DECODE

DECODE(Arg0, Arg1, Arg2, ...)

When Arg0 is Arg1, outputs Arg2. When Arg0 is Arg3, outputs Arg4. Repeats until it runs out of pairs of arguments. At this point, if there is one remaining argument, this is used as a default value. If not, then the output is NULL.

Note

Treats NULL as a literal value that can be matched on.

Therefore, the following:

DECODE(A, NULL, 0, 'x', 1, 'y', 2, 'z', 3, -1)

Is logically equivalent to:

CASE WHEN A IS NULL THEN 0
     WHEN A = 'x' THEN 1
     WHEN A = 'y' THEN 2
     WHEN A = 'z' THEN 3
     ELSE -1 END