現(xiàn)在我們先來(lái)把test表中的一條記錄的birth字段設(shè)置為空。

mysql> update test set t_birth=null where t_id=1;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

OK，執(zhí)行成功！
設(shè)置一個(gè)字段值為空時(shí)的語(yǔ)法為：set <字段名>=NULL
說(shuō)明一下，這里沒(méi)有大小寫(xiě)的區(qū)分，可以是null，也可以是NULL。

下面看看結(jié)果：

mysql> select * from test;
+------+--------+----------------------------------+------------+
| t_id | t_name | t_password                       | t_birth    |
+------+--------+----------------------------------+------------+
|    1 | name1  | 12345678901234567890123456789012 | NULL       |
|    2 | name2  | 12345678901234567890123456789012 | 2013-01-01 |
+------+--------+----------------------------------+------------+
2 rows in set (0.00 sec)

接下來(lái)分別查詢(xún)一下字段t_birth值為空或不為空的記錄：

mysql> select * from test where t_birth is null;
+------+--------+----------------------------------+---------+
| t_id | t_name | t_password                       | t_birth |
+------+--------+----------------------------------+---------+
|    1 | name1  | 12345678901234567890123456789012 | NULL    |
+------+--------+----------------------------------+---------+
1 row in set (0.00 sec)

mysql> select * from test where t_birth is not null;
+------+--------+----------------------------------+------------+
| t_id | t_name | t_password                       | t_birth    |
+------+--------+----------------------------------+------------+
|    2 | name2  | 12345678901234567890123456789012 | 2013-01-01 |
+------+--------+----------------------------------+------------+
1 row in set (0.00 sec)

說(shuō)明：
1、查詢(xún)字段值為空的語(yǔ)法：where <字段名> is null
2、查詢(xún)字段值不為空的語(yǔ)法：where <字段名> is not null

關(guān)于MySQL查詢(xún)空字段或非空字段（is null和not null），本文就介紹這么多，希望對(duì)大家有所幫助，謝謝！ 

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