SQL foundation tutorial (2nd edition) Chapter 6 function, predicate, CASE expression: 6-3 CASE expression

● Although the CASE expression in the ELSE clause can be omitted, but in order to make the SQL statements easier to understand, or that we should not be omitted. 
● the CASE expression END can not be omitted.
● Use CASE expression can be SELECT statement are combined.

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What is CASE expression
CASE expression is used in distinguishing cases, distinguished in such a case generally referred to programming (conditional) branch .

CASE expression syntax
Let's hurry to learn about search CASE expression syntax right.

CASE use expressions
ELSE clause also can be omitted, then will default to ELSE NULL . However, in order to prevent the leakage was read, still I hope that we can show to write  ELSE  clause.
 

 

--MySQL
-- MySQL中使用IF代替CASE表达式
SELECT  product_name,
        IF( IF( IF(product_type = '衣服',  CONCAT('A：', product_type), NULL)
                    IS NULL AND product_type = '办公用品', CONCAT('B：', product_type), 
                IF(product_type = '衣服',  CONCAT('A：', product_type), NULL))
                    IS NULL AND product_type = '厨房用具', CONCAT('C：', product_type), 
                    IF( IF(product_type = '衣服',  CONCAT('A：', product_type), NULL)
                    IS NULL AND product_type = '办公用品', CONCAT('B：', product_type), 
                IF(product_type = '衣服',  CONCAT('A：', product_type), NULL))) AS abc_product_type
  FROM Product;

■ CASE表达式的书写位置