Decode Function and Case Statement are used to transform data values at retrieval time. DECODE and CASE are both analogous to the "IF THEN ELSE" conditional statement.
Before version 8.1, the DECODE was the only thing providing IF-THEN-ELSE functionality in Oracle SQL. Because DECODE can only compare discrete values (not ranges), continuous data had to be contorted into discreet values using functions like FLOOR and SIGN. In version 8.1, Oracle introduced the searched CASE statement, which allowed the use of operators like > and BETWEEN (eliminating most of the contortions) and allowing different values to be compared in different branches of the statement (eliminating most nesting). In version 9.0, Oracle introduced the simple CASE statement, that reduces some of the verbosity of the CASE statement, but reduces its power to that of DECODE.
Example with DECODE function
Say we have a column named REGION, with values of N, S, W and E. When we run SQL queries, we want to transform these values into North, South, East and West. Here is how we do this with the decode function:
Note that Oracle decode starts by specifying the column name, followed by set of matched-pairs of transformation values. At the end of the decode statement we find a default value. The default value tells decode what to display if a column values is not in the paired list.
Example with CASE statement
when ‘N’ then ’North’
when ‘S’ then ’South’
when ‘E’ then ’East’,
when ‘W’ then ’West’
Difference between DECODE and CASE:
Everything DECODE can do, CASE can. There is a lot more that you can do with CASE, though, which DECODE cannot. Following is the list of differences -
1. DECODE can work with only scaler values but CASE can work with logical oprators, predicates and searchable subqueries.
2. CASE can work as a PL/SQL construct but DECODE is used only in SQL statement.CASE can be used as parameter of a function/procedure.
3. CASE expects datatype consistency, DECODE does not.
4. CASE complies with ANSI SQL. DECODE is proprietary to Oracle.
5. CASE executes faster in the optimizer than does DECODE.
6. CASE is a statement while DECODE is a function.