Using subquery clauses which affect the number or order of the rows in the subquery, for example

SELECT * FROM t1 WHERE t1.column1 IN
  (SELECT column1 FROM t2 ORDER BY column1);
SELECT * FROM t1 WHERE t1.column1 IN
  (SELECT DISTINCT column1 FROM t2);
SELECT * FROM t1 WHERE EXISTS
  (SELECT * FROM t2 LIMIT 1);

Replacing a join with a subquery, for example

SELECT DISTINCT column1 FROM t1 WHERE t1.column1 IN (
  SELECT column1 FROM t2);

instead of

SELECT DISTINCT t1.column1 FROM t1, t2
  WHERE t1.column1 = t2.column1;

Moving clauses from outside to inside the subquery, for example:

SELECT * FROM t1
  WHERE s1 IN (SELECT s1 FROM t1 UNION ALL SELECT s1 FROM t2);

instead of

SELECT * FROM t1
  WHERE s1 IN (SELECT s1 FROM t1) OR s1 IN (SELECT s1 FROM t2);

Para outro exemplo:

SELECT (SELECT column1 + 5 FROM t1) FROM t2;

em vez de:

SELECT (SELECT column1 FROM t1) + 5 FROM t2;

Using a row subquery instead of a correlated subquery, for example:

SELECT * FROM t1
  WHERE (column1,column2) IN (SELECT column1,column2 FROM t2);

instead of

SELECT * FROM t1
  WHERE EXISTS (SELECT * FROM t2 WHERE t2.column1=t1.column1
  AND t2.column2=t1.column2);

Using NOT (a = ANY (...)) rather than a <> ALL (...).

Using x = ANY (table containing {1,2}) rather than x=1 OR x=2.

Using = ANY rather than EXISTS