CASE 语法

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    WHEN conditionN THEN resultN
    ELSE result
END; 

将表中的内容转换为右边的形式：

1、创建表，创建数据

CREATE TABLEchapter10_7 (
	order_id VARCHAR(255) NULL,
	price VARCHAR(255) NULL,
	deal_date VARCHAR(255) NULL,
	area VARCHAR(255) NULL
)
ENGINE=InnoDB
DEFAULT CHARSET=utf8mb4
COLLATE=utf8mb4_0900_ai_ci;

INSERT INTOchapter10_7 (order_id, price, deal_date, area) VALUES('S001', '10', '2019/1/1', 'A区');
INSERT INTOchapter10_7 (order_id, price, deal_date, area) VALUES('S002', '20', '2019/1/1', 'B区');
INSERT INTOchapter10_7 (order_id, price, deal_date, area) VALUES('S003', '30', '2019/1/1', 'C区');
INSERT INTOchapter10_7 (order_id, price, deal_date, area) VALUES('S004', '40', '2019/1/2', 'A区');
INSERT INTOchapter10_7 (order_id, price, deal_date, area) VALUES('S005', '10', '2019/1/2', 'B区');
INSERT INTOchapter10_7 (order_id, price, deal_date, area) VALUES('S006', '20', '2019/1/2', 'C区');
INSERT INTOchapter10_7 (order_id, price, deal_date, area) VALUES('S007', '30', '2019/1/3', 'A区');
INSERT INTOchapter10_7 (order_id, price, deal_date, area) VALUES('S008', '40', '2019/1/3', 'C区');

2、SQL解决：

-- 选择 deal_date 和三个区域的订单数量  
SELECT  
    a.deal_date,  -- 选择 deal_date 列  
    COUNT(CASE WHEN a.area = 'A区' THEN order_id ELSE NULL END) AS A区,  -- 计算 A区的订单数量  
    COUNT(CASE WHEN a.area = 'B区' THEN order_id ELSE NULL END) AS B区,  -- 计算 B区的订单数量  
    COUNT(CASE WHEN a.area = 'C区' THEN order_id ELSE NULL END) AS C区  -- 计算 C区的订单数量    
FROM  
    chapter10_7 a  -- 从 chapter10_7 表中选择数据，并给这个表起了一个别名 'a'    
GROUP BY  
    a.deal_date;  -- 根据 deal_date 进行分组，这样我们可以为每个 deal_date 计算三个区域的订单数量

CASE when 实战练习拓展：

SELECT
	order_id,
	price,
CASE		
		WHEN price <= 10 THEN '价格低' 
		WHEN price >= 10 AND price <= 20 THEN '价格中等'
		WHEN price >= 30 THEN'价格高等'
		ELSE '价格有问题' 
		END AS price_category 
FROM
	demo.chapter10_7