SQL Querying Multiple Tables
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External
Internal
Overview
More than one table can be used in a FROM query clause, and when that happens, it is said that the query performs a join.
Examples
All examples provided in this article are based on three tables (person, address and country), with the following schema:
person
Column | Type id | name | company_id
-----------+--------- ------+------------------+-------------
id | integer # primary key 1 | Alice | 10
name | text 2 | Bob | 20
company_id | integer # foreign key that references company(id) 3 | Charlie | 30
company
Column | Type id | name | city_id
-----------+--------- ------+------------------+-------------
id | integer # primary key 10 | Moonphone | 100
name | text 20 | Vortextime | 200
city_id | integer # foreign key that references city(id) 30 | Bluestone | 300
city
Column | Type id | name | company_id
-----------+--------- ------+------------------+-------------
id | integer # primary key 1 | Alice | 10
name | text 2 | Bob | 20
3 | Charlie | 30
Join Condition
If more than one table appears in the FROM clause, then the condition used to link the tables must be included as well. This is the ANSI-approved method. of joining multiple tables, and it is the most portable across various database servers.
SELECT person.name, company.name
FROM person
INNER JOIN company ON person.id = company.id
Foreign key