Question

Basically I have this query:

( SELECT * FROM tbl WHERE type = 'word1' )
  UNION
( SELECT * FROM tbl WHERE type = 'word2' ) // Run this query if there are no results with type = 1

Basically I would like to run the second query only if the first hasn’t any results. is it possible?

Answer 1

The FIRST “PreCheck” query does a count of how many records ARE of type = 1. After that, if the count is greater than 1, then return 1, otherwise return 2.

Now, THAT answer can be used in the join (which is always a single row via COUNT(*)) which will either have a 1 or 2 value. THAT value will be the second value is the EQUALITY conditon. So, if there IS an entry of 1, the result will be as if

WHERE t1.Type = 1

Thus never allowing any 2 in the test. HOWEVER, if NO entries are found, it will have a value of 2 and thus create a WHERE clause of

WHERE t1.type = 2

select t1.* 
   from 
      ( select if( count(*) > 0, 1, 2 ) IncludeType
           from tbl t2
           where t2.type = 1 ) preCheck,
      tbl t1
   where 
      t1.type = preCheck.IncludeType

If there is an index on the “type” column, the first query should be almost instantaneous.