mysql null值的特殊处理

Posted 2020-08-25 THISISPAN

实例

尝试以下实例:

[email protected]# mysql -u root -p password;
Enter password:*******
mysql> use RUNOOB;
Database changed
mysql> create table tcount_tbl
    -> (
    -> runoob_author varchar(40) NOT NULL,
    -> runoob_count  INT
    -> );
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO tcount_tbl
    -> (runoob_author, runoob_count) values (‘mahran‘, 20);
mysql> INSERT INTO tcount_tbl
    -> (runoob_author, runoob_count) values (‘mahnaz‘, NULL);
mysql> INSERT INTO tcount_tbl
    -> (runoob_author, runoob_count) values (‘Jen‘, NULL);
mysql> INSERT INTO tcount_tbl
    -> (runoob_author, runoob_count) values (‘Gill‘, 20);

mysql> SELECT * from tcount_tbl;
+-----------------+----------------+
| runoob_author | runoob_count |
+-----------------+----------------+
| mahran          |             20 |
| mahnaz          |           NULL |
| Jen             |           NULL |
| Gill            |             20 |
+-----------------+----------------+
4 rows in set (0.00 sec)

mysql>

以下实例中你可以看到 = 和 != 运算符是不起作用的：

mysql> SELECT * FROM tcount_tbl WHERE runoob_count = NULL;
Empty set (0.00 sec)
mysql> SELECT * FROM tcount_tbl WHERE runoob_count != NULL;
Empty set (0.01 sec)

实例

查找数据表中 runoob_count 列是否为 NULL，必须使用IS NULL和IS NOT NULL，如下实例：

mysql> SELECT * FROM tcount_tbl 
    -> WHERE runoob_count IS NULL;
+-----------------+----------------+
| runoob_author | runoob_count |
+-----------------+----------------+
| mahnaz          |           NULL |
| Jen             |           NULL |
+-----------------+----------------+
2 rows in set (0.00 sec)
mysql> SELECT * from tcount_tbl 
    -> WHERE runoob_count IS NOT NULL;
+-----------------+----------------+
| runoob_author | runoob_count |
+-----------------+----------------+
| mahran          |             20 |
| Gill            |             20 |
+-----------------+----------------+
2 rows in set (0.00 sec)