Join Vs Sub Query

Practice 1:

You can improve performance by avoiding joins altogether.Consider this SELECT:

SELECT COUNT (*) FROM Table1, Table2
WHERE Table1.column1 = Table2.column1

If Table1.column1 is unique, you could replace the join with a sub query.

Transform the SELECT to:

SELECT COUNT (*) FROM Table2
WHERE Table2.column1 IN
(SELECT Table1.column1 FROM Table1)
GAIN: 4/6

WARNING
Don't do this for Oracle; it shows a loss.

Practice 2: