MySQL的几个length函数以及聚合函数对于NULL的处理

数据库基于MySQL，其他数据库有些不一样

最近跟着CSDN上的一位博主的MySQL系列文章学习了下，做下笔记复习巩固下。

王大锤砸 

length函数：

char_length:字符个数

length:字节个数

bit_length:二进制个数

mysql> select length('a'),char_length('a'),bit_length('a');
+-------------+------------------+-----------------+
| length('a') | char_length('a') | bit_length('a') |
+-------------+------------------+-----------------+
|           1 |                1 |               8 |
+-------------+------------------+-----------------+
1 row in set (0.00 sec)

mysql> select length('啊'),char_length('啊'),bit_length('啊');
+---------------+--------------------+-------------------+
| length('啊')  | char_length('啊')  | bit_length('啊')  |
+---------------+--------------------+-------------------+
|             3 |                  1 |                24 |
+---------------+--------------------+-------------------+
1 row in set (0.00 sec)

聚合函数对于NULL

COUNT(*):会对所有的NULL计数，

COUNT(1):会对所有的NULL计数,

COUNT(某列):会忽略所有的NULL,

COUNT(NULL):返回0

SUM():会忽略NULL

MAX():忽略NULL

MIN():忽略NULL

AVG():忽略NULL

上面这4个都会忽略null比较好理解：毕竟NULL无法参与计数，但是要注意是忽略NULL而不是把NULL当成0，通过这个就能说明：

mysql> select avg(v) from test_count;
+--------+
| avg(v) |
+--------+
| 1.5000 |
+--------+
1 row in set (0.04 sec)

mysql> select * from test_count;
+------+
| v    |
+------+
|    1 |
|    2 |
| NULL |
+------+
3 rows in set (0.00 sec)

#平均数结果是1.5，所以能证明除以的是2而不是3，能证明NULL被忽略了

group by:这个以前我还没注意，NULL列会在查询之后放在最前面

mysql> select * from test_count;
+------+
| v    |
+------+
|    1 |
|    2 |
| NULL |
+------+
3 rows in set (0.00 sec)

mysql> select * from test_count group by v;
+------+
| v    |
+------+
| NULL |
|    1 |
|    2 |
+------+
3 rows in set (0.01 sec)

order by:NULL会被认为是最小值 （工作需要，用了postgresql，order by ,null会在最后，order by desc，null在最前面）

mysql> select * from test_count order by v;
+------+
| v    |
+------+
| NULL |
| -128 |
|    1 |
|    2 |
+------+
4 rows in set (0.00 sec)

distinct:NULL会被消重

mysql> select distinct * from test_count;
+------+
| v    |
+------+
|    1 |
|    2 |
| NULL |
| -128 |
+------+
4 rows in set (0.02 sec)

mysql> select * from test_count;
+------+
| v    |
+------+
|    1 |
|    2 |
| NULL |
| -128 |
| NULL |
| NULL |
+------+
6 rows in set (0.00 sec)

+:与NULL相加结果都为NULL

mysql> select * from test_count;
+------+
| v    |
+------+
|    1 |
|    2 |
| NULL |
| -128 |
| NULL |
| NULL |
+------+
6 rows in set (0.00 sec)

mysql> select 1+v from test_count;
+------+
| 1+v  |
+------+
|    2 |
|    3 |
| NULL |
| -127 |
| NULL |
| NULL |
+------+
6 rows in set (0.00 sec)

<,>,<>,=:与NULL比较都返回NULL，需要用is null 或者not null

is null和is not null：结果为真返回1，结果为假返回0

mysql> select 0<v,0>v,0<>v,0=v,v is null, v is not null from test_count;
+------+------+------+------+-----------+---------------+
| 0<v  | 0>v  | 0<>v | 0=v  | v is null | v is not null |
+------+------+------+------+-----------+---------------+
|    1 |    0 |    1 |    0 |         0 |             1 |
|    1 |    0 |    1 |    0 |         0 |             1 |
| NULL | NULL | NULL | NULL |         1 |             0 |
|    0 |    1 |    1 |    0 |         0 |             1 |
| NULL | NULL | NULL | NULL |         1 |             0 |
| NULL | NULL | NULL | NULL |         1 |             0 |
+------+------+------+------+-----------+---------------+
6 rows in set (0.03 sec)

mysql> select * from test_count;
+------+
| v    |
+------+
|    1 |
|    2 |
| NULL |
| -128 |
| NULL |
| NULL |
+------+
6 rows in set (0.00 sec)

 如果想统计NULL一共有多少行行，可以使用if函数：

mysql> select sum(if(v is null, 1, 0)) NULL一共多少行 from test_count;
+---------------------+
| NULL一共多少行      |
+---------------------+
|                   4 |
+---------------------+
1 row in set (0.03 sec)

mysql> select sum(if(v is null, 0, 1)) 非NULL一共多少行 from test_count;
+------------------------+
| 非NULL一共多少行       |
+------------------------+
|                      3 |
+------------------------+
1 row in set (0.00 sec)

mysql> select * from test_count;                                  unt;t;
+------+
| v    |
+------+
|    1 |
|    2 |
| NULL |
| -128 |
| NULL |
| NULL |
| NULL |
+------+
7 rows in set (0.00 sec)