Sql joins
From wikinotes
Joins
Overview
Joins let you
- query rows from multiple tables at once
- filter your table based on an association to another table
SELECT * # select all rows from 'users' and from 'department_users' tables FROM users INNER JOIN department_users # only return subset of rows where the 'ON' condition is met ON department_users.user_id = users.id # what determines a match? multiple conditions can be definedTypes
There are generally at least 3x types of joins.
INNER JOIN
- rows where match-condition is metLEFT JOIN
- all rows in src-table, columns in join-table without a match are all NULLRIGHT JION
- all rows in join-table, columns in src-tabl without a match are all NULLSubqueries
You can also join on a nested query.
See sql queries for an example.