MySQL 字段 NOT NULL

create table table_2 ( `id` INT (11) NOT NULL,user_name varchar(20) NOT NULL)
create table table_3 ( `id` INT (11) NOT NULL,user_name varchar(20))
insert into table_2 values (4,"zhaoliu_2_1"),(2,"lisi_2_1"),(3,"wangmazi_2_1"),(1,"zhangsan_2"),(2,"lisi_2_2"),(4,"zhaoliu_2_2"),(3,"wangmazi_2_2")
insert into table_3 values (1,"zhaoliu_2_1"),(2, null)
-- 1、NOT IN子查询在有NULL值的情况下返回永远为空结果，查询容易出错select user_name from table_2 where user_name not in (select user_name from table_3 where id!=1)
mysql root@10.48.186.32:t_test_zz5431> select user_name from table_2 where user_name not
                                -> in (select user_name from table_3 where id!=1);
+-------------+
| user_name   |
|-------------|
+-------------+
0 rows in set
Time: 0.008s
mysql root@10.48.186.32:t_test_zz5431>
-- 2、单列索引不存null值，复合索引不存全为null的值，如果列允许为null，可能会得到“不符合预期”的结果集
-- 如果name允许为null，索引不存储null值，结果集中不会包含这些记录。所以，请使用not null约束以及默认值。
select * from table_3 where name != 'zhaoliu_2_1'
-- 3、如果在两个字段进行拼接：比如题号+分数，首先要各字段进行非null判断，否则只要任意一个字段为空都会造成拼接的结果为null。
select CONCAT("1",null) from dual; -- 执行结果为null。
-- 4、如果有 Null column 存在的情况下，count(Null column)需要格外注意，null 值不会参与统计。
mysql root@10.48.186.32:t_test_zz5431> select * from table_3;
+------+-------------+|   id | user_name   ||------+-------------||    1 | zhaoliu_2_1 ||    2 | <null>      ||   21 | zhaoliu_2_1 ||   22 | <null>      |+------+-------------+
4 rows in set
Time: 0.007s
mysql root@10.48.186.32:t_test_zz5431> select count(user_name) from table_3;+--------------------+|   count(user_name) ||--------------------||                  2 |+--------------------+
1 row in set
Time: 0.007s
-- 5、注意 Null 字段的判断方式， = null 将会得到错误的结果。
mysql root@localhost:cygwin> create index IDX_test on table_3 (user_name);
Query OK, 0 rows affectedTime: 0.040smysql root@localhost:cygwin>  select * from table_3 where user_name is nullG
***************************[ 1. row ]***************************
id        | 2
user_name | None
1 row in set
Time: 0.002s
mysql root@localhost:cygwin> select * from table_3 where user_name = nullG
0 rows in set
Time: 0.002smysql root@localhost:cygwin> desc select * from table_3 where user_name = 'zhaoliu_2_1'G
***************************[ 1. row ]***************************
id            | 1select_type   | SIMPLEtable         | table_3type          | refpossible_keys | IDX_testkey           | IDX_testkey_len       | 23ref           | constrows          | 1Extra         | Using where
1 row in setTime: 0.006s
mysql root@localhost:cygwin> desc select * from table_3 where user_name = nullG
***************************[ 1. row ]***************************
id            | 1select_type   | SIMPLEtable         | Nonetype          | Nonepossible_keys | Nonekey           | Nonekey_len       | Noneref           | Nonerows          | NoneExtra         | Impossible WHERE noticed after reading const tables
1 row in set
Time: 0.002smysql root@localhost:cygwin> desc select * from table_3 where user_name is nullG
***************************[ 1. row ]***************************id            | 1select_type   | SIMPLEtable         | table_3type          | refpossible_keys | IDX_testkey           | IDX_testkey_len       | 23ref           | constrows          | 1Extra         | Using where
1 row in set
Time: 0.002s

alter table table_3 add index idx_user_name (user_name);alter table table_2 add index idx_user_name (user_name);explain select * from table_2 where user_name='zhaoliu_2_1';explain select * from table_3 where user_name='zhaoliu_2_1';