Question

此查询有什么问题？

sql error: #1222 - The used SELECT statements have a different number of columns

SELECT `id`,`firstname`,`lastname` FROM `users` WHERE `firstname` LIKE 
'%ned%' OR `lastname` LIKE '%ned%' 
OR CONCAT(`firstname`," ", `lastname`) LIKE '%ned%' 
OR CONCAT(`lastname`," ", `firstname`) LIKE '%ned%' 
UNION SELECT `users`.`id` FROM `users` LEFT JOIN `friends` ON `users`.`id`=`friends`.`user_id`

用户表

+------------------------------------------+
|   id   |  firstname | lastname | ....    |
|   1    |   Nedim    |  Kanat   |  ....   |
+------------------------------------------+

朋友表

+-------------------------------------------+
|   id   |  user_id   | friend_id | ....    |
|   1    |   1        |  2        |  ....   |
+-------------------------------------------+

Answer 1

您不应该使用UNION。您应该以这种方式使用查询：

SELECT `id`,`firstname`,`lastname`, (
    SELECT `users`.`id` FROM `users` LEFT JOIN `friends` ON `users`.`id`=`friends`.`user_id`
) AS `UserID` FROM `users` WHERE `firstname` LIKE 
'%ned%' OR `lastname` LIKE '%ned%' 
OR CONCAT(`firstname`," ", `lastname`) LIKE '%ned%' 
OR CONCAT(`lastname`," ", `firstname`) LIKE '%ned%'

<强>差分

UNION用于添加新行。
, () AS Column用于添加为同一行中的另一列。

Answer 2

联合是集之间的数学运算。这种操作要求两个表是COMPATIBLE，这意味着它们必须在数量和类型上具有相同的列。显而易见的是，您正在尝试在3列SELECT和1列SELECT语句之间建立联合（LEFT JOIN不会像您期望的那样添加其他2列导致SELECT项目加入后的表格）

SQL SELECT语句 - 不同的列数

2 个答案: