DECODE
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DECODE is a single-row function in Oracle which applies conditional logic to SQL queries. This function works very much like an IF-THEN-ELSE statement.
Syntax:
DECODE (expression1, value1, return_value1, value2, return_value2, ... valueN, return_valueN, [default_return_value])
[edit] How it works
If expression1 equals value1 then return_value1 is returned by DECODE function, and if expression1 equals value2 then return_value2 is returned and so on…..
The optional default_return_value is returned if expression1 does not match with any of value1,value2,…..valueN values.
In short
IF expression1 = valueN THEN return_valueN ELSE default_return_value