Can I use more than one column in subquery?

Question

I want to show the names of all employees from the EMPLOYEES table who are working on more than three projects from the PROJECT table.

PROJECTS.PersonID is a a foreign key referencing EMPLOYEES.ID:

SELECT NAME, ID 
FROM EMPLOYEES
WHERE ID IN
    ( 
    SELECT PersonID, COUNT(*)
    FROM PROJECTS
    GROUP BY PersonID
    HAVING COUNT(*) > 3
    )

Can I have both PersonID, COUNT(*) in that subquery, or there must be only one column?

Answer 1

Not in an IN clause (or at least not the way you are trying to use it. Some RDBMSs allow tuples with more than one column in the IN clause but it wouldn't help your case here)

You just need to remove the COUNT(*) from the SELECT list to achieve your desired result.

SELECT NAME, ID 
FROM EMPLOYEES
WHERE ID IN
    ( 
    SELECT PersonID
    FROM PROJECTS
    GROUP BY PersonID
    HAVING COUNT(*) > 3
    )

If you wanted to also return the count you could join onto a derived table or common table expression with more than one column though.

SELECT E.NAME,
       E.ID,
       P.Cnt
FROM   EMPLOYEES E
       JOIN (SELECT PersonID,
                    Count(*) AS Cnt
             FROM   PROJECTS
             GROUP  BY PersonID
             HAVING Count(*) > 3) P
         ON E.ID = P.PersonID

Answer 2

To answer your question, you can only have 1 column for the IN subquery. You could get your results using the query below:

SELECT e.ID
      ,e.Name 
FROM dbo.Projects p
LEFT OUTER JOIN dbo.Employees e
    ON p.PersonID = e.ID
GROUP BY e.ID
        ,e.Name 
HAVING COUNT(*) > 3