SQL Querying Multiple Tables: Difference between revisions

From NovaOrdis Knowledge Base
Jump to navigation Jump to search
Line 26: Line 26:
  <b>city</b>
  <b>city</b>
   
   
  Column    | Type                                                  id | name            | company_id
  Column    | Type                                                  id | name             
  -----------+---------                                          ------+------------------+-------------
  -----------+---------                                          ------+------------------
  id        | integer <font color=teal># primary key</font>                               1  | Alice            | 10
  id        | integer <font color=teal># primary key</font>                               100 | San Francisco
  name      | text                                                      | Bob              | 20  
  name      | text                                                      200 | New York  
                                                                         3  | Charlie          | 30
                                                                         300 | Chicago
</font>
</font>



Revision as of 20:47, 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 | 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             
-----------+---------                                           ------+------------------
id         | integer # primary key                               100 | San Francisco
name       | text                                                      200 | New York  
                                                                       300 | Chicago

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