mysql - 如何向此查询添加 If Else End If

Question

以下查询在未找到值时返回 NULL。我怎样才能If在他们两个周围分别添加一个，所以如果它Null那么Not Found返回查询的结果？

SET @id = 26;
SELECT 
(SELECT id  FROM devTable WHERE id <= @id AND country = 'US' ORDER BY id DESC LIMIT 9,1) AS `prev`,
(SELECT id  FROM devTable WHERE id >= @id AND country = 'US' ORDER BY id ASC  LIMIT 1,1) AS `next`

Answer 1

使用IFNULL()：

SET @id = 26;
SELECT 
IFNULL(
  (SELECT id FROM devTable WHERE id <= @id AND country = 'US' ORDER BY id DESC LIMIT 9,1),
  'Not Found') AS prev,
IFNULL(
  (SELECT id FROM devTable WHERE id >= @id AND country = 'US' ORDER BY id ASC  LIMIT 1,1),
  'Not Found') AS next

Answer 2

查找 T-SQL Exists 命令。

喜欢：

IF EXISTS( SELECT STUFF FROM JUNK WHERE ITEMS=5) THEN
BEGIN
SELECT "HAVE JUNK"  -- OR PRINT, SET, ...
END
ELSE
BEGIN
SELECT "SORRY, DON'T HAVE JUNK" -- OR PRINT, SET, ...
END
END