SQL Querying Multiple Tables: Difference between revisions

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Line 12: Line 12:
  Column    | Type                                                id        | name    | company_id
  Column    | Type                                                id        | name    | company_id
  -----------+---------                                          -----------+---------+-------------
  -----------+---------                                          -----------+---------+-------------
  id        | integer  <font color=teal># primary key</font>.                              1       | Alice    |  10
  id        | integer  <font color=teal># primary key</font>.                              1 | Alice    |  10
  name      | text                                                  2       | Bob      |  20  
  name      | text                                                  2 | Bob      |  20  
  company_id | integer  <font color=teal># foreign key that references company(id)</font>    3       | Charlie      |  30  
  company_id | integer  <font color=teal># foreign key that references company(id)</font>    3   | Charlie      |  30  
   
   
  <b>company</b>
  <b>company</b>

Revision as of 20:39, 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        | 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         | 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.

 SELECT person.name, company.name
 FROM person 
   INNER JOIN company ON person.id = company.id


Foreign key

Join