mysql - 选择查询不起作用时的情况

Question

我不确定情况如何与 mysql 一起使用，任何人都可以解释这个查询有什么问题吗？

CASE WHEN SELECT COUNT(*) from websites where website_id = 171 and master = 2 > 0
SELECT timestamp from websites where website_id = 171 and master = 2 
ELSE SELECT NOW() as timestamp

Answer 1

查看 MySQL Control Flow Functions

CASE...WHEN 需要有 THEN 关键字才能工作。

您可能想要:

CASE 
    WHEN (SELECT COUNT(*) from websites where website_id = 171 and master = 2) > 0
      THEN (SELECT timestamp from websites where website_id = 171 and master = 2) 
    ELSE NOW()
END as timestamp

如果只有两种可能性，那么使用 IF 会更好:

IF((SELECT count(*) from websites where website_id = 171 and master = 2) > 0,  
   (SELECT timestamp from websites where website_id = 171 and master = 2),  
   NOW()) AS TIMESTAMP

或者，您可以使用 IFNULL 并跳过计数(*)

IFNULL((SELECT timestamp from websites where website_id = 171 and master = 2),
   NOW()) AS TIMESTAMP