          | Collation | Nullable | Default 
----------+------------------------+-----------+----------+---------
 name     | character varying(255) |           | not null | 
 username | character varying(255) |           | not null | 
Indexes:
    "users_username_key" UNIQUE CONSTRAINT, btree (username)
Copy to clipboard

Unfortunately, there is no SHOW CREATE TABLE tablename. However, if you are using PHP to try and retrieve information about the tables, then one can use the following to get details about the columns:

SELECT * FROM information_schema.columns
WHERE table_name = 'my_table_name';
Copy to clipboard

.... and the following to get information about the foreign keys:

SELECT
  tc.table_schema, 
  tc.constraint_name, 
  tc.table_name, 
  kcu.column_name, 
  ccu.table_schema AS foreign_table_schema,
  ccu.table_name AS foreign_table_name,
  ccu.column_name AS foreign_column_name 
FROM 
  information_schema.table_constraints AS tc 
  JOIN information_schema.key_column_usage AS kcu
    ON tc.constraint_name = kcu.constraint_name
    AND tc.table_schema = kcu.table_schema
  JOIN information_schema.constraint_column_usage AS ccu
    ON ccu.constraint_name = tc.constraint_name
    AND ccu.table_schema = tc.table_schema
WHERE tc.constraint_type = 'FOREIGN KEY' 
AND tc.table_name='my_table_name';
Copy to clipboard

... and the following to get the table's primary key:

SELECT
  tc.table_schema, 
  tc.constraint_name, 
  tc.table_name, 
  kcu.column_name,
  tc.constraint_type
FROM 
  information_schema.table_constraints AS tc 
  JOIN information_schema.key_column_usage AS kcu
    ON tc.constraint_name = kcu.constraint_name
    AND tc.table_schema = kcu.table_schema
WHERE tc.constraint_type = 'PRIMARY KEY' 
AND tc.table_name='my_table_name';
Copy to clipboard

Show / List Sequences

\ds
Copy to clipboard

Alternatively, if you need to use an SQL statement, becuase you need to run this from something like a PHP script, you would run:

SELECT sequence_schema, sequence_name 
FROM information_schema.sequences 
ORDER BY sequence_name 
Copy to clipboard

Exit / Logout

\q
Copy to clipboard

It is easier just to use the keyboard shortcut Ctrl + D.

Output Formatting

Flip Rows And Columns

If you are trying to fetch just one row and the table has a lot of columns, you may find it easier if you flip the output so that each column becomes a newline. To do this use:

\x
Copy to clipboard

If there are a lot of results, PostgreSQL will implement scrolling by default. To scroll down through the results, one presses spacebar. However, if like me, you find this annoying, you can turn off paging with:

\pset pager off
Copy to clipboard

No Wrap Output

If you have lots of columns in your tables and you don't want word wrapping in your output, you can run the following command inside PostgreSQL to use less as your pager, and tell it not to wrap lines.

\setenv PAGER 'less -S'
Copy to clipboard

If you are using a PostgreSQL docker container it won't have less installed, so you would need to install less, and then set the environment variable like so:

PAGER="/usr/bin/less -S"
Copy to clipboard

Output Results To File

You can use \o as an output buffer that will output the results to a file or pipe. E.g.

db=>\o out.txt
db=>\dt
db=>\o
Copy to clipboard

The second \o turns off the output.

You could put as many queries in there as you like.

Clear Screen

If you want to clear the screen of previous results so that the next query and result will appear from the top of the screen, then do the following:

\! clear
Copy to clipboard

User Management

I use the word "user" to save confusion. Technically, PostgreSQL has no concept of "users", only "roles". More info.

Create User

CREATE USER programster WITH PASSWORD 'thisismypassword';
Copy to clipboard

By default, when this user logs in, PostgreSQL will try to connect them to the programster database, and they will have the ability to see and create tables in the other databases.

Create Role

CREATE ROLE $ROLE_NAME with password '$USER_PASSWORD' login;
Copy to clipboard

Remove login if you don't want that to be loginable, or be explicit with the use of nologin.

Delete Role

DROP ROLE $ROLE_NAME;
Copy to clipboard

You will need to remove this role's ownership of anything before you can perform this action.

Change User Password

\password [user]
Copy to clipboard

This is most easily done by logging in as the admin user first with sudo -u postgres psql postgres

If you forgot to add the login part when creating a user, you can retrospectively change a role/user so they can log in.

ALTER ROLE "username" WITH LOGIN;
Copy to clipboard

List Users / Show Users

\du
Copy to clipboard

Example output:

                                    List of roles
 Role name  |                         Attributes                         | Member of 
------------+------------------------------------------------------------+-----------
 myusername |                                                            | {}
 postgres   | Superuser, Create role, Create DB, Replication, Bypass RLS | {}

Copy to clipboard

Rename User / Role

ALTER ROLE "old_role_name" RENAME TO "new_role_name";
Copy to clipboard

Grant User Superuser Privileges

ALTER USER $USERNAME WITH SUPERUSER;
Copy to clipboard

Remove User Superuser Privileges

ALTER USER $USERNAME WITH NOSUPERUSER;
Copy to clipboard

Grant User Full Access To Database

GRANT ALL ON database $DATABASE_NAME TO $ROLE_OR_USER;
Copy to clipboard

Grant User Read-Only Access to Database

GRANT pg_read_all_data ON database $DATABASE_NAME TO $ROLE_OR_USER;
Copy to clipboard

pg_read_all_data came in with PostgreSQL 14, so if you are using an older version of PostgreSQL, this will not work.

Grant Ability To Create Databases

The command below will give the user named username the ability to create their own databases.

ALTER USER $USERNAME CREATEDB;
Copy to clipboard

Grant Access To Public Schema

PostgreSQL 15 changes things so that the public schema does not have CREATE privileges granted by default anymore. Thus, if you create a user, and they need to operate on a database in the public schema, then you may need to grant them the ability to work in the public schema. Pick one of the following that is appropriate for your use case:

GRANT USAGE ON SCHEMA public TO my_database_user;

GRANT CREATE ON SCHEMA public TO my_database_user;

GRANT USAGE, CREATE ON SCHEMA public TO my_database_user;

GRANT ALL ON SCHEMA public TO my_database_user;
Copy to clipboard

Add User To Role

GRANT $ROLE_NAME TO $USERNAME;
Copy to clipboard

For example, the PostgreSQL AWS RDS instances have a role called rds_superuser which you need to add users to if you want them to have superuser privileges:

GRANT rds_superuser TO my_database_username;
Copy to clipboard

Remove User From Role

REVOKE $ROLE_NAME FROM $USER;
Copy to clipboard

e.g.

REVOKE rds_superuser TO my_database_username;
Copy to clipboard

Change Database Owner

ALTER DATABASE my_database_name OWNER TO "new_owner_name";
Copy to clipboard

Changing the database owner does not implicitly change the ownership of its tables.

Change Table Owner

ALTER TABLE my_table_name OWNER TO new_owner_name;
Copy to clipboard

Mass Change Ownership

If you want to mass change ownership of items from one role to another, then you can do the following:

REASSIGN OWNED BY "original_owner" TO "new_owner";
Copy to clipboard

You will need to connected on a superuser account to do this, something that you cannot have when using AWS RDS.

Delete User / Role

DELETE USER "my_users_name"
Copy to clipboard

This will fail if the user is the owner of anything, and will output all the things that user owns.

General Administration

Show Running Queries

SELECT * FROM pg_stat_activity;
Copy to clipboard

List User Connections

If you want to see who has an open connection to the database, run the following query:

select 
    pid, 
    usename, 
    datname as database_name, 
    client_addr as client_address, 
    application_name,
    backend_start,
    state,
    state_change
from pg_stat_activity;
Copy to clipboard

Show Settings / Variables

If you want to see all the settings for postgresql, then run:

SHOW ALL;
Copy to clipboard

If you want to see a specific setting then specify it instead of all, like:

SHOW max_connections;
Copy to clipboard

Kill A Users Connection / Session

SELECT pg_terminate_backend($PROCESS_ID);
Copy to clipboard

$PROCESS_ID is the pid output in listing connections.

Dump / Backup Database

pg_dump  \
  --host $HOST \
  --port "5432" \
  --username $USERNAME \
  --password \
  --no-acl \
  --no-owner \
  --clean \
  --create \
  --file /path/to/dump/file.sql \
  $DB_NAME
Copy to clipboard

--no-owner - Do not output commands to set ownership of objects to match the original
database.
--no-acl - Prevent dumping of access privileges (grant/revoke commands).
--clean - Output commands to clean (drop) database objects prior to outputting the commands for creating them.
--create - Begin the output with a command to create the database itself and reconnect to the created database.

Don't use the --create or --clean switches if you wish to import the dump file later to a different database name! E.g. if you are creating a copy of the database, instead of creating a backup of a host for later restoration.

You can add --compress=9 to have pg_dump compress the output at the most-compressed level as if having been run through gzip. The default is effectively 0.

Custom Format

I found that using the method above resulted in me losing the use of complicated constraints that were using the btree_gist extension. To resolve this, I used the same command, but with the --format=custom parameter, which results in creating a binary dump. This also means that options like --clean and --create have no effect, as one needs to specify them in the pg_restore command instead which we will get onto later.

pg_dump  \
  --host $HOST \
  --port "5432" \
  --username $USERNAME \
  --password \
  --no-acl \
  --no-owner \
  --format=custom \
  --file /path/to/dump/file.dump \
  $DB_NAME
Copy to clipboard

Restore / Import Database

From SQL File

To restore a dump taken using the way we created in the first pg_dump command which generated a plaintext SQL file, you would just run the SQL file like so:

psql \
  --host $HOST \
  --port "5432" \
  --username $USERNAME \
  --password \
  -d $DATABASE \
  -f $FILEPATH
Copy to clipboard

There is a pg_restore command, but it wouldn't work on a plain SQL dump like we created.

From Binary Dump

If you used the --format=custom option when performing the pg_dump, then you will need to use the pg_restore command in order to restore the database from this file.

This is the method I generally use:

PORT="5432"
USERNAME="username_here"
DB_NAME="replacement_database"
HOST="127.0.0.1"
FILEPATH=/path/to/dump/file.dump

pg_restore \
  --host $HOST \
  --port $PORT \
  --username $USERNAME \
  --format=custom \
  --password \
  --no-owner \
  --exit-on-error \
  --dbname=$DB_NAME \
  $FILEPATH
Copy to clipboard

Here I am restoring to a new/empty database I created specified by $DB_NAME. If one uses the --clean and/or --create options, then the --dbname option is pretty much ignored, and the name of the database at the point of creating the database dump will be used, which will error out if you have --clean specified and it doesn't exist.

One can specify the schema with --schema. However, doing so will likely cuase you issues if your database dump relies on extensions. In such a case, you can work around this by setting up the extensions before running the import.

Here is the manual for pg_restore.

Extensions

List Extensions

CLI Command

If you are using the pgsql CLI, then one can simply run:

\dx
Copy to clipboard

Below is the example output:

                           List of installed extensions
    Name    | Version |   Schema   |                  Description                  
------------+---------+------------+-----------------------------------------------
 btree_gist | 1.6     | public     | support for indexing common datatypes in GiST
 plpgsql    | 1.0     | pg_catalog | PL/pgSQL procedural language
(2 rows)
Copy to clipboard

You do not need the superuser privilege to execute this.

SQL Query Form

However, if you need to run a traditional query (such as if executing PHP code), then you would need the following:

SELECT * FROM pg_extension;
Copy to clipboard

Example output:

  oid  |  extname   | extowner | extnamespace | extrelocatable | extversion | extconfig | extcondition 
-------+------------+----------+--------------+----------------+------------+-----------+--------------
 14703 | plpgsql    |       10 |           11 | f              | 1.0        |           | 
 35190 | btree_gist |       10 |         2200 | t              | 1.6        |           | 
(2 rows)
Copy to clipboard

You do not need the superuser privilege to execute this.

Install Extension

The following command will create the extension if it doesn't already exist:

CREATE EXTENSION IF NOT EXISTS extension_name_here.
Copy to clipboard

You need the superuser privilege to execute this, even if the extension is already installed!

Databases

Create A Database

CREATE DATABASE my_database_name
ENCODING 'UTF8' 
LC_COLLATE = 'en_GB.UTF-8'
LC_CTYPE = 'en_GB.UTF-8'
TEMPLATE template0;
Copy to clipboard

If you get a locale error, then run the following commands from your BASH shell:

sudo apt-get install language-pack-en
sudo locale-gen en_GB.UTF-8
sudo update-locale 
sudo service postgresql restart
Copy to clipboard

If you don't need utf8 and are fine with LATIN encoding, then you can just use:

CREATE DATABASE my_database_name;
Copy to clipboard

Delete/Drop Database

DROP DATABASE my_database_name;
Copy to clipboard

If there is a connection to the database because it is in use, then the command will fail. If you wish to force the dropping of the database in such a scenario, you can do so with:

DROP DATABASE my_database_name WITH (FORCE)
Copy to clipboard

Change Database Name

ALTER DATABASE $CURRENT_DB_NAME RENAME TO $NEW_DB_NAME
Copy to clipboard

Create Table

Basic Create a table Example

CREATE TABLE broker (
    id serial NOT NULL,
    name varchar(255) NOT NULL,
    redirect_url text NOT NULL,
    secret varchar(30) NOT NULL,
    modified_timestamp timestamp DEFAULT CURRENT_TIMESTAMP,
    PRIMARY KEY (id)
);
Copy to clipboard

There is no AUTO_INCREMENT but a special serial and big_serial type. There is no unsigned type If you wish to wrap your table or column names, you would need to use double quotes (") instead of the ` character.

Creating Tables With Foreign Keys

CREATE TABLE cities (
    city     varchar(80) primary key,
    location point
);

CREATE TABLE weather (
    city      varchar(80) references cities(city),
    temp_lo   int
);
Copy to clipboard

By default, foreign keys will be set to use NO ACTION for both update and delete actions (resulting in the same behavioiur as RESTRICT). You can manually specify what you want like so:

CREATE TABLE weather (
    city      varchar(80) references cities(city) ON UPDATE CASCADE ON DELETE RESTRICT,
    temp_lo   int
);
Copy to clipboard

Columns that are foreign keys are not automatically indexed. PostgreSQL only requires the column that is referenced is an index. It is probably a good idea to make your foreign key columns indexes, because otherwise it can slow down operations like CASCADE delete, as the database will have to scan the whole table to find the row to delete.

Creating Tables With Enums

CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');

CREATE TABLE person (
    name text,
    current_mood mood
);
Copy to clipboard

Creating Tables With Unique Column

CREATE TABLE users (
    name varchar(255) NOT NULL,
    username varchar(255) UNIQUE NOT NULL
);
Copy to clipboard

This will automatically create an index on the unique column.

Create Table With Combined Unique Key

CREATE TABLE myTable (
    id uuid PRIMARY KEY,
    col1 INT NOT NULL,
    col2 INT NOT NULL,
    UNIQUE (col1, col2)
)
Copy to clipboard

Editing Tables

Rename Table

ALTER TABLE original_table_name 
RENAME TO new_table_name;
Copy to clipboard

Add Column

ALTER TABLE my_table 
ADD COLUMN "new_column" 
varchar NOT NULL;
Copy to clipboard

Edit / Alter / Modify Column

In the MySQL world, one would define everything about the column in one statement. In PostgreSQL one uses a step for each part. E.g. one wanted to change the type, set to not being null, and remove the default value, one would do:

ALTER TABLE my_table_name 
ALTER COLUMN my_column_name TYPE varchar(255),
ALTER COLUMN my_not_nullable_column_name SET NOT NULL,
ALTER COLUMN my_nullable_column_name DROP NOT NULL,
ALTER COLUMN my_column_name DROP DEFAULT;
Copy to clipboard

Drop Default

To remove a default from column, you can run the following:

ALTER TABLE my_table_name
ALTER COLUMN my_column_name DROP DEFAULT;
Copy to clipboard

Set Not Null / Nullable

To make a column nullable, or not null, do the following:

ALTER TABLE my_table_name
ALTER COLUMN my_nullable_column DROP NOT NULL,
ALTER COLUMN my_not_nullable_column_name SET NOT NULL;
Copy to clipboard

Rename Column

ALTER TABLE my_table 
RENAME COLUMN "original_name" 
TO "new_column_name";
Copy to clipboard

Drop Column

ALTER TABLE my_table 
DROP COLUMN "my_column_name";
Copy to clipboard

If you wish to drop multiple columns then you can do this like so:

ALTER TABLE my_table 
DROP COLUMN "my_first_column",
DROP COLUMN "my_second_column";
Copy to clipboard

If the column is used by other views/procedures etc, then you may wish to cascade the removals:

ALTER TABLE my_table 
DROP COLUMN "my_first_column" CASCADE;
Copy to clipboard

Indexes / Constraints

Create Index

You cannot specify an INDEX in the create table definition. However, if you specify a column as unique, then an index is automatically created for that column. If you have a column that is not unique but also needs to be indexed for quick selections, then you can add an index after the table has been created with:

CREATE INDEX on tableName ("column_name");
Copy to clipboard

Add Combined Unique Key Constraint

ALTER TABLE my_table
ADD CONSTRAINT constraint_name UNIQUE (column1, column2);
Copy to clipboard

Add Foreign Key Constraint

If you need to update an existing table column, to turn it into a foreign key:

ALTER TABLE my_table
ADD CONSTRAINT my_constraint_name 
FOREIGN KEY (my_table_column) 
REFERENCES my_other_table (id);
Copy to clipboard

Add Check Constraint

If you need to add a check constraint to an existing table:

ALTER TABLE my_table
ADD CONSTRAINT time_all_or_nothing
CHECK ((time_start IS NOT NULL AND time_end IS NOT NULL) OR (time_start IS NULL AND time_end IS NULL))
Copy to clipboard

Create Table With Check Constraint

If you want to create a table with a check constraint in the definition, rather than having to add it afterwards, here is an example whereby we have a table of events, and an event can be all day, in which case it does not have a start or end time, or it can be part of a day, in which case it must have both a start and an end time.

CREATE TABLE events (
    id uuid PRIMARY KEY,
    date_start date NOT NULL,
    date_end date NOT NULL,
    time_start time,
    time_end time,
    name varchar(255) NOT NULL,
    CHECK ((time_start IS NOT NULL AND time_end IS NOT NULL) OR (time_start IS NULL AND time_end IS NULL))
);
Copy to clipboard

Remove / Drop Constraint

If you want to remove a constraint, such as an index or a foreign key, then you would do it like so:

ALTER TABLE my_table DROP CONSTRAINT "my_constraint_name";
Copy to clipboard

Remove / Drop Constraint

If you want to remove a constraint, such as an index or a foreign key, then you would do it like so:

ALTER TABLE my_table DROP CONSTRAINT "my_constraint_name";
Copy to clipboard

Enums / Types

List Types

If you wish to list out all the user created types / enums (not the tables), then run the following psql command:

\dt
Copy to clipboard

Describe Type / Enum

If you wish to find out about a specific type, such as find out the possible values for an enum, then do the following:

\dT+ my_custom_type
Copy to clipboard

Create Type (Enum)

CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');
Copy to clipboard

Drop/Remove Type (Enum)

If you want to remove a type (such as an Enum you created), then you would do it like so:

DROP TYPE my_type;
Copy to clipboard

Selecting Data

Select Distinct Values {#select-distinct-values)

The following query will output only the unique/distinct values for the myColumnName column in a table.

SELECT DISTINCT myColumnName
FROM my_table_name
Copy to clipboard

Select Distinct Combinations

The following query will output only the unique/distinct combination of column1, column2 values in a table.

SELECT DISTINCT column1, column2
FROM my_table_name
Copy to clipboard

Select First Row Of Distinct Value

The query below will order a table by column3, and select the first row for each distinct value of column1. E.g. there may be multiple column2 values for a single value in column1, but it will only show the first of them when ordered by column3.

SELECT 
  DISTINCT ON(column1) column1_alias
  column2
FROM my_table_name
ORDER BY column3
Copy to clipboard

One does not have to have an ORDER BY clause, but you almost certainly want one in order to achieve repeatable results due to how the DISTINCT ON clause will only return the first row for each unique value of column1

Inserting Data

CREATE TABLE cities (
    city     varchar(80) primary key,
    location point
);

INSERT INTO cities (city, location)
VALUES 
    ('Houston', '29.7604, -95.3698'),
    ('Dallas', '32.7767, -96.7970')
;
Copy to clipboard

Pay close attention to the use of (or lack of ) various quotation marks.

Updating Data

UPDATE cities 
SET city='Washington' 
WHERE city='Houston';
Copy to clipboard

Deleting Data

DELETE FROM cities 
WHERE city='Houston';
Copy to clipboard

Schemas

In PostgreSQL, a schema is a namespace that contains named database objects such as tables, views, indexes, data types, functions, stored procedures and operators. A database can contain any number of schemas. A database will contain a default schema called public.

Create Schema

CREATE SCHEMA 'my_schema_name';
Copy to clipboard

Show Current Schema

SELECT current_schema();
Copy to clipboard

Switch / Set Schema

SET SCHEMA 'my_schema_name';
Copy to clipboard

Please note: Setting the schema also sets your search path accordingly. E.g. the following commands will set the search_path to hello, public before then setting the schema to hello. When we output the search_path, one sees that the search path is set to just hello.

bob=# SET search_path TO hello,public;
SET
bob=# SET SCHEMA 'hello';
SET
bob=# SHOW search_path;
 search_path 
-------------
 hello
(1 row)
Copy to clipboard

Rename Schema

ALTER SCHEMA 'my_schema_name'
RENAME TO 'new_schema_name';
Copy to clipboard

If a schema with the new name already exists, then the operation will fail.

Delete / Drop Schema

DROP SCHEMA 'my_schema_name';
Copy to clipboard

Set Search Path

Usually, users just refer to the table name, rather than prefixing with the schema name. E.g. myTableName instead of mySchemaName.myTableName. When referencing without the schema, this is referred to as an unqualified name. When PostgreSQL encounters an unqualified name, the system determines the table by following the search path. The default search path is just set to public, which is also the default schema.

Thus, if you wish to refer to tables in a schema using unqualified names, you can just include it in your search path like so:

SET search_path TO mySchema,public;
Copy to clipboard

Order may be important! The system will use the first table it comes across in the search path. Thus, if you have two tables with the same name in different schemas, the one within the schema that was listed first will be the one that is used. It would be a good idea to ensure you don't have tables with the same name in the schemas listed in your search path.

Setting Search Path Changes Current Schema

If one sets the search path, one is also setting the current schema. The current schema will change to whatever is the first schema listed in the search path. For example, if one was to set the schema to hello, but then set the search path to 'public','hello', one would have changed the current schema back to public as shown below:

bob=# SET SCHEMA 'hello';
SET
bob=# SET search_path TO 'public','hello';
SET
bob=# SELECT current_schema();
 current_schema 
----------------
 public
(1 row)
Copy to clipboard

Show Search Path

SHOW search_path;
Copy to clipboard

Working With JSON

If you use the JSON or JSONB data types for a column, there are special operations you can do with it. For example, I created a server_dump column that holds the json_encode of the PHP $_SERVER superglobal and can then select the requestor's IP address from this with:

SELECT server_dump->'REMOTE_ADDR' FROM api_requests;
Copy to clipboard

As a rule of thumb, use jsonb instead of json data type wherever possible. The json data type simply checks the text is valid json and stores it in plain text, rather than actually processing it. On the other hand, jsonb gets compressed to binary, and allows things like more advanced indexing. Info source 2.

PostgreSQL 14 added support for subscripting syntax, so you can now do:

SELECT server_dump['REMOTE_ADDR'] FROM api_requests;
Copy to clipboard

... or for an example that goes two layers down:

UPDATE shirts
SET details['attributes']['color'] = '"neon blue"'
WHERE id = 123;
Copy to clipboard

Working With Time

Convert Unix Timestamp to Timestamp

Use the to_timestamp function.

SELECT to_timestamp(my_unix_timestamp_column) as my_timestamp FROM my_table;
Copy to clipboard

Convert Unix Timestamp to Date

To convert a unix timestamp (epoch) to a date (such as to group by a day of the year):

SELECT DATE(to_timestamp(my_unix_timestamp_column)) as my_date FROM my_table;
Copy to clipboard

Convert Timestamp to Unix Timestamp (Epoch)

The following will convert the created_at field from a timestamp with timezone field, to a Unix timestamp.

SELECT 
  EXTRACT(EPOCH FROM created_at)::integer as created_at 
FROM my_table;
Copy to clipboard

You do not have to have the ::integer at the end. This simply converts it from a double precision to an integer.

Selecting Between Dates

If you have a table full of rows that have dates / timestamps / etc, and you want to select the rows between (and including) two dates/times, then one can just use the BETWEEN keyword like so:

SELECT * FROM my_table 
WHERE my_date_field BETWEEN '2019-01-01' AND '2022-01-01'
Copy to clipboard

Grouping Dates By Month

The following example will convert the table's my_date_field into the corresponding month using the DATE_TRUNC function. We can then group by that date and order by it. This is particularly useful for stats/reporting.

SELECT 
    DATE_TRUNC('month', my_date_field)::date as month,
    fome_field
FROM my_table
GROUP BY month
ORDER BY month ASC;
Copy to clipboard

Working With Network Types

PostgreSQL has native support for network types as outlined below. This can make your life much easier, and probably make your code much faster if you move some of the operations to the database from the application layer.

Network Types

inet

The inet type holds a single ipv4 or ipv6 address. E.g. '192.168.0.1' or ff06:ffff:300c:326b:0000:0000:0000:0000. It supports ipv6 shortening formats, so one could specify the ipv6 address like ff06:ffff:300c:326b:: instead for the ipv6 address before, because the double colon means the rest is filled with 0's.

Just to not input the cidr format, even if it is for one address. E.g. 192.168.0.1/32. For that you want the cidr type as outlined below.

cidr

This format is used for ipv4 or ipv6 CIDRs like:

192.168.0.1/32
192.168.0.1/24
2001:4f8:3:ba:2e0:81ff:fe22:d1f1/128

Since this type can represent a range of network addresses, one can use it for some pretty cool network operations, like checking if an IP address is within a range etc. This is most useful for things like specifying the IP address ranges of countries for something like a Geo-blocker. Thus you don't have to specify a row for every possible ipv4 or ipv6 address on the internet, which would not be pragmatic, especially given how larve ipv6 is.

This type will automatically derive the value from a given input. For example a value of 192.168.1 would be converted to 192.168.1.0/24, and 192.168/24 would be converted to 192.168.0.0/24. However, I would not recommend this, as you are likely to just confuse yourself and others as it might not default to what you are expecting. I would always plug in the full CIDR to be safe.

macaddr

This type represents MAC addresses which are usually found on ethernet cards, but MAC addresses can be used for other purposes as well.

PostgreSQL supports having colons, hyphens, or dots. like so (all of the following below are the same value):

08:00:2b:01:02:03
08-00-2b-01-02-03
08002b:010203
08002b-010203
0800.2b01.0203
0800-2b01-0203
08002b010203

macaddr8

This type is for 8 byte length mac addresses instead of just 6 bytes (above). However, it will accept your 6 byte length mac addresses as well. (Your computer MAC address is almost definitely just 6 bytes unless things have changed since I wrote this).

Operations

The full list of operations can be found in the online docs. However I will outline the ones I find most useful below.

Find If Network Address is Within or Equals

You can use the <<= to find out if the left side is contained within or equals the right side. This would expect the left side to be either a network address or a cidr, but the right side to be a cidr. E.g. let's imagine you have the cidr's that you allow into your website in a column called my_cidr_column and you want to check if a visitor who came from '179.62.56.51' is allowed into your website. You would run:

SELECT count(*) as num_rows FROM my_whitelist_table
WHERE '179.62.56.51' <<= my_cidr_column
Copy to clipboard

... and then you could just check that num_rows is greater than 0.

You could do the same thing in reverse with >>= which checks if the left side contains or equals the right side. Taking our example from above, we would simply do:

SELECT count(*) as num_rows FROM my_whitelist_table
WHERE my_cidr_column >>= '179.62.56.51';
Copy to clipboard

Addition and Subtraction

Believe it or not, you could add/subtract two network addresses. E.g. the following would result in inserting a value of 192.168.0.200 into your table:

INSERT INTO my_table (id, ipv4_address)
VALUES ('9b2119db-c4b8-4e67-b3e1-94aa5c4dc716', ('192.168.0.1' + 199));
Copy to clipboard

You can even add/subtract two inet addresses:

'192.168.1.43' - '192.168.1.19'
Copy to clipboard

Greater Than and Less Than

You can use >, <, >=, and <= on two inet types and it will act exactly as you expect.

Comments

Table Comments

You can add comments to a table for others to read later. E.g.

COMMENT ON TABLE mytable IS 'This table is for...';
Copy to clipboard

Column Comments

You can retrospectively add a comment to a table column like so:

COMMENT ON COLUMN my_table.column_name IS 'This is my comment';
Copy to clipboard

If you want to specify the comment at the point of creating the table, then you could do:

CREATE TABLE something (
    id serial NOT NULL,
    column1 varchar(255) IS 'comment describing column here',
    PRIMARY KEY (id)
);
Copy to clipboard

SQL Comments

You can add a comment to a line using -- like so:

-- comment goes here  creating the programster user.
CREATE USER programster WITH PASSWORD 'thisismypassword';
Copy to clipboard

Multi-line Comments

You can add a multiline comment anywhere in SQL by using /* and */ which signify the start and end of the comment.

/* 
  * multiline comment here
  * creating the programster user.  
  */
CREATE USER programster WITH PASSWORD 'thisismypassword';
Copy to clipboard

... or alternatively:

CREATE TABLE something (
    id serial NOT NULL,
    column1 varchar(255), /* comment here - deliberately not setting NOT NULL for some reason */
    PRIMARY KEY (id)
);
Copy to clipboard

PostGIS

The content within this section expects you to have installed the PostGIS extension in your PostgreSQL database, or be running the PostGIS Docker image.

Latitude and Longitude Order

PostGIS uses longitude, latitude ordering for everything, whilst services such as Google maps, and the EPSG:4326 standard use the latitude, longitude ordering.

Spatial Reference Identifier (SRID)

When looking at the documentation for many PostGIS functions, you will see an optional srid
parameter. This specifies what coordinate system one is working within and one always needs to make sure that one is operating across matching coordinate systems. E.g. when trying to find if a point is within a shape, both the shape and the point need to be defined using the same coordinate system.

A common SRID that you will often see, and is a default in many of the PostGIS functions is 4326, which represents spatial data using longitude and latitude coordinates on the Earth's
surface as defined in the WGS84 standard, which is also used for the Global Positioning System (GPS).

Inserting Geometries

The following snippet shows you how you can insert various geometric types into the database.

CREATE TABLE geometries (name varchar, geom geometry);

INSERT INTO geometries VALUES
  ('Point', 'POINT(0 0)'),
  ('Linestring', 'LINESTRING(0 0, 1 1, 2 1, 2 2)'),
  ('Polygon', 'POLYGON((0 0, 1 0, 1 1, 0 1, 0 0))'),
  ('PolygonWithHole', 'POLYGON((0 0, 10 0, 10 10, 0 10, 0 0),(1 1, 1 2, 2 2, 2 1, 1 1))'),
  ('Collection', 'GEOMETRYCOLLECTION(POINT(2 0),POLYGON((0 0, 1 0, 1 1, 0 1, 0 0)))');

SELECT name, ST_AsText(geom) FROM geometries;
Copy to clipboard

ST_Contains

The ST_Contains function checks whether the first geometry fully contains the second geometry specified. For example, the code below shows how to check if the geometry column of my_table contains the GPS coordinates of Big Ben.

$longitude = -0.1246
$latitude = 51.5007
$srid = 4326

$query = 
    "SELECT *
    FROM my_table
    WHERE ST_Contains(
        my_geometry_column, 
        ST_GeomFromText('POINT({$longitude} {$latitude})', {$srid})
    );";
Copy to clipboard

Notice how there is no comma between the longitude and latitude when specifying the POINT coordinates.

ST_GeomFromGeoJSON

The ST_GeomFromGeoJSON function converts a GeoJSON string into a geometry object. This is incredibly useful for taking online GeoJSON data sources (such as this), and importing them into your database.

$geoJsonString = '{
       "type": "Polygon",
       "coordinates": [
            [ 
                [100.0, 0.0], 
                [101.0, 0.0], 
                [101.0, 1.0], 
                [100.0, 1.0], 
                [100.0, 0.0] 
            ]
        ]
    }';

$escapedGeoJsonString = pg_escape_literal($conn, $geoJsonString);

$query = "INSERT INTO my_table (name, geometry_column) VALUES (
    'myShape',
     (SELECT ST_GeomFromGeoJSON({$escapedGeoJsonString}))
);";
Copy to clipboard

This gracefully handles all kinds of shapes as defined in the GeoJSON, not just the Polygon type in the example. E.g. this includes the MultiPolygon etc.

ST_Within

ST_Within is the inverse of contains, in that it checks if the first parameter is within the second, instead of checking if the second parameter contains the second. (order of the parameters).

Below is an example where I dynamically check if a table points latitude and logitude are within the provided GeoJSON polygon (used from a Leaflet codebase).

ST_Within(
    ST_SetSRID(ST_MakePoint(table_longitude_column, table_latitude_column), 4326),
    ST_GeomFromGeoJSON('{.....}')
);
Copy to clipboard

Misc

Sometimes one needs to be able to run commands from a script, and thus one cannot perform the interactive password input. Thus, one needs to be able to "provide" the password in a secure manner (specifying --password=something doesn't work). There are two main ways to do this as outlined below:

PGPASS File

Create a .pgpass file in your home directory with the connection details of your databases like so:

HOST:PORT:DB_NAME:USERNAME:PASSWORD
Copy to clipboard

e.g.

localhost:5432:postgres:myadmin:Str0ngP@ssw0rd
Copy to clipboard

Then you should be able to log in without entering a password, if you provide the same three following attributes that match up:

database name
host
user

This will note work if you add --password to your psql command, so don't forget to not include that.

One can specify wildcards in your .pgpass file. E.g.

*:*:*:postgres:myadmin:Str0ngP@ssw0rd
Copy to clipboard

If you don't have a home directory, or if you need to specify a custom location, you can set the location of your .pgpass file by setting an environment variable like so:

export PGPASSFILE='/custom/path/to/.pgpass'
Copy to clipboard

Finally, your .pgpass file will only work if you set its permissions to 600 (like your SSH keys).

sudo chmod 600 .pgpass
Copy to clipboard

Environment Variable

You can temporarily set your password by setting the PGPASSWORD environment variable like so:

export PGPASSWORD
Copy to clipboard

This will note work if you add --password to your psql command, so don't forget to not include that.

Quotation Marks

Double quotes (") are used to denote column names, and single quotes (') are also only used to denote values. When working with UUIDs, its best to always wrap in single quotes.

UPDATE my_table 
SET "my_column" = 'f68b8ef1-ed46-42bd-9619-4ae37dae3eb3' 
WHERE "uuid"='97c681fc-d217-4bcb-970d-e54793d8fd94';
Copy to clipboard

You don't always have to use quotes, but you will need to use them if you have a column name that clashes with a reserved word, such as when.

Output of Nulls

Unlike MySQL which will show "null" as a value when showing data, like so:

mysql> select * from cities;
+---------+----------+
| city    | location |
+---------+----------+
| Houston |     NULL |
+---------+----------+
1 row in set (0.00 sec)
Copy to clipboard

...PostgreSQL will just show emptiness like so:

postgres=# select * from cities;
    city    |      location      
------------+--------------------
 Dallas     | (32.7767,-96.797)
 Washington | (29.7604,-95.3698)
 London     | 
(3 rows)
Copy to clipboard

Get Table Stats

The command below is a good way to get some stats about your tables. It can be quite useful when you want to see how "active" your tables are, or see how many "dead tuples" are taking up space and need deleting by the vacuum process. (Dead tuples are rows that have been deleted but have not been removed from disk yet by the vacuum process).

SELECT 
    relname as table_name,
    n_tup_ins as "inserts",
    n_tup_upd as "updates",
    n_tup_del as "deletes",
    n_live_tup as "live_tuples",
    n_dead_tup as "dead_tuples",
    last_vacuum,
    last_autovacuum,
    last_analyze,
    last_autoanalyze,
    vacuum_count,
    autovacuum_count,
    analyze_count,
    autoanalyze_count
FROM pg_stat_user_tables
ORDER BY table_name;
Copy to clipboard

References

Last updated: 26th January 2025
First published: 16th August 2018

This blog is created by Stuart Page

I'm a freelance web developer and technology consultant based in Surrey, UK, with over 10 years experience in web development, DevOps, Linux Administration, and IT solutions.

Need support with your infrastructure or web services?

Get in touch

Programster's Blog

Tutorials focusing on Linux, programming, and open-source

PostgreSQL Cheatsheet

Related Posts

Table of Contents

Basics

Installation

Login As Master/Root User

Login As Normal User

List / Show Databases

Switch Database

Show / List Tables

Describe Table

Show / List Sequences

Exit / Logout

Output Formatting

Flip Rows And Columns

Scrolling Results - Turn Off Paging

No Wrap Output

Output Results To File

Clear Screen

User Management

Create User

Create Role

Delete Role

Change User Password

Allow User To Login

List Users / Show Users

Rename User / Role

Grant User Superuser Privileges

Remove User Superuser Privileges

Grant User Full Access To Database

Grant User Read-Only Access to Database

Grant Ability To Create Databases

Grant Access To Public Schema

Add User To Role

Remove User From Role

Change Database Owner

Change Table Owner

Mass Change Ownership

Delete User / Role

General Administration

Show Running Queries

List User Connections

Show Settings / Variables

Kill A Users Connection / Session

Dump / Backup Database

Custom Format

Restore / Import Database

From SQL File

From Binary Dump

Extensions

List Extensions

CLI Command

SQL Query Form

Install Extension

Databases

Create A Database

Delete/Drop Database

Change Database Name

Create Table

Basic Create a table Example

Creating Tables With Foreign Keys

Creating Tables With Enums

Creating Tables With Unique Column

Create Table With Combined Unique Key

Editing Tables

Rename Table

Add Column

Edit / Alter / Modify Column

Drop Default

Set Not Null / Nullable

Rename Column

Drop Column

Indexes / Constraints

Create Index

Add Combined Unique Key Constraint

Add Foreign Key Constraint

Add Check Constraint

Create Table With Check Constraint