SQL Querying Multiple Tables: Difference between revisions
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<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>city</b> | |||
Column | Type | Column | Type | ||
Revision as of 20:19, 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
city
Column | Type
------------+------------------------
id | smallint
name | character varying(30)
birthday | date
address_id | smallint
Indexes:
"person_pkey" PRIMARY KEY (id)
Foreign-key constraints:
"person_address_id_fkey" FOREIGN KEY (address_id) REFERENCES address(id)
Foreign key