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谈谈关系数据库中null的处理

谈谈关系数据库中null的处理

作者: chimpansee | 来源:发表于2019-08-02 17:49 被阅读0次

    我们在设计数据库时，常常会允许一些字段为null。但是在后续处理null时，由于null的特殊性，可能在算术运算、逻辑运算中带来一些始料不及的情况，我们来总结写null的处理特例情况。

    • 算术运算，比较运算

    null在算术运算代表不知道的值，所以任何算术表达式中出现了null，其结果也是一个不知道的值，也就是null

    mysql> select * from person;
+------+------+-------+------+
| id   | name | memo  | age  |
+------+------+-------+------+
|    1 | mike | memo1 |   30 |
|    2 | john | memo2 | NULL |
|    3 | tom  | memo3 |   25 |
|    4 | mary | NULL  | NULL |
|    5 | mary | NULL  | NULL |
+------+------+-------+------+
5 rows in set (0.00 sec)

mysql> select age*2 from person;
+-------+
| age*2 |
+-------+
|    60 |
|  NULL |
|    50 |
|  NULL |
|  NULL |
+-------+
5 rows in set (0.00 sec)

mysql>
mysql> select age+2 from person;
+-------+
| age+2 |
+-------+
|    32 |
|  NULL |
|    27 |
|  NULL |
|  NULL |
+-------+
5 rows in set (0.00 sec)

mysql>
mysql> select null>1 ;
+--------+
| null>1 |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

mysql> select null<1;
+--------+
| null<1 |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

mysql> select null=1;
+--------+
| null=1 |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

    • 逻辑运算

    • 与运算：
      true and null= null;
      false and null=false;
      null and null=null;

    mysql> select true and null ;
+---------------+
| true and null |
+---------------+
|          NULL |
+---------------+
1 row in set (0.00 sec)

mysql> select false and null;
+----------------+
| false and null |
+----------------+
|              0 |
+----------------+
1 row in set (0.00 sec)

mysql> select null and null;
+---------------+
| null and null |
+---------------+
|          NULL |
+---------------+
1 row in set (0.00 sec)

mysql>
    • 或运算
      true or null=true;
      flase or null=null;
      null or null=null;
    mysql> select true or null;
+--------------+
| true or null |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)

mysql> select false or null
    -> ;
+---------------+
| false or null |
+---------------+
|          NULL |
+---------------+
1 row in set (0.00 sec)

mysql> select null or null;
+--------------+
| null or null |
+--------------+
|         NULL |
+--------------+
1 row in set (0.00 sec)

mysql>
    • 非运算
      not(null)=null;
    mysql> select not(null);
+-----------+
| not(null) |
+-----------+
|      NULL |
+-----------+
1 row in set (0.00 sec)

mysql>

    • where 语句

    如果想通过select将期待的结果集查询出来，其where条件必须是true。其他值包括（null，false）一概不予返回。

    mysql> select * from person ;
+------+------+-------+------+
| id   | name | memo  | age  |
+------+------+-------+------+
|    1 | mike | memo1 |   30 |
|    2 | john | memo2 | NULL |
|    3 | tom  | memo3 |   25 |
|    4 | mary | NULL  | NULL |
|    5 | mary | NULL  | NULL |
+------+------+-------+------+
5 rows in set (0.00 sec)

mysql> select * from person where age>25;
+------+------+-------+------+
| id   | name | memo  | age  |
+------+------+-------+------+
|    1 | mike | memo1 |   30 |
+------+------+-------+------+
1 row in set (0.00 sec)

mysql>
mysql> select * from person where age is null;
+------+------+-------+------+
| id   | name | memo  | age  |
+------+------+-------+------+
|    2 | john | memo2 | NULL |
|    4 | mary | NULL  | NULL |
|    5 | mary | NULL  | NULL |
+------+------+-------+------+
3 rows in set (0.00 sec)

    • join语句

    在join条件中，如果有null值存在，结果是不匹配。

    mysql> select * from person ;
+------+------+-------+------+
| id   | name | memo  | age  |
+------+------+-------+------+
|    1 | mike | memo1 |   30 |
|    2 | john | memo2 | NULL |
|    3 | tom  | memo3 |   25 |
|    4 | mary | NULL  | NULL |
|    5 | mary | NULL  | NULL |
+------+------+-------+------+
5 rows in set (0.00 sec)

mysql> select * from teacher;
+------+------+------+
| id   | name | age  |
+------+------+------+
|    1 | mike |   30 |
|    4 | mary |   35 |
+------+------+------+
2 rows in set (0.00 sec)
mysql> select * from person natural join teacher;
+------+------+------+-------+
| id   | name | age  | memo  |
+------+------+------+-------+
|    1 | mike |   30 | memo1 |
+------+------+------+-------+
1 row in set (0.00 sec)

mysql>

    • 集合语句

    集合语句中，多个null会被当作重复值来处理，有点类似空格似的。

    mysql> select * from person;
+------+------+-------+------+
| id   | name | memo  | age  |
+------+------+-------+------+
|    1 | mike | memo1 |   30 |
|    2 | john | memo2 | NULL |
|    3 | tom  | memo3 |   25 |
|    4 | mary | NULL  | NULL |
|    5 | mary | NULL  | NULL |
+------+------+-------+------+
5 rows in set (0.00 sec)

mysql> select * from teacher;
+------+------+------+
| id   | name | age  |
+------+------+------+
|    1 | mike |   30 |
|    4 | mary |   35 |
|    2 | john | NULL |
+------+------+------+
3 rows in set (0.00 sec)

mysql> select name ,age from person union select name,age from teacher;
+------+------+
| name | age  |
+------+------+
| mike |   30 |
| john | NULL |
| tom  |   25 |
| mary | NULL |
| mary |   35 |
+------+------+
5 rows in set (0.00 sec)

mysql> select age from person union select age from teacher;
+------+
| age  |
+------+
|   30 |
| NULL |
|   25 |
|   35 |
+------+
4 rows in set (0.00 sec)

mysql>

    • group by 语句

    在group by语句中，多个null会被当作重复值。

    mysql> select * from person;
+------+------+-------+------+
| id   | name | memo  | age  |
+------+------+-------+------+
|    1 | mike | memo1 |   30 |
|    2 | john | memo2 | NULL |
|    3 | tom  | memo3 |   25 |
|    4 | mary | NULL  | NULL |
|    5 | mary | NULL  | NULL |
+------+------+-------+------+
5 rows in set (0.00 sec)
mysql> select count(*) from person group by age;
+----------+
| count(*) |
+----------+
|        1 |
|        3 |
|        1 |
+----------+
3 rows in set (0.00 sec)

mysql> select count(*) from person group by name,age;
+----------+
| count(*) |
+----------+
|        1 |
|        1 |
|        1 |
|        2 |
+----------+
4 rows in set (0.00 sec)

    • select 语句

    select对于字段的处理默认不会去重，对null也不例外。

    mysql> select age from person;
+------+
| age  |
+------+
|   30 |
| NULL |
|   25 |
| NULL |
| NULL |
+------+
5 rows in set (0.00 sec)

mysql> select distinct age from person;
+------+
| age  |
+------+
|   30 |
| NULL |
|   25 |
+------+
3 rows in set (0.00 sec)

mysql>

    • 聚集函数

    聚集函数在处理数据前，会剔除掉null，这与算术处理中逻辑不同，这是因为考虑到聚集函数结果不会仅仅因为一个null而成为null，从而失去实际的计算意义。但是如果处理结果是null，结果也就显示null。

    mysql> select * from person;
+------+------+-------+------+
| id   | name | memo  | age  |
+------+------+-------+------+
|    1 | mike | memo1 |   30 |
|    2 | john | memo2 | NULL |
|    3 | tom  | memo3 |   25 |
|    4 | mary | NULL  | NULL |
|    5 | mary | NULL  | NULL |
+------+------+-------+------+
5 rows in set (0.00 sec)

mysql> select count(age) from person;
+------------+
| count(age) |
+------------+
|          2 |
+------------+
1 row in set (0.00 sec)
mysql>
mysql> select count(age) from person where age is null;
+------------+
| count(age) |
+------------+
|          0 |
+------------+
1 row in set (0.00 sec)

mysql> select max(age) from person where age is null;
+----------+
| max(age) |
+----------+
|     NULL |
+----------+
1 row in set (0.00 sec)

mysql> select avg(age) from person where age is null;
+----------+
| avg(age) |
+----------+
|     NULL |
+----------+
1 row in set (0.00 sec)

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