SQL Joins

SQL Joins

I'm always at a loss when asked to explain the differences between the different types of (SQL) joins. I know what I need to use but in my muddled mind can't ever explain why. This post should help to explain the differences for those that don't know and help remind those that do.

Joins are either inner or outer, outer can be full outer, left outer, or right outer. Let’s start with a couple of tables to help visualise what each join will do.

Table A User Id Name 1 Adrian Williams 2 Steve Sidwell 3 James Harper 4 Jamie Cureton 5 Nicky Forster 6 Barry Hunter 7 Jimmy Quinn

Table B User Id Login 3 Jharper 4 Jcureton 5 nforster 6 Bhunter 7 Jquinn 8 Scoppell 9 brodgers

Assume that all queries are asked to return user id (from Table A), name, and login.

Each of the joins will return the following

Left Outer Join

The left outer join will return all records from Table A and values from Table B where they exist, otherwise a null is returned.

TSQL:

select

      tablea.userid as userid,

      name,

      login

from

      TableA

left outer join

      TableB on TableA.userid = TableB.userid

Results:

User Id	Name	Login
1	Adrian Williams	NULL
2	Steve Sidwell	NULL
3	James Harper	jharper
4	Jamie Cureton	jcureton
5	Nicky Forster	nforster
6	Barry Hunter	bhunter
7	Jimmy Quinn	jquinn

Right Outer Join

The right outer join will return all records from Table B and values from Table A where they exist, otherwise a null is returned.

TSQL:

select

      tablea.userid as userid,

      name,

      login

from

      TableA

right outer join

      TableB on TableA.userid = TableB.userid

Results:

User Id	Name	Login
3	James Harper	jharper
4	Jamie Cureton	jcureton
5	Nicky Forster	nforster
6	Barry Hunter	bhunter
7	Jimmy Quinn	jquinn
NULL	NULL	scoppell
NULL	NULL	brodgers

Full Outer Join

The full outer join will return all records from Table A and Table B, where there isn’t a corresponding record in null will be returned.

TSQL:

select

      tablea.userid as userid,

      name,

      login

from

      TableA

Full outer join

      TableB on TableA.userid = TableB.userid

Results:

User Id	Name	Login
1	Adrian Williams	NULL
2	Steve Sidwell	NULL
3	James Harper	jharper
4	Jamie Cureton	jcureton
5	Nicky Forster	nforster
6	Barry Hunter	bhunter
7	Jimmy Quinn	jquinn
NULL	NULL	scoppell
NULL	NULL	brodgers

Inner Join

The inner join will only return data from rows where there is a match in both Table A and Table B.

TSQL:

select

      tablea.userid as userid,

      name,

      login

from

      TableA

inner join

      TableB on TableA.userid = TableB.userid

Results:

User Id	Name	Login
3	James Harper	jharper
4	Jamie Cureton	jcureton
5	Nicky Forster	nforster
6	Barry Hunter	bhunter
7	Jimmy Quinn	jquinn