Just wanted to point out that you can make that SQL quite a bit less repetitive. Depending on the data stored in t2 and t3 this may or may not hold true

SELECT t1.col1, t2.col5
  FROM t1
  JOIN t2 ON t2.tid = t1.tid
  WHERE t2.col3 = 99 AND ...
UNION
SELECT t1.col1, t3.col5
  FROM t1
  JOIN t3 ON t3.tid = t1.tid
  WHERE t3.col4 = 42 AND ...
[download]

is often equivalent to

SELECT t1.col1, coalesce(t2.col5, t3.col5) AS col5
FROM t1
LEFT JOIN t2 ON t2.tid = t1.tid AND t2.col3 = 99
LEFT JOIN t3 ON t3.tid = t1.tid AND t3.col4 = 42
WHERE ...
[download]

which is probably much easier for the database server to execute. The EXCEPT can be done with a left anti-join.

But do heed the other nameless monk's advice. Taking column names directly from a form is a security hole.