The ultimate PostgreSQL cheatsheet

So I had been working with Django, Flask and Express.js for a while now, and my database of choice for every single project has of course always been Postgres. What makes these frameworks great (regardless of language) is the ability to use an ORM (Object Relational Mapper) that sits between your code and the DB. It does all the heavy lifting for you and takes care of executing SQL queries on your behalf.

This is great but I couldn’t stop thinking “what if I have to manually debug something directly into postgres ?” This is when reality slapped me in the face, I barely knew the SQL Language. So I thought It’d be fun to create a cheatsheet that I (and you) could keep as a reference for these times when you absolutely need to set this column to UNIQUE and you don’t know how.

Enjoy ! (and yeah I went all in with the pokemon references)

Create a User (or Role)

/* create a user without privileges*/
CREATE ROLE zabana;

/* create a user with privileges*/
CREATE ROLE zabana LOGIN CREATEDB CREATEROLE REPLICATION;

/* Add privileges to existing user*/
ALTER ROLE zabana WITH LOGIN CREATEROLE CREATEDB REPLICATION;

Make a user superuser (bump their privileges)

ALTER ROLE zabana WITH superuser;

Rename an existing user

ALTER ROLE psyduck RENAME TO brock;

Create a DB

CREATE DATABASE pokemons;

Rename a DB

ALTER DATABASE pokemons RENAME TO charizard;

Create a table in a DB

CREATE TABLE trainers(

    id INT PRIMARY KEY NOT NULL,
    pokemon_type CHAR(50),
    name CHAR(50) NOT NULL UNIQUE,
    gender CHAR(50) NOT NULL UNIQUE
);

Delete a DB

DROP DATABASE pokemons;

Delete a user

/* assuming there's a pikachu role on the system */
DROP ROLE pikachu;

Change DB ownership

ALTER DATABASE pokemons OWNER TO zabana;

Rename a table

ALTER TABLE trainers RENAME TO gym_trainers;

Change column type

ALTER TABLE gym_trainers ALTER COLUMN pokemon_type TYPE TEXT; /* it makes zero
sense to want to change this column type to TEXT but YOLO */

Rename a column

ALTER TABLE gym_trainers ALTER COLUMN name RENAME TO trainer_name;

Add a column to a table

ALTER TABLE gym_trainers ADD COLUMN bio TEXT;

/*
ALTER TABLE <table_name>
ADD COLUMN <column_name> <data_type> <constraints if any>
*/

Add a column with a UNIQUE constraint

ALTER TABLE gym_trainers ADD COLUMN age INT UNIQUE; /* cause why not */

Add a column with a NOT NULL constraint

ALTER TABLE gym_trainers ADD COLUMN main_pokemon CHAR(60) NOT NULL;

Remove NOT NULL CONSTRAINT from a column

ALTER TABLE gym_trainers ALTER COLUMN main_pokemon DROP NOT NULL;

Add a column with a NOT NULL constraint and a DEFAULT value

ALTER TABLE gym_trainers ADD COLUMN city CHAR(80) NOT NULL DEFAULT 'Indigo
Plateau';

Add a column with a CHECK constraint


CREATE TABLE trainers(

    id INT PRIMARY KEY,
    trainer_name CHAR(50) NOT NULL,

    /*
        method 1
        Add the check yolo style
    */
    age INT NOT NULL CHECK (age > 18)

    /*
        method 2
        Add a named constraint for better error handling
    */
    age INT CONSTRAINT legal_age CHECK (age > 18) NOT NULL

    /*
        method 3
        Add the constraint at the end
        for more clarity
    */
    age INT NOT NULL,
    CONSTRAINT legal_age CHECK (age > 18) NOT NULL
);

Add a CONSTRAINT to an existing column

ALTER TABLE trainers ADD CONSTRAINT unique_name UNIQUE (trainer_name);

Remove a named CONSTRAINT from a table

ALTER TABLE trainers DROP CONSTRAINT unique_name;

Insert a row into a table

INSERT INTO trainers VALUES (1, 23, 'brock');

/* or */

INSERT INTO trainers (age, trainer_name)
VALUES (1, 23, 'brock');

/*
    Note that in the second case we don't have pass
    the id. Postgres will automatically generate and autoincrement
    it for us. To omit the id column we must use named inserts otherwise an
    error is raised.
*/

Insert multiple rows into a table


INSERT INTO trainers (age, trainer_name) VALUES
(19, 'misty'),
(22, 'chen'),
/* ... */
/* ... */

Clear a table (without deleting it)

TRUNCATE trainers;

Set the primary key type to a serial (An auto incrementing integer)


/* Considering this table structure */

CREATE TABLE pokemon_list (

    id INT PRIMARY KEY,
    /* ... */
    /* ... */
    /* ... */
);


/*
    1.
    Create a sequence for the auto generating prinary key
    It follows the tablename_columnname_seq
*/
CREATE SEQUENCE pokemon_list_id_seq;

/* 2. Set the id column to not null */
ALTER TABLE pokemon_list ALTER COLUMN id SET NOT NULL;

/* 3. Set the default value to the next value in the sequence*/
ALTER TABLE pokemon_list
ALTER COLUMN pokemon_list
SET DEFAULT nextval('pokemon_list_id_seq');

/* 4. Link the sequence to the correct table and column */
ALTER SEQUENCE pokemon_list_id_seq OWNED BY pokemon_list.id;

Import data from a file

/*
    Must use absolute path and the user must have appropriate permissions
    Defaults to importing using TAB as the default parameter.
    We'll use a csv file as an example
*/

COPY pokemon_list FROM '/path/to/yourfile.csv' DELIMITER ',';

/*
    This only works if a pk is specified for each row
    The (my) prefered way to do it is the following
*/

COPY pokemon_list (name, level, type) FROM '/path/to/yourfile.csv' DELIMITER ',';

/* It's much more flexible because you control what data you actually import*/

Export a table to a file

To be able to export a table to a file, we need to ensure that postgres has write permissions to the file.

sudo chmod 777 /path/to/directory # This is just an example, edit this as needed

We can now safely copy the table to the file.

COPY pokemon_list TO '/path/to/file.csv' DELIMITER ',';

Select columns by using aliases

SELECT name AS pokemon_name, type AS pokemon_type
FROM pokemon_list;

Select elements based on a criteria

SELECT *
FROM pokemon_list
WHERE id > 3;

Select elements based on string comparison

SELECT *
FROM pokemon_list
WHERE type LIKE '%water%';

Select all results and order them by id in reverse

SELECT *
FROM pokemon_list
ORDER BY id DESC;

Select all results and order them by a column name

SELECT *
FROM pokemon_list
ORDER BY level;

/* If the column you're ordering by is not of type INT then the ordering will be
done alphabetically */

Select DISTINCT column from table

SELECT DISTINCT type AS pokemon_type
FROM pokemon_list;

Limit the results from a SELECT query

SELECT *
FROM pokemon_list
LIMIT 3;

Select the last 3 items

SELECT *
FROM pokemon_list
ORDER BY id DESC
LIMIT 3;

Create two tables with a foreign key relationship

CREATE TABLE pokemon_types(

    id SERIAL PRIMARY KEY,
    type_name CHAR(120) NOT NULL
)

CREATE TABLE pokemon_list(

    id serial PRIMARY KEY,
    pokemon_name CHAR(120) NOT NULL,
    pokemon_level INT NOT NULL,
    pokemon_type INT REFERENCES pokemon_types(id) NOT NULL,
    CONSTRAINT pokemon_level_not_zero CHECK (pokemon_level > 0)

);

Perform Joins based on a criteria

SELECT name, level, pokemon_types.name AS type
FROM pokemon_list
JOIN pokemon_types
ON pokemon_type_id  = pokemon_types.id
WHERE pokemon_type_id = 1;

/* Will return the name, level and type name for all water pokemons */

Perform joins

SELECT name, level, pokemon_types.name AS type
FROM pokemon_list
JOIN pokemon_types
ON pokemon_type_id  = pokemon_types.id;

Create a VIEW based on a JOIN

CREATE VIEW pokemonswithtypes AS
SELECT name, level pokemon_types.name AS type
FROM pokemon_list
JOIN pokemon_types
ON pokemon_type_id = pokemon_types.id;

/* To see the data */

SELECT * FROM pokemonswithtypes;

Update the VIEW (Change the query)

SELECT name, pokemon_types.name AS type
FROM pokemon_list
JOIN pokemon_types
ON pokemon_type = pokemon_types.id
WHERE pokemon_type = 1;

/* only show the name and type for water pokemons */

Delete the VIEW

DROP VIEW pokemonswithtypes;

Use aggregate functions (MIN, MAX, SUM, COUNT, AVG)

/* MAX */
SELECT MAX(pokemon_level)
FROM pokemon_list;

/* MIN */
SELECT MIN(pokemon_level)
FROM pokemon_list;

/* AVG */
SELECT AVG(pokemon_level)
FROM pokemon_list;

/* ROUND */
SELECT ROUND(AVG(pokemon_level))
FROM pokemon_list;

/* COUNT */
SELECT COUNT(*)
FROM pokemon_list;

/* SUM */
SELECT SUM(pokemon_level)
FROM pokemon_list;

Use boolean aggregate functions

/* Add a column is_legendary of type boolean to table pokemon_list */
ALTER TABLE pokemon_list ADD COLUMN is_legendary BOOL NOT NULL DEFAULT TRUE;

/*  BOOL_AND
    returns a result if **ALL** records have that column set to true
*/

SELECT BOOL_AND(is_legendary) FROM pokemon_list;


/*
    BOOL_OR
    returns a result if one or more records have that column set to true
*/

SELECT BOOL_OR(is_legendary) FROM pokemon_list;

Update a table and change all column values

UPDATE pokemon_list
SET is_legendary = FALSE;

Update a table and change value based on a criteria

UPDATE pokemon_list
SET is_legendary = TRUE
WHERE id = 2;

Delete row with specific id

DELETE FROM pokemon_list WHERE id = 4

Delete rows withing a range of ids

DELETE FROM pokemon_list
WHERE id BETWEEN 1 AND 4;

Delete all rows

DELETE FROM pokemon_list;

Note: The difference between DELETE and DROP or TRUNCATE is that the former can be undone (rolled back) the latter can’t

Alter a table to drop a CONSTRAINT if it exist

ALTER TABLE pokemon_types
DROP CONSTRAINT IF EXISTS unique_type_name;

Note: If you find errors, typos or would like to add new tips, feel free to reach out to me on twitter. I’m @zabanaa_. Thank you for reading ! And if you find this useful, share it with your friends and coworkers !

zabana@karimc:~$ ls otherposts/