Question

I’m having a problem with mysql that you might be able to help. I have 2 tables with the following structure, and a couple of sample rows

TABLE 1
-------
LISTID     NAME        10_OTHER_COLUMNS
---------------------------------------
1          List1
2          List2
3          List3

TABLE 2
-------
LISTID      LISTTYPE(ENUM, 4 options)
------------------------------------------------
1           type1
1           type2
2           type3
3           type1
3           type2
3           type3

The relation is one to many from TABLE 1 to TABLE 2. I want to do a select on TABLE 1 where the rows are return ONLY, and ONLY IF they have exactly one match on TABLE 2. Explaining better, a list may have more than one type, if I simply do a:

SELECT t1.*
FROM table_1 t1
LEFT JOIN table_2 t2 USING (listid)
WHERE t2.listtype = 'type3'

It returns List2 and List3. I want to remove List3 from the results and only have List2. Any ideas?

Answer 1

Ok, I THINK I got it… You are asking for ex: “type3”, an English description to clarify would be…

Give me a list of all Tables (ie: ListID) that AT MOST have ONLY the one code I’m looking for and nothing else, not associated with any other “types”.

select 
      t1.*
   from
      ( select
              t2.ListID,
              count(*) as TotalTypesForTable,
              sum( if( t2.type = 'type3', 1, 0 )) as TypeWanted
           from
              Table2 t2
           group by
              t2.ListID ) PreQuery

      JOIN Table1 t1
         on PreQuery.ListID = t1.ListID
   where
          PreQuery.TypeWanted = 1
      AND PreQuery.TotalTypesForTable = 1

FEEDBACK OPTION…

Per your comment about a couple million records, I would adjust it this way which may also be faster overall too.

select 
      t1.*
   from
      Table2 t2Required

         LEFT JOIN Table2 as t2Extra
            on t2Required.ListID = t2Extra.ListID
           AND NOT t2Extra.type = 'type3'

         JOIN Table1 t1
            on t2Required.ListID = t1.ListID
   where
          t2Required.type = 'type3'
      and t2Extra.ListID IS NULL

I’d like to also know the performance difference on the second one. This second approach actually works based on an INTENTIONAL expected “don’t find me”, but doesn’t require a sub-select. By joining table2 to itself on same ID, but an “any other” type than the one we’re looking for, the ONLY one we want is the entry where WE DONT find in the t2Extra instance. THEN, get the table name from t1…