mysql - CASE Mysql 的 Null 检查

Question

我尝试检查值，如果它为空(没有任何内容可显示)，我会推断出空字符串。我正在尝试:

SELECT nullif(order_id, ' ') as order_id FROM label_pass_voucher WHERE order_id = 3447;
select CASE WHEN order_id then order_id else ' ' end as order_id FROM label_pass_voucher WHERE order_id = 3447;

它不会推断出任何结果。我的错在哪里？提前致谢

Answer 1

我假设您要查找的查询是:

SELECT IFNULL(order_id, '') AS order_id
FROM label_pass_voucher
WHERE order_id = 3447;

MySQL官方文档:

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.

NULLIF() 做的事情略有不同:

Returns NULL if expr1 = expr2 is true, otherwise returns expr1. This is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.