SQL Querying Multiple Tables: Difference between revisions
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| Line 15: | Line 15: | ||
<font size=-2> | <font size=-2> | ||
<b>person</b> | <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> | <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> | <b>city</b> | ||
Column. | Type | |||
-----------+--------- | |||
id | integer <font color=teal># primary key</font> | |||
name | text | |||
</font> | </font> | ||
Revision as of 20:22, 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.
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.
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
Foreign key