SQL Querying Multiple Tables

Internal

• Querying Multiple Tables

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

The mechanism of linking two or more tables is referred to as a join. For two tables, the most generic way of combining rows is to take every row from the first table and combine it with every row with the other table. This is known as the cartesian product of two relations, or a cross join. For two tables with N and M rows respectively, the result will contain N x M rows. In case of three tables, with N, M and P rows respectively, the result will contain N x M x P rows. See the Cross Join section below for an example of how that works.

If more than one table appears in the FROM clause, then the conditions 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. The syntax requires that all the tables are specified in the FROM clause. One of the table does not need any qualifiers, then for each additional table, an:

JOIN table_name ON <join_condition>

is added as in the following example:

 FROM table_one
   JOIN table_two ON <join_condition>
   JOIN table_three ON <join_condition>
   ...

Note that a JOIN keyword without any qualifier defaults to an inner join.

To inner join two tables:

 SELECT person.name AS name, company.name AS company
 FROM person 
   JOIN company ON person.company_id = company.id

  name   |  company
---------+------------
 Alice   | Moonphone
 Bob     | Vortextime
 Charlie | Bluestone

To inner join three tables:

 SELECT person.name AS name, company.name AS company, city.name AS city 
 FROM person
   JOIN company ON person.company_id = company.id 
   JOIN city ON company.city_id = city.id;

  name   |  company   |     city
---------+------------+---------------
 Alice   | Moonphone  | San Francisco
 Bob     | Vortextime | New York
 Charlie | Bluestone  | Chicago

Using Table Aliases in Joins

The SQL syntax permits to alias each of the join-participating tables, so you can identify which tables you are referring to when you reference columns in the SELECT clause, by using table aliases:

 FROM table_one AS table_one_alias
   JOIN table_two AS table_two_alias ON <join_condition>
   JOIN table_three AS table_three_alias ON <join_condition>
   ...

For permanent, temporary and virtual tables aliasing is optional, their original names are sufficient to reference columns. However for derived tables, the alias is useful:

SELECT person.name, derived_table_alias.name 
FROM person 
  JOIN (SELECT id, UPPER(name) AS name FROM company) AS derived_table_alias ON person.company_id = derived_table_alias.id;

Join Condition

Tables are usually joined using foreign keys. The join condition states that we should associate the rows from the first table with rows from other table where the foreign key column value from the first table row is equal with the primary key column value from other table row.

Join Types

Inner Join

When JOIN is specified without any qualifier, it implies an inner join. For clarity, INNER can be specified:

 SELECT ...
 FROM ...
   INNER JOIN sometable ON ...

Cross Join

A cross join produces the cartesian product of the tables involved in join.

For two tables:

 
SELECT * FROM person CROSS JOIN company;

 id |  name   | company_id | eye_color | id |    name    | city_id
----+---------+------------+-----------+----+------------+---------
  1 | Alice   |         10 | blue      | 10 | Moonphone  |     100
  2 | Bob     |         20 | black     | 10 | Moonphone  |     100
  3 | Charlie |         30 | black     | 10 | Moonphone  |     100
  1 | Alice   |         10 | blue      | 20 | Vortextime |     200
  2 | Bob     |         20 | black     | 20 | Vortextime |     200
  3 | Charlie |         30 | black     | 20 | Vortextime |     200
  1 | Alice   |         10 | blue      | 30 | Bluestone  |     300
  2 | Bob     |         20 | black     | 30 | Bluestone  |     300
  3 | Charlie |         30 | black     | 30 | Bluestone  |     300
(9 rows)