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decode [Oracle SQL]

decode (expression, search_1, result_1)
decode (expression, search_1, result_1, search_2, result_2)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n)

decode (expression, search_1, result_1, default)
decode (expression, search_1, result_1, search_2, result_2, default)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n, default)
decode compares expression to the search_x expressions and, if matches, returns result_x. If not, returns default, or, if default is left out, null.
This is demonstrated with the following example: 
create table a (x int, b int);

insert into a values (1,2);
insert into a values (1,3);
insert into a values (2,4);
insert into a values (2,5);
insert into a values (3,6);

select * from a where b = decode (x,1,2,2,4);

         X          B
---------- ----------
         1          2
         2          4

Links

See also case when which allows to do similar things, but is more powerful.
On the lazy evaluation of decode and the eager evaluation of nvl.