SQL VQ22 判断其是否有过购买记录

题目 : https://www.nowcoder.com/practice/5cddad8995974b0c915ab89961428ee0?tpId=341

数据

drop table if exists customers_info;

CREATE TABLE `customers_info` (
  `customer_id` int NOT NULL, -- 客户id
  `gender` varchar(10) NOT NULL, -- 客户性别
  `city` varchar(32) NOT NULL, -- 客户所在城市
  `country` varchar(32) NOT NULL, -- 客户所在国家
  `age` int, -- 客户年龄
  `latest_place_order_date` date 
); -- 客户最近购买日期

INSERT INTO customers_info VALUES(18903, 'male', '北京', '中国', 23, '2021-03-01');
INSERT INTO customers_info VALUES(21089, 'female', '纽约', '美国', 66, '2022-05-09');
INSERT INTO customers_info VALUES(22869, 'male', '上海', '中国', null, '2022-03-09');
INSERT INTO customers_info VALUES(16739, 'null', '杭州', '中国', 36, null);
INSERT INTO customers_info VALUES(34992, 'female', '北京', '中国', 19, '2020-11-09');
INSERT INTO customers_info VALUES(22889, 'male', '墨尔本', '澳大利亚', 26, null);

需求

查询客户 id , 并加一列 , 判断该客户是否有过购买记录(latest_place_order_date 1 为有 , 0 为没有)

查询结果 :

customer_id|if_placed_order
18903|1
21089|1

解决

case 语法 :

CASE 字段 
	WHEN 预期值1 THEN 结果1
	WHEN 预期值2 THEN 结果2
	-- ...
  ELSE 结果3
END as 别名


CASE 
	WHEN 判断1 THEN 结果1
	WHEN 判断2 THEN 结果2
	-- ...
  ELSE 结果3
END as 别名

if判断 :

-- expr : 条件表达式
-- 当为 true，就返回 result_true，否则返回 result_false
if(expr, result_true, result_false)

方法1 :

• 对时间进行俩次判断

select customer_id,
    case
        when latest_place_order_date is null then 0
        when latest_place_order_date is not null then 1
    end as if_placed_order
from customers_info;

方法2 :

• 对时间判断一次 , 否则就 else

select customer_id,
	case 
		when latest_place_order_date is null then 0
  	else 1
	end as if_placed_order
from customers_info

方法3 :

select customer_id,
    if(latest_place_order_date is null, 0, 1) as if_placed_order
from customers_info;