SQL --- CASE expression

CASE expression is used to determine the condition returns a value when the condition is satisfied, another value is returned when the condition is not satisfied.

 

grammar:

CASE WHEN ... THEN ...

           WHEN ... THEN ...

           ELSE ... 

END

(Note: The data type of each of the branches back must be the same.)

(Note: when exclusivity clause must be, because when the when clause is true, when the remaining clauses will be ignored.)

 

CASE
    WHEN ComparsionCondition THEN result
    WHEN ComparsionCondition THEN result
    ELSE other
 END

 

CASE expression uses:

1, the conventional coding method conversion

example:

student_name  score
Ming          80
Red           92
Huzi          100
Mung          60

Score student_name 
Ming good 
Red preferably 
Huzi preferably 
Mung in

SELECT student_name,
            (CASE WHEN score>=90 THEN '优'
                      WHEN score<90 AND score>=75 THEN '良'
                      WHEN score<75 AND score>=60 THEN '中'
             ELSE '不及格' END) AS score
FROM Score;

 

2, Cross-Tab Construction

example:

The Result DATE
  1900-1-1 wins
  1900-1-1 wins
  1900-1-1 negative
  1900-1-2 wins
  1900-1-2 wins

          Outcome
  1900-1-121 
 1900-1-2 2 0

SELECT DISTINCT Date,
SUM(CASE WHEN Result='胜' THEN 1 ELSE 0 END) AS '胜',
SUM(CASE WHEN Result='负' THEN 1 ELSE 0 END) AS '负'
FROM Test
GROUP BY Date;

 

3, the ranks of the converted

product_type sum_price
clothes       5000
office        600
kitchen       10000

sum_price_clothes sum_price_office sum_price_kitchen
5000              600              10000

SELECT SUM(CASE WHEN product_type = '衣服' THEN sale_price
                ELSE 0 END) AS sum_price_clothes,
            SUM(CASE WHEN product_type = '办公用品' THEN sale_price
                ELSE 0 END) AS sum_price_office,
            SUM(CASE WHEN product_type = '厨房用品' THEN sale_price
                ELSE 0 END) AS sum_price_kitchen
FROM Product;

 

4, generate a specific ordering

Examples: The table of the sorted sequence B, D, C, A of the

item xyz 
A     1 2 3 
B     5 5 2 
C     4 7 1 
D     3 3 8

SELECT item, x, y, z
FROM t
ORDER BY ( CASE WHEN item="B" THEN 1
                           WHEN item="D" THEN 2
                           WHEN item="C" THEN 3
                           WHEN item="A" THEN 4
                           ELSE NULL
                  END );