Question

我不确定案例如何与mysql一起使用，有人能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 ...需要THEN关键字才能运作。

你可能想要：

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

但是，如果只有两种可能性，那么你最好使用IF：

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

Answer 2

我认为您可以重写您的查询，如：

SELECT COALESCE(timestamp, NOW()) from websites where website_id = 171 and master = 2

Answer 3

使用嵌入选择获取值时，需要将select包装在括号中。此外，CASE需要使用END关闭试试这个：

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 (SELECT NOW())
END as timestamp

查询不起作用时选择案例

3 个答案: