Question

I have a table

form (
int id )
webformsystemflags ( int id, int formid int sysflagid )

sysflag ( int id, name char(10) )

form table is a table which has all the forms

webform is a table which has the forms which have flags applied to it. It has a foreign key formid which is id to the form table and sysflagid which is foreign key to the sys flag table

sys flag is the table which contains the flags. Lets say I have flags defined as 1,2,3
I can have forms which don’t have all the flags applied to it, some may have 1, some may have 2 or some may have 3 applied to it or some may have none.

How can I find all the forms which have either flag 1 or flag 2 or flag 3 applied to it ?

Answer 1

This is a common trick to find EXCLUSION. The value I have below of “FlagYouAreExpectingTo_NOT_Exist” is explicitly the one you expect NOT to be there. Here’s how it works.

Get every form and LEFT JOIN to the Web System Flags table WITH FINDING the matching form, and flag setting you DO NOT want. If it finds a valid entry for the form and flag, the “formid” in the (wsf) table will exist. So, we want all that DON’T exist, hence the closing WHERE wsf.formid is null.

It will be NULL for those where it is NOT already flagged.

select
      f.ID
   from
      forms f
         left join webformsystemflags wsf
            on  f.id = wsf.formid
           AND wsf.sysflagid = FlagYouAreExpectingTo__NOT__Exist
   where
      wsf.formid is null