DECODE
is considered the most powerful function in Oracle. Oracle 8i release
introduced the CASE expression. The CASE expression can do all that DECODE does
plus lot of other things including IF-THEN analysis, use of any comparison
operator and checking multiple conditions, all in a SQL query itself. Moreover,
using the CASE function, multiple conditions provided in separate SQL queries
can be combined into one, thus avoiding multiple statements on the same table
(example given below). The function is available from Oracle 8i onwards.
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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