Usage of the key word in MYSQL when the then else end

Explain:

1 SELECT            2      Case                   -------------If3      whenSex='1'  Then 'male' -------------sex= ' 1 ', the return value ' male '4      whenSex='2'  Then 'female' -------------sex= ' 2 ', the return value ' female '5     Else 'other'                 -------------Other return ' other '6     End                    -------------End7  fromSys_user--------Overall Understanding: In the Sys_user table if sex= ' 1 ', then the return value ' male ' if sex= ' 2 ', then return the value ' female ' otherwise return ' other '

---usage one:

1 SELECT 2              Case  whenState= '1'  Then 'Success' 3                   whenState= '2'  Then 'failed'4             ELSE 'other' END  5              fromSys_scheduler

---usage II:

1 SELECT State2              Case  when '1'  Then 'Success' 3                   when '2'  Then 'failed'4             ELSE 'other' END  5              fromSys_scheduler

Example:

1 have employee table Empinfo2 ( 3Fempnovarchar2(Ten) not NULLPK,4Fempnamevarchar2( -) not NULL, 5Fage Number  not NULL, 6Fsalary Number  not NULL 7 ); 8 if the amount of data is about 10 million; write a SQL that you think is most efficient, and use a SQL to calculate the following four types of people:9Fsalary>9999  andFage>  * TenFsalary>9999  andFage<  *  OneFsalary<9999  andFage>  *  AFsalary<9999  andFage<  *  - the number of employees per type; - Select sum( Case  whenFsalary> 9999  andFage>  * the  Then 1 - Else0End) as"Fsalary>9999_fage> *", - sum( Case  whenFsalary> 9999  andFage<  * -  Then 1 + Else 0 - End) as"Fsalary>9999_fage< *", + sum( Case  whenFsalary< 9999  andFage>  * A  Then 1 at Else 0 - End) as"Fsalary<9999_fage> *", - sum( Case  whenFsalary< 9999  andFage<  * -  Then 1 - Else 0 - End) as"Fsalary<9999_fage< *" in  fromEmpinfo;

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Usage of the key word in MYSQL when the then else end