SQL SELECT 语句 - 不同的列数

Question

What's wrong with this query?

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`

Users table

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

Friends table

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

Answer 1

You shouldn't use UNION for this. You should be using the query this way:

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%'

Difference

• UNION is for adding as a new row.
• , () AS Column is for adding as another column in the same row.

Answer 2

An union is a mathematical operation between sets. This kind of operations requires the two tables to be COMPATIBLE, which means that they have to have the same columns in number and types. And it's clearly obvious that you're trying to make an union between a 3 columns SELECT and 1 column SELECT statements (the LEFT JOIN isn't adding other 2 columns as you expect cause the SELECT projects the table after the join is made)