SQL Querying Multiple Tables: Difference between revisions
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=Overview= | =Overview= | ||
More than one table can be used in a <code>[[SQL_SELECT#Querying_Multiple_Tables|FROM]]</code> query clause, and when that happens, it is said that the query performs a '''join'''. | More than one table can be used in a <code>[[SQL_SELECT#Querying_Multiple_Tables|FROM]]</code> 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 (<code>person</code>, <code>address</code> and <code>country</code>), with the following schema: | |||
<font size=-2> | |||
<b>person</b> | |||
Column | Type | |||
-----------+--------- | |||
id | integer <font color=teal># primary key</font> | |||
name | text | |||
company_id | integer <font color=teal># foreign key that references company(id)</font> | |||
<b>company</b> | |||
Column | Type | |||
-----------+--------- | |||
id | integer <font color=teal># primary key</font> | |||
name | text | |||
city_id | integer <font color=teal># foreign key that references city(id)</font> | |||
<b>city</b> | |||
Column | Type | |||
-----------+--------- | |||
id | integer <font color=teal># primary key</font> | |||
name | text | |||
</font> | |||
=Join Condition= | =Join Condition= | ||
Revision as of 20:24, 23 May 2024
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 | integer # primary key name | text company_id | integer # foreign key that references company(id) company Column | Type -----------+--------- id | integer # primary key name | text city_id | integer # foreign key that references city(id) city Column | Type -----------+--------- id | integer # primary key name | text
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.
Foreign key