简单的 MySQL SELECT 查询和子句

Question

Reviews table:

+--------------------+
| ID | DESC |  MISC  |
+--------------------+
| 1  | EDR  | NULL   | 
| 2  | EDR  | Mumba  | 
| 3  | EDR  | NULL   | 
| 4  | YTO  | Mumba  | 
| 5  | YTO  | Mumba  | 
+--------------------+

Query:

SELECT ID
FROM Reviews
WHERE DESC = 'EDR' AND MISC <> 'Sent'

I'm expecting my query to return:

+----+
| ID |
+----+
| 1  |
| 2  |
| 3  |
+----+

But instead, I get:

+------+
|  ID  |
+------+
| NULL |
+------+

What gives?

Answer 1

Almost any comparison to NULL returns NULL -- which is treated as FALSE -- even <> .

So, use a NULL -safe comparison:

SELECT ID
FROM Reviews
WHERE DESC = 'EDR' AND NOT (MISC <=> 'Sent')

And, I'll point out that there is a standard NULL -safe comparison in SQL. In your case, that would be: MISC IS NOT DISTINCT FROM 'Sent' . But MySQL has its own syntax.

Answer 2

A value can't be equal to NULL, nor can it be different from NULL.

It's easier to understand if you see a NULL as 'unknown'.

But you can check with IS NULL

SELECT ID
FROM Reviews
WHERE DESC = 'EDR' 
AND (MISC IS NULL OR MISC <> 'Sent')