Case of SQL when

Basically learned the next SQL function statement, case when similar to the C language in the Switich cases statement, multi-branch selection, different values can have a different amount of return. For example, in a table that contains a sex field, the possible value is 1, 2, and after the subordinate statement is executed, a value of 1 returns the male, where the value is 2 to return the female.

--Simple case function when '         1 ' then ' Male ' when         ' 2 ' then ' women ' else ' other ' end--case search function case when sex = ' 1 ' Then ' man ' when         Sex = ' 2 ' Then ' women ' else ' other ' END

At the same time, case statements are implemented in two ways. The following tables are available:

Country (country)	Population (population)
China	600
United States	100
Canada	100
United Kingdom	200
France	300
Japan	250
Germany	200
Mexico	50
India	250

Now to count the number of people on each continent, you can use the following statement:

SELECT  SUM (population), case country when '                China ' Then ' Asia ' when '                India ' then     ' Asia ' when '                Japan ' Then     ' Asia '                When ' us ' then ' North America ' when                ' Canada ' Then ' North America ' when                ' Mexico ' then  ' North America '        Else ' other ' endfrom    Table_agroup by case country when '                China ' then ' Asia ' when '                India ' then     ' Asia ' when                ' Japan ' then     ' Asia ' C22/>when ' Us ' then ' North America ' when '                Canada ' then ' North America ' when                ' Mexico ' then  ' North America ' else '        other ' END;

The test code and results for SQLite data tables in Sqliteman are as follows:

Similarly, the method can be used to determine the number of countries on each continent, the code and results are as follows:

Here, sum (perosn) sums the number of people in each continent and returns the total number of values (total); count (perosn) counts the number of countries on each continent, returning the number of rows that match the specified criteria.

Refer to the above Information section from: https://www.cnblogs.com/prefect/p/5746624.html

Case of SQL when