5.0 KiB

Raw Blame History

Intro to SQL

Lesson Objectives tools

Connect to Postgres through CLI
Create a Database
Create a table
Insert into the table
Select from table
Update the table
Delete from table

Connect to Postgres through CLI

If you used homebrew to install, use this command to start up the postgres enviornment.

brew services start postgresql

You can use this command to stop the service if you would like or you can just leave it running in the background at all times (it will disconnect when you shut down your machine).

brew services stop postgresql

Lets start up the postgres shell:

psql -l (list all subdatabses) 
psql db_name (start psql shell, using the sub database db_name)
psql (start psql shell connecting to your username)

Once inside the psql app, you can list the sub databases like this:

\l

Create a Database

Postgres has "sub-databases" (probably one for each app that you build)

Create a new database

CREATE DATABASE foo;

Delete (drop) a database:

DROP DATABASE foo;

Connect to a different database:

\connect test_db;

Data types

Postgres has the following data types:

int - whole number
decimal - float/decimal
bool - boolean
varchar(n) - small text
text - large text
timestamp - date

Create a table

Inside a database, we have tables, which are just like a spreadsheet or grid. Rows are entries, and columns are properties of each row.
You have to tell Postgres what data type each column is. It's very 'strict'

Create a table called 'foo' with one column called 'name' which is a small text column:

CREATE TABLE foo ( name varchar(20) );

List (describe) your tables:

\dt

Delete (drop) a table:

DROP TABLE foo;

Create table with multiple columns

id is a serial, which is a special integer that increments each time a new row is created)
name is a 20 character string
age is an integer
email is a 32 character string

CREATE TABLE users ( id serial, name varchar(20), age int, email varchar(32) ); -

Describe the columns of the users table

\d users;

Insert into the table

Make sure your values are in the same order that you specify they will be. You can move them around as you like, as long as you're consistent

INSERT INTO users ( name, age, email ) VALUES ( 'Matt', 36, 'matt.huntington@generalassemb.ly');

Select from table

There are lots of ways to alter how you retrieve data from a table:

Select all rows from the users table. Display only the name column

SELECT name FROM users;

Select all rows from the users table. Display all columns

SELECT * FROM users;

Select all rows from the user table where the name column is set to 'Matt'

SELECT * FROM users WHERE name = 'Matt';

Select all rows from the user table where the name column contains 'Matt' as a substring

SELECT * FROM users WHERE name LIKE '%Matt%';

Select all rows from the user table where the name column is set to 'Matt' AND the email column is set to matt.huntington@gmail.com

SELECT * FROM users WHERE name = 'Matt' AND email = 'matt.huntington@gmail.com';

Select all rows from the user table where either the name column is set to 'Matt' OR the email column is set to matt.huntington@gmail.com

SELECT * FROM users WHERE name = 'Matt' OR email = 'matt.huntington@gmail.com';

Select all rows from the user table where the age column is set to 36

SELECT * FROM users WHERE age = 36;

Select all rows from the user table where the age column is not set to 16

SELECT * FROM users WHERE age != 16;

Select all rows from the user table where the age column is greater than 26

SELECT * FROM users WHERE age > 26;

Select all rows from the user table where the age column is less than 46

SELECT * FROM users WHERE age < 46;

Select all rows from the user table where the age column is less than or equal to 36

SELECT * FROM users WHERE age <= 36;

Select all rows from the user table where the age column is greater than or equal to 36

SELECT * FROM users WHERE age >= 36;

Select all rows from the user table where the age column has no value

SELECT * FROM users WHERE age IS NULL;

Select all rows from the user table where the age column has any value

SELECT * FROM users WHERE age IS NOT NULL;

Update the table

Update the users table. Set the name column to 'Matthew' for every row that has the id column set to 1. VERY IMPORTANT If you do not specify a WHERE condition, or in some other way limit it, the query will alter every row in your table. WATCH OUT!!

UPDATE users SET name = 'Matthew' WHERE id = 1;

Delete from table

Delete all rows from the users table that have the id column set to 1. VERY IMPORTANT If you do not specify a WHERE condition, or in some other way limit it, the query will delete every row in your table. WATCH OUT!!

DELETE FROM users WHERE id = 1;

5.0 KiB Raw Blame History