Re: Difference between Decode and Case?
Answer Decode
# 4 The default operator for comparison is '=',
for e.g select deptno,decode
(deptno,10,'dept10',20,dept20,default)
so if the deptno=10 then the value is replaced
by dept10
and so on,
whereas in Case:
we can use any operator other than '=',
for e.g
select deptno,
case when deptno=10 and sal>1000 then 'dept10'
else
'dept20'
Re: Difference between Decode and Case?
Answer Decode
# 3 1.It’s a function
2.can compare only discrete vaules
3.Pl/Sql standards
4.cannot process null
Case
1.It's an Expression
2.can hande range values
3.ANSI Standards
4.Processing time is faster when compared to
Decode
5.can process null
DECODE and CASE statements in Oracle both provide a conditional construct, of this form:
if A = n1 then A1
else if A = n2 then A2
else X
Databases before Oracle 8.1.6 had only the DECODE function. CASE was introduced in Oracle 8.1.6,
as a standard, more meaningful and more powerful function.
Everything DECODE can do, CASE can. There is a lot more that you can do with CASE, though,
which DECODE cannot – which we’ll see in this article.
1. CASE can work with logical operators other than ‘=’
DECODE can do an equality check only. CASE is capable of more logical comparisons such as < >
etc. To achieve the same effect with DECODE, ranges of data had to be forced into discrete form
making unwieldy code.
An example of putting employees in grade brackets based on their salaries – this can be done
elegantly with CASE.
SQL> select ename
2 , case
3 when sal < 1000
4 then 'Grade I'
5 when (sal >=1000 and sal < 2000)
6 then 'Grade II'
7 when (sal >= 2000 and sal < 3000)
8 then 'Grade III'
9 else 'Grade IV'
10 end sal_grade
11 from emp
12 where rownum < 4;
ENAME SAL_GRADE
---------- ---------
SMITH Grade I
ALLEN Grade II
WARD Grade II
2. CASE can work with predicates and searchable subqueries
DECODE works with expressions which are scalar values only. CASE can work with predicates and
subqueries in searchable form.
An example of categorizing employees based on reporting relationship, illustrating these two uses of
CASE.
SQL> select e.ename,
2 case
3 -- predicate with "in"
4 -- mark the category based on ename list
5 when e.ename in ('KING','SMITH','WARD')
6 then 'Top Bosses'
7 -- searchable subquery
8 -- identify if this emp has a reportee
9 when exists (select 1 from emp emp1
10 where emp1.mgr = e.empno)
11 then 'Managers'
12 else
13 'General Employees'
14 end emp_category
15 from emp e
16 where rownum < 5;
ENAME EMP_CATEGORY
---------- -----------------
SMITH Top Bosses
ALLEN General Employees
WARD Top Bosses
JONES Managers
3. CASE can work as a PL/SQL construct
DECODE can work as a function inside SQL only. CASE can be a more efficient substitute for IF-
THEN-ELSE in PL/SQL.
SQL> declare
2 grade char(1);
3 begin
4 grade := 'b';
5 case grade
6 when 'a' then dbms_output.put_line('excellent');
7 when 'b' then dbms_output.put_line('very good');
8 when 'c' then dbms_output.put_line('good');
9 when 'd' then dbms_output.put_line('fair');
10 when 'f' then dbms_output.put_line('poor');
11 else dbms_output.put_line('no such grade');
12 end case;
13 end;
14 /
PL/SQL procedure successfully completed.
CASE can even work as a parameter to a procedure call, while DECODE cannot.
SQL> var a varchar2(5);
SQL> exec :a := 'THREE';
PL/SQL procedure successfully completed.
SQL>
SQL> create or replace procedure proc_test (i number)
2 as
3 begin
4 dbms_output.put_line('output = '||i);
5 end;
6 /
Procedure created.
SQL> exec proc_test(decode(:a,'THREE',3,0));
BEGIN proc_test(decode(:a,'THREE',3,0)); END;
ERROR at line 1:
ORA-06550: line 1, column 17:
PLS-00204: function or pseudo-column 'DECODE' may be used inside a SQL
statement only
ORA-06550: line 1, column 7:
PL/SQL: Statement ignored
SQL> exec proc_test(case :a when 'THREE' then 3 else 0 end);
output = 3
PL/SQL procedure successfully completed.
4. Careful! CASE handles NULL differently
Check out the different results with DECODE vs NULL.
SQL> select decode(null
2 , null, 'NULL'
3 , 'NOT NULL'
4 ) null_test
5 from dual;
NULL
----
NULL
SQL> select case null
2 when null
3 then 'NULL'
4 else 'NOT NULL'
5 end null_test
6 from dual;
NULL_TES
--------
NOT NULL
The “searched CASE” works as does DECODE, though.
SQL> select case
2 when null is null
3 then 'NULL'
4 else 'NOT NULL'
5 end null_test
6* from dual
SQL> /
NULL_TES
--------
NULL
5. CASE expects datatype consistency, DECODE does not
Compare the two examples – DECODE gives you a result, CASE gives a datatype mismatch error.
SQL> select decode(2,1,1,
2 '2','2',
3 '3') t
4 from dual;
----------
SQL> select case 2 when 1 then '1'
2 when '2' then '2'
3 else '3'
4 end
5 from dual;
when '2' then '2'
ERROR at line 2:
ORA-00932: inconsistent datatypes: expected NUMBER got CHAR
6. CASE is ANSI SQL-compliant
CASE complies with ANSI SQL. DECODE is proprietary to Oracle.
7. The difference in readability
In very simple situations, DECODE is shorter and easier to understand than CASE, as in:
SQL> -- An example where DECODE and CASE
SQL> -- can work equally well, and
SQL> -- DECODE is cleaner
SQL> select ename
2 , decode (deptno, 10, 'Accounting',
3 20, 'Research',
4 30, 'Sales',
5 'Unknown') as department
6 from emp
7 where rownum < 4;
ENAME DEPARTMENT
---------- ----------
SMITH Research
ALLEN Sales
WARD Sales
SQL> select ename
2 , case deptno
3 when 10 then 'Accounting'
4 when 20 then 'Research'
5 when 30 then 'Sales'
6 else 'Unknown'
7 end as department
8 from emp
9 where rownum < 4;
ENAME DEPARTMENT
---------- ----------
SMITH Research
ALLEN Sales
WARD Sales
In complex situations, CASE is shorter and easier to understand. Complicated processing in
DECODE, even if technically achievable, is a recipe for messy, unreadable code – while the same can
be achieved elegantly using CASE.
