在mysql中选择查询总是返回空结果

Question

I run this simple query with PHP and also phpMyAdmin:

SELECT * FROM `rdm_order` WHERE `aff_result` != "xyc"

I checked. table,column and rows are exist.

but the query return me empty result.

aff_result is this format: VARCHAR(10) COLLATE utf8_persian_ci

It seems all thing is correct. what is wrong?

Server: Localhost via UNIX socket
Server type: MySQL
Server version: 5.7.17-0ubuntu0.16.04.1-log - (Ubuntu)
Protocol version: 10
Server charset: UTF-8 Unicode (utf8)

Apache/2.4.25 (Unix) OpenSSL/1.0.2g
Database client version: libmysql - mysqlnd 5.0.12-dev 
PHP extension: mysqliDocumentation curlDocumentation mbstringDocumentation
PHP version: 7.0.16

Answer 1

It's because the values are NULL . NULL cannot be captured by an = or != comparison. Use the following instead:

SELECT   * 
FROM     `rdm_order` 
WHERE    `aff_result` != 'xyc'
OR       `aff_result` IS NULL

Answer 2

Use the NULL -safe comparison operator:

SELECT *
FROM `rdm_order`
WHERE not `aff_result` <=> 'xyc';

The ANSI standard operator is is distinct from .

Answer 3

If you do not have any good reason to set default value of aff_result as NULL, you can modify your aff_result column and set default value as '' (blank string) using mysql alter command and your query will return the expected result set.
As explained by @Siyual, NULL cannot be captured by an = or != comparison.
And using OR operator in where clause is not index friendly.