Question

I have a query that is returning systems and areas from my database like so:

SELECT  Areas.ID AreaID,
    Areas.Name AreaName,
              Systems.*

FROM Systems
INNER JOIN Areas ON Areas.ID = Systems.AreaID
WHERE ....

THis returns data that looks like the following:

|  AreaID   |  AreaName   | SystemName  | ...
|     1     |    area1    |     sys1    |
|     1     |    area1    |     sys2    |
|     1     |    area1    |     sys3    |
|     1     |    area1    |     sys4    |
|     2     |    area2    |     sys5    |
|     2     |    area2    |     sys6    |

I would like to return an additonal column containing the number of systems in each area returned, so that I end up with something like this:

|  AreaID   |  AreaName   | SystemName  |  noOfSystems  | ...
|     1     |    area1    |     sys1    |       4       |
|     1     |    area1    |     sys2    |       4       |
|     1     |    area1    |     sys3    |       4       |
|     1     |    area1    |     sys4    |       4       |
|     2     |    area2    |     sys5    |       2       |
|     2     |    area2    |     sys6    |       2       |

I.E. There are 4 systems with the area id of 1 and 2 with the area id of 2.

How can this be done? I’m sure i’ve heard of a built in function that does this but I can’t find quite what I want.

Answer 1

You will want use the aggregate function COUNT() and then GROUP BY. This can be done in a correlated subquery:

SELECT  Areas.AreaID AreaID,
    Areas.areaname AreaName,
    s1.SystemName,
    (select count(systemname) 
     from systems s2
     where s1.areaid = s2.areaid
     group by areaid) noOfSystems
FROM Systems s1
INNER JOIN Areas 
    ON Areas.AreaID = s1.AreaID
WHERE ....

See SQL Fiddle with Demo

Or you can use a subquery that you join to get the total count:

SELECT  Areas.ID AreaID,
    Areas.Name AreaName,
    s1.SystemName,
    s2.NoOfSystems
FROM Systems s1
INNER JOIN Areas 
    ON Areas.ID = s1.AreaID
INNER JOIN
(
    select COUNT(SystemName) NoOfSystems,
        AreaID
    from systems
    group by AreaID
) s2
    on s1.AreaID= s2.AreaID
WHERE ....

See SQL Fiddle with Demo

This version uses a subquery to get the total count and then you join that back to the Systems table to return additional columns, if needed.

Or, you can use windowing functions if your RDBMS has this option by using Count() over():

SELECT  Areas.ID AreaID,
    Areas.Name AreaName,
    Systems.SystemName,
    COUNT(SystemName) over(partition by Areas.ID, Areas.AreaName) as NoOfSystems
FROM Systems
INNER JOIN Areas 
    ON Areas.ID = Systems.AreaID
WHERE ....

See SQL Fiddle with Demo