Question

我有这张桌子：

title1  title2   type
---------------------
qwe1    xcv2     3
asd1    tzu      7
fgh1    (empty)  4

我有这个问题：

SELECT `title1` AS `title`, `type`
FROM `table`
UNION
SELECT `title2` AS `title`, `type`
FROM `table`
ORDER BY `title` ASC, type

（请参阅fiddle here）。

如果title2为空，如何转换此查询以使其不显示任何行？不应显示第1行。

我试图实现一个IF语句，但我总是遇到一个mysql错误。

Answer 1

这应该覆盖空字符串并且为null：

SELECT `title1` AS `title`, `type`
FROM `table`
UNION
SELECT `title2` AS `title`, `type`
FROM `table` where title2 is not null and title2 <> ''
ORDER BY `title` ASC, type

Answer 2

您应该使用WHERE语句为查询定义条件

SELECT `title1` AS `title`, `type`
FROM `table`
WHERE `title1` IS NOT NULL AND LENGTH(`title1`) > 0
UNION
SELECT `title2` AS `title`, `type`
FROM `table`
WHERE `title2` IS NOT NULL AND LENGTH(`title2`) > 0
ORDER BY `title` ASC, type

Answer 3

使用UNION时，删除重复项。您确定不想使用UNION ALL吗？另外，你有什么理由需要title2吗？它对我来说似乎没有正常化。

SELECT title1 AS title, `type`
FROM `table`
UNION ALL
SELECT title2 AS title, `type`
FROM `table`
WHERE title2 IS NOT NULL AND title2 <> ''

See the demo

Answer 4

我认为，而不是检查title2 is not null和title2 <> ''，最简单的方法是使用COALESCE将任何Null值转换为空，然后检查COALESCE的结果是否为＆lt;＆gt; ; ''，像这样：

SELECT title1 AS title, `type`
FROM `table`
UNION
SELECT title2 AS title, `type`
FROM `table`
WHERE COALESCE(title2, '') <> ''

如果您不需要删除重复项，我认为最好使用UNION ALL而不是UNION。

MySQL检索数据IF列不为空

4 个答案:

See the demo