oracle decode

from: http://www.adp-gmbh.ch/ora/sql/decode.html 

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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.

a sample: 

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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