跟燕十八学习php-第二十六天-奇怪的null
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mysql> #建表时,列后面 not null default '',default 0,这是什么意思
mysql> #答:就是让这个列值不为null,如果某个列确实没填值,也有默认值,也不为null
mysql> # 为什么不希望让列的值为null呢?
mysql> create table test9 (
    -> sname varchar(20)
    -> )engine myisam charset utf8;
query ok, 0 rows affected (0.20 sec)
mysql> insert into test9
    -> values
    -> ('lisi','wangwu','null');
error 1136 (21s01): column count doesn't match value count at row 1
mysql> insert into test9
    -> values
    -> ('lisi'),('wangwu',('null');
error 1064 (42000): you have an error in your sql syntax; check the manual that corresponds
to your mysql server version for the right syntax to use near '' at line 3
mysql> insert into test9
    -> values
    -> ('lisi'),('wangwu'),('null');
query ok, 3 rows affected (0.00 sec)
records: 3  duplicates: 0  warnings: 0
mysql> select * from test9;
+--------+
| sname  |
+--------+
| lisi   |
| wangwu |
| null   |
+--------+
3 rows in set (0.00 sec)
mysql> insert into test9 values (null);
query ok, 1 row affected (0.02 sec)
mysql> select * from test9;
+--------+
| sname  |
+--------+
| lisi   |
| wangwu |
| null   |
| null   |
+--------+
4 rows in set (0.00 sec)
mysql> delete from test9 where sname='null';
query ok, 1 row affected (0.03 sec)
mysql> select * from test9;
+--------+
| sname  |
+--------+
| lisi   |
| wangwu |
| null   |
+--------+
3 rows in set (0.00 sec)
mysql> #查询出用户名不为null的行
mysql> select * from test9 where sname!=null;
empty set (0.00 sec)
mysql> #lisi wangwu没查出来? 为什么
mysql> #查sname为null的行
mysql> select * from test9 where sname=null;
empty set (0.00 sec)
mysql> #又是空
mysql> select 2>1;
+-----+
| 2>1 |
+-----+
|   1 |
+-----+
1 row in set (0.00 sec)
mysql> select 2+-----+
| 2+-----+
|   0 |
+-----+
1 row in set (0.00 sec)
mysql> select 39+-------+
| 39+-------+
|     1 |
+-------+
1 row in set (0.00 sec)
mysql> select 'lisi'=null;
+-------------+
| 'lisi'=null |
+-------------+
|        null |
+-------------+
1 row in set (0.00 sec)
mysql> # null为假, lisi=null是假
mysql> select null=null;
+-----------+
| null=null |
+-----------+
|      null |
+-----------+
1 row in set (0.00 sec)
mysql> # null=null,还是null,还是假
mysql> select null!=null;
+------------+
| null!=null |
+------------+
|       null |
+------------+
1 row in set (0.00 sec)
mysql> # null 是空,四大皆空的空. 
mysql> # null的比较需要用特殊的运算符  is null ,is not null
mysql> select * from sname where sname is not null;
error 1146 (42s02): table 'test.sname' doesn't exist
mysql> select * from test9 where sname is not null;
+--------+
| sname  |
+--------+
| lisi   |
| wangwu |
+--------+
2 rows in set (0.00 sec)
mysql> select * from test9 where sname is null;
+-------+
| sname |
+-------+
| null  |
+-------+
1 row in set (0.00 sec)
mysql> exit
燕十八老师太幽默了, 昨天的视频如下:
http://www.tudou.com/programs/view/-a_s1emsnds/