I love it when a plan comes together
- by DavidWimbush
I'm currently working on an application so that our Marketing department can produce most of their own mailing lists without my having to get involved. It was all going well until I got stuck on the bit where the actual SQL query is generated but a rummage in Books Online revealed a very clean solution using some constructs that I had previously dismissed as pointless.
Typically we want to email every customer who is in any of the following n groups. Experience shows that a group has the following definition:
<people who have done A>
[(AND <people who have done B>) | (OR <people who have done C>)]
[APART FROM <people who have done D>]
When doing these by hand I've been using INNER JOIN for the AND, UNION for the OR, and LEFT JOIN + WHERE D IS NULL for the APART FROM. This would produce two quite different queries:
-- Old OR select A.PersonID from ( -- A select PersonID from ... union -- OR -- C select PersonID from ... ) AorC left join -- APART FROM ( select PersonID from ... ) D on D.PersonID = AorC.PersonID where D.PersonID is null -- Old AND select distinct main.PersonID from ( -- A select PersonID from ... ) A inner join -- AND ( -- B select PersonID from ... ) B on B.PersonID = A.PersonID left join -- APART FROM ( select PersonID from ... ) D on D.PersonID = A.PersonID where D.PersonID is null
But when I tried to write the code that can generate the SQL for any combination of those (along with all the variables controlling what each SELECT did and what was in all the optional bits of each WHERE clause) my brain started to hurt. Then I remembered reading about the (then new to me) keywords INTERSECT and EXCEPT. At the time I couldn't see what they added but I thought I would have a play and see if they might help. They were perfect for this. Here's the new query structure:
-- The way forward select PersonID from ( ( ( -- A select PersonID from ... ) union -- OR intersect -- AND ( -- B/C select PersonID from ... ) ) except ( -- D select PersonID from ... ) ) x
I can easily swap between between UNION and INTERSECT, and omit B, C, or D as necessary. Elegant, clean and readable - pick any 3! Sometimes it really pays to read the manual.
