一个sql题目, 统计每年每月的信息

1 题目描述

2 建表和插入数据

CREATE TABLE sales(  id INT AUTO_INCREMENT PRIMARY KEY,  YEAR VARCHAR(10),  MONTH VARCHAR(10),  amount FLOAT(2,1));INSERT INTO sales VALUES (NULL,'1991','1',1.1),(NULL,'1991','2',1.2),(NULL,'1991','3',1.3),(NULL,'1991','4',1.4),(NULL,'1992','1',2.1),(NULL,'1992','2',2.2),(NULL,'1992','3',2.3),(NULL,'1992','4',2.4);

3 select语句

SELECT s.year ,(SELECT t.amount FROM  sales AS t WHERE t.month = '1' AND s.year = t.year) AS m1,(SELECT t.amount FROM  sales AS t WHERE t.month = '2' AND s.year = t.year) AS m2,(SELECT t.amount FROM  sales AS t WHERE t.month = '3' AND s.year = t.year) AS m3,(SELECT t.amount FROM  sales AS t WHERE t.month = '4' AND s.year = t.year) AS m4FROM sales AS s GROUP BY s.year