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SELECT t1.col1, t2.col5
  FROM t1
...
  FROM t1
  JOIN t3 ON t3.tid = t1.tid
  WHERE t3.col4 = 42 AND ...

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 ...