To CTE or not to CTE

Question

Having been stuck with SQL2000 for far too long, I've not really had a lot of exposure to Common Table Expressions.

The answers I've given here (#4025380) and here (#4018793) have gone against the flow in that they didn't use a CTE.

I appreciate that for recursion they are the beez kneez, and there are a few queries that can be greatly simplified by their use, but at what point is their use just frivolous? Do they have a great performance benefit over a subquery or a join? Do they really simplify code and make it more maintainable?

In short, when is it good practice to use a CTE over a 'lesser' syntax.

Answer 1

You should generally use a CTE over a normal subquery if:

• Your query requires recursion (as you noted)
• The subquery is large or complex
• The containing query is large or complex
• The subquery is repeated (or at least several subqueries can be simplified by performing different simple operations on a common subquery)

In short, yes they do make queries more readable when well-used.

Answer 2

Personally, once I got comfortable using them, I think that they produce cleaner, more readable code. As an example, compare your answer to mine on #4018793. We essentially did the same thing; I used a CTE and you didn't.

Your answer without CTE:

SELECT
    course,
    section,
    grade,
    gradeCount
FROM
    table
INNER JOIN
    (SELECT
        grade,
        Max(gradeCount) as MaxGradeCount
    FROM
        table
    ) MaxGrades
    ON  table.grade = MaxGrades.grade
        AND table.gradeCount = MaxGrades.MaxGradeCount
ORDER BY 
    table.grade

My answer with CTE:

;with cteMaxGradeCount as (
    select 
        grade, 
        max(gradeCount) as MaxGradeCount
    from @Test
    group by grade
)
select 
    t.course, 
    t.SECTION, 
    t.grade, 
    t.gradeCount
from cteMaxGradeCount c
inner join @Test t
    on  c.grade = t.grade
        and c.MaxGradeCount = t.gradeCount
order by t.grade

Answer 3

They are syntactic sugar, with the exception of hierarchical/recursive queries.

However, not everything that can be done recursively should be - date generation via recursive CTE was barely better than a cursor -- the NUMBERS table trick scaled much better.

Answer 4

CTE yields a faster result in a recursive scenario. The result of CTE is repeatedly used to get the final resultset. So since you have taken your where clause or subquery in CTE, definitely it is going to show performance improvement.
Reference : http://msdn.microsoft.com/en-us/library/ms190766(v=sql.105).aspx

Just a note, in many scenarios, temp tables gives better performance then CTE also, so you should give a try to temp tables as well.
Reference : http://social.msdn.microsoft.com/Forums/en/transactsql/thread/d040d19d-016e-4a21-bf44-a0359fb3c7fb