Basic syntax

CASE expression syntax is similar to an IF-THEN-ELSE statement. Oracle checks each condition starting from the first condition (left to right). When a particular condition is satisfied (WHEN part) the expression returns the tagged value (THEN part). If none of the conditions are matched, the value mentioned in the ELSE part is returned. The ELSE part of the expression is not mandatory-- CASE expression will return null if nothing is satisfied.

case when <condition> then <value>
when <condition> then <value>
...
else <value>
end 

Examples

The following examples will make the use of CASE expression more clear.

E.g.: Returning categories based on the salary of the employee.

select sal, case when sal < 2000 then 'category 1' 
                 when sal < 3000 then 'category 2' 
                 when sal < 4000 then 'category 3' 
                 else 'category 4' 
            end 
from emp; 

E.g.: The requirement is to find out the count of employees for various conditions as given below. There are multiple ways of getting this output. Five different statements can be written to find the count of employees based on salary and commission conditions, or a single select having column-level selects could be written.

select count(1) 
from   emp 
where  sal < 2000 
and    comm is not null; 
 
select count(1) 
from   emp 
where  sal < 2000 
and    comm is null; 

select count(1) 
from   emp 
where  sal < 5000 
and    comm is not null; 

select count(1) 
from   emp 
where  sal < 5000 
and    comm is null; 

select count(1) 
from   emp 
where  sal > 5000; 

(or)

select (select count(1)
        from   emp
        where  sal < 2000
        and    comm is not null) a,
       (select count(1)
        from   emp
        where  sal < 2000
        and    comm is null) b,
       (select count(1)
        from   emp
        where  sal < 5000
        and    comm is not null) c,
       (select count(1)
        from   emp
        where  sal < 5000
        and    comm is null) d,
(select count(1)
from   emp
where  sal > 5000) e
from dual

With CASE expression, the above multiple statements on the same table can be avoided.

select count(case when sal < 2000 and comm is not null then 1 
                  else null 
             end), 
       count(case when sal < 2000 and comm is null then 1 
                  else null 
             end), 
       count(case when sal < 5000 and comm is not null then 1 
                  else null 
             end), 
       count(case when sal < 5000 and comm is null then 1 
                  else null 
             end), 
       count(case when sal > 5000 then 1 
                  else null 
             end) 
from emp; 

(or)

select count(case when sal < 2000 and comm is not null then 1 
             end) cnt1, 
       count(case when sal < 2000 and comm is null then 1 
             end) cnt2, 
       count(case when sal < 5000 and comm is not null then 1 
             end) cnt3, 
       count(case when sal < 5000 and comm is null then 1 
             end) cnt4, 
       count(case when sal > 5000 then 1 
             end) cnt5 
from emp;

E.g.: CASE expression can also be nested.

select (case when qty_less6months < 0 and qty_6to12months < 0 then
                            (case when season_code in ('0', '1', '2', '3', '4') then 'value is negative'
                                  else 'No stock'
                             end)
             when qty_1to2years < 0 and qty_2to3years < 0 then
                            (case when season_code in ('A', 'B', 'C', 'D', 'E') then 'value is negative'
                                  else 'No stock'
                             end)
             else 'Stock Available'
        end) stock_check
from   jnc_lots_ageing_mexx_asof
where  rownum < 20
and    qty_less6months < 0 and qty_6to12months < 0

E.g.: The data types of the returned values should be the same. In the example below, one argument is assigned a numeric value resulting in an error.

SQL> select sal, case when sal < 2000 then 'category 1'
  2                   when sal < 3000 then 0
  3                   when sal < 4000 then 'category 3'
  4                   else 'category 4'
  5              end
  6  from emp;
                 when sal < 3000 then 0
                                      *
ERROR at line 2:
ORA-00932: inconsistent datatypes

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