SQL---Case

In a database, there are two ways to compare data,

First: No conditional judgment on any column

SELECT student_id  As study number, Chinese as English results,case when Chinese >=" excellent "when中文版 >= and english<=' pass 'when中文版<  - ' inferior lattice ' END as describes the FROM dbo. Score

The result of the execution is:

School Number	English score	Describe
1	120	Excellent
2	100	Pass
3	55	Fail to pass
4	59	Fail to pass
5	60	Pass

Judging from case, judging what? I don't know, when the time begins, we know what to judge, similar to the If...esle If...else in C #.

Second: Direct case column, value judgment　

1 ' School Number is 1 '  2' study number 2'3' study number 3'   4 ' School number is 4 ' endfrom dbo.   Student

The result of the execution is:

(No column name)
School Number is 1
School Number is 2
School Number is 3
School Number is 4
Null

If you see that the last one is null, neither satisfies the case condition, and if there is no else statement, the default is to return a null value.

From the case start, know to judge the value of the StudentID, if it is 1, the output "study number 1", followed by and so on ...

Equivalent to switch (conditional) case in C #

SELECT Case Studentidwhen1Then'School Number is 1' when2Then'School Number is 2' when3Then'School Number is 3' when4Then'School Number is 4'ELSE'Anonymous'endfrom dbo. Student

The result of this SQL execution is:

(No column name)
School Number is 1
School Number is 2
School Number is 3
School Number is 4
John doe

Case for multiple columns:

SQL---Case