PostgreSQL-Cheat-Sheet

Cheat Sheet for PostgreSQL, notes taken from freeCodeCamp.org

A database is a collection of data organized in a way that makes it easy to access and manipulate the data.

POSTGRE is the Database

SQL is the Structured Query Language ( Programming Lanuage)

How data is stored

Stores data in tables
Columns
Rows

Relational Database

A database that has relationships between tables

Getting Things Started

The postgresql package will by default create a user called postgres
Switch to the postgres user by sudo -iu postgres

sudo -iu logs you in as , without having to know 's password.

Look at the Initial cofiguration section to create a database cluster
exit out of the postgres user by exit
start the postgresql.service using systemctl start postgresql
type psql
type \q to exit
psql — PostgreSQL interactive terminal
Look at the Familiarize with PostgreSQL section to understand how to access database shell

Next Time

Command	Description
`systemctl start postgresql`	Start the Service
`sudo -iu postgres`	Change to the postgres user
`psql -d databasename`	Start the database shell
`\q`	Exit the database shell

Commands

Command	Description
`help`	Gives a small help guide
`\?`	Gives a big help guide
`\l`	Lists all databases
`CREATE DATABASE testname;`	Create a database by the name of testname
`\c databasename`	change to database
`\d`	(Describe) Show all the tables in Database
`\d tablename`	(Describe) Describe the table
`\dt`	Show only the table types (not other types)
`DROP TABLE tablename`	Drops the table
`DROP DATABASE dbname`	Drops the whole Database
`\i path_to_sqlfile`	Executes commands from a file
`SELECT col1, col2 FROM table`	select col1, col2 from table
`SELECT DISTINCT col FROM table`	select disctinct values from col only
`SELECT * FROM person ORDER BY col ASC`	Sort by col (ascending, default)
`SELECT * FROM person ORDER BY col DESC`	Sort by col (descending)
`SELECT * from table WHERE cond1 and cond2`	Filter based on multiple conditions
`WHERE col IN ('China', 'Poland', 'Japan')`	Filter if col is in the list
`WHERE col BETWEEN DATE '2000-01-01' AND '2015-01-01'`	Filter if col is between the dates

Connec to a Database

psql --help gives a list of options

Method 1 : psql -h hostname -p port -U username is a sample method to connect to a database, add other options accordingly
Method 2 : go to psql and type \c databaseaname to connect to a database, the same command can be used to change the database

A very Dangerous Command

To delete a database,

DROP DATABASE test;

MAKE SURE YOU REALLY WANT TO DO IT!

Create Table

CREATE TABLE table_name(
  Column name + data type + constraints if any
);

Ex:

CREATE TABLE person (
  id int ,
  first_name VARCHAR(50),
  last_name VARCHAR(50),
  gender VARCHAR(6),
  date_of_birth TIMESTAMP,
);

All Data Types in SQL

DOCUMENTAION : List of All the datatypes

Create Table with Constraints

Specify extra information that must be followed for every entry that is being addded

CREATE TABLE person (
  id BIGSERIAL NOT NULL PRIMARY KEY ,
  first_name VARCHAR(50) NOT NULL,
  last_name VARCHAR(50) NOT NULL,
  gender VARCHAR(6) NOT NULL,
  date_of_birth TIMESTAMP NOT NULL,
);

Insert records into tables

INSERT INTO tablename(
  col_name
)
VALUES ('value')

INSERT INTO person (
  first_name,
  last_name,
  gender,
  date_of_birth)
VALUES('Anne', 'Smith', 'FEMALE', DATE `1998-01-09');

Order in VALUES must match as given before

Generate 1000 Rows with Mockaroo

mockaroo website to create fake data

person.sql is the file generate using mockaroo.com

You can directly add the table from the sql file using the following command

\i path_to_loc\person.sql

Select From

SELECT * FROM table -- select all the columns from table

SELECT col_name FROM table -- select col_name from table

SELECT col1, col2 FROM table -- select col1, col2 from table

Order By

SELECT * FROM person ORDER BY col ASC -- Sort by col (ascending, default)

SELECT * FROM person ORDER BY col DESC -- Sort by col (descending)

Multiple Columns

SELECT * FROM person ORDER BY col1, col2 ASC -- Sort by col1 then by col2 (ascending, default)

Distinct

SELECT DISTINCT col FROM table

Distinct removes duplicate values from the table

Where Clause and AND

Filter based on condition

SELECT * from table WHERE cond1 and cond2

Eg:

SELECT * FROM table WHERE gender = 'FEMALE';

SELECT * FROM table WHERE gender = 'FEMALE' and country_of_birth = 'Poland'

Comparision Operators

<> is not equals symbol

Limit, Offset & Fetch

SELECT * FROM person LIMIT 10 -- Limit the results by 10 records

SELECT * FROM person OFFSET 5 LIMIT 5 -- Limit the results by 5 records and skip the first 5 records

LIMIT is not an official SQL command, use FETCH instead

SELECT * FROM person OFFSET 5 FETCH FIRST 5 ROWS ONLY

IN Keyword

Insstead of this

SELECT * FROM person WHERE country_of_birth = 'China' OR country_of_birth = 'Poland' OR country_of_birth = 'Japan'

Use IN keyword

SELECT * FROM person WHERE country_of_birth IN ('China', 'Poland', 'Japan')

Between Keyword

SELECT * FROM person WHERE date_of_birth BETWEEN DATE '2000-01-01' AND '2015-01-01'

Like and iLike Operators

SELECT * FROM person WHERE email LIKE '%.com'

'%' is a wildcard character, it matches any number of characters

SELECT * FROM person WHERE email LIKE '--------.com'

Number of - is the number of characters to be matched

ILIKE is case insensitive

Group By

SELECT country_of_birth, COUNT(*) FROM person GROUP BY country_of_birth

Groups by and gives the count

Group By and Having Keyword

Extra layer of filtering

SELECT country_of_birth, COUNT(*) FROM person GROUP BY country_of_birth HAVING COUNT(*) >5

List of Aggregate Functions

Avg, Min & Max Functions

Add a car table with the sql file given

SELECT MAX(price) FROM car -- Get the max car price

SELECT AVG(price) FROM car -- Get the average car price

SELECT ROUND(AVG(price)) FROM car -- Rounds the value

SELECT make, model, MIN(price) FROM car GROUP BY make, model -- Get the min price for each make and model

SELECT make, model, MIN(price), MAX(price), AVG(price) FROM car GROUP BY make, model

Sum Function

SELECT SUM(price) FROM car -- Get the sum of all car prices

SELECT make, SUM(price) FROM car GROUP BY make -- Get the sum of all car prices from each make

Basics of Arthemetic Operators

SELECT 10 + 2;

SELECT 10 * 2;

SELECT 10 / 2;

SELECT 10 ^ 2;

SELECT 5!;

SELECT 10 % 2;

Arthemetic Operators - Round

SELECT id, make, model, price, price * .10 FROM car; A new column for the price and 10% of the price

SELECT id, make, model, price, ROUND(price * .10, 2) FROM car; See the usage of ROUND

Alias

SELECT id, make, model, price AS original_price, ROUND(price * .10, 2) AS discounted_price FROM car; See the usage of AS

Coalesce

SELECT COALESCE(1);

SELECT COALESCE(null, 1); Still gives 1

SELECT COALESCE(null, 1, 10); Still gives 1 ( first non null value )

SELECT COALESCE(email, 'email not found') from person;

SathvikJoel / PostgreSQL-Cheat-Sheet