Use of case when in Oracle

Http://www.cnblogs.com/eshizhan/archive/2012/04/06/2435493.html

1. Two forms of the case-when expression:

--Simple case function case  sex when  ' 1 ' then ' Male ' when  ' 2 ' Then ' Women '  Else ' other ' END  --case search function  casewhen sex = ' 1 ' Then ' man ' when  sex = ' 2 ' Then ' women ' else '  other ' END  

2. When the case is used in different locations of the SQL statement:

2.1 Select Case

SELECT   grade, count (case if sex = 1 then 1      /*sex 1 for boys, 2 girls *                       /ELSE NULL                       END) Number of boys,                count (case when s ex = 2 then 1                       ELSE NULL                       END) Number of schoolgirls from    students GROUP by grade;

2.2 where case

SELECT t2.*, t1.* from   T1, T2  WHERE (case when t2.compare_type = ' A ' and                   T1. Some_type like ' nothing% ' then                1 when              t2.compare_type! = ' A ' and                   T1. Some_type ' nothing% ' then                1              ELSE 0           END) = 1

2.3 Group by case

SELECT case when  salary <= "1" when  salary > Salary <= and  "2" when  salary > Salary <=  3 ' when  salary > Salary <= and ' 4 '  ELSE NULL END Salar Y_class,--alias name Count (*) from table_a GROUP by case when  salary <= and ' 1 ' when  salary &G T and salary <=  2 ' when  salary > Salary <=, then  ' 3 ' when  salary > Salary <= and 4 '  

3. Other implementations of If-then-else

3.1 Decode function:

Select decode (Sex, ' M ', ' Male ', ' F ', ' Female ', ' Unknown ') from   employees;

Use of case when in Oracle