mysql的高阶语句介绍（排序，查询和视图）
mysql高阶语句介绍（正则和运算）
mysql高阶语句（函数）
mysql的高阶语句介绍（排序，查询和视图）
mysql的高阶语句介绍
- 按关键字排序
-
- 升序排列
- 降序排列
- 结合判断语句排序
- 多字段的排序
- 区间判断查询
- 去重查询
- 对结果进行分组
-
- 结合函数分组
- 结合排序分组
- 限制结果的条目
-
- 结合排序限制
- 设置别名查询
-
- as设置别名
- as作为克隆作用
- 通配符
- 子查询
-
- 相同表子查询
- 不同表查询
- 不同表插入
- 修改时使用子查询
- 删除时使用子查询
- existe结合子查询
- 别名的子查询
- 数据库的视图
-
- 单表视图
- 多表视图
- null值
-
- null值与空值的区别
mysql除了增删改查之外，有时候需要对查询结果进行处理，这就衍生出了高级的查询语句
按关键字排序
- 对查询结果进行排序
升序排列
- asc：是按照升序进行排序，默认的排序方式，可以省略
mysql> select * from test;
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 60.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+------------+--------+
7 rows in set (0.00 sec)
mysql> select id,name,score from test order by score; //指定score默认排序是升序
+------+-----------+-------+
| id | name | score |
+------+-----------+-------+
| 6 | hanmeimei | 40.00 |
| 7 | lilei | 50.00 |
| 3 | wangwu | 60.00 |
| 1 | zhangsan | 80.00 |
| 2 | lisi | 90.00 |
| 5 | wangdi | 98.00 |
| 4 | tianqi | 99.00 |
+------+-----------+-------+
7 rows in set (0.00 sec)
降序排列
- desc：是按降序方式进行排列，需要指定
mysql> select id,name,score from test order by score desc; //查询score指定为降序方式
+------+-----------+-------+
| id | name | score |
+------+-----------+-------+
| 4 | tianqi | 99.00 |
| 5 | wangdi | 98.00 |
| 2 | lisi | 90.00 |
| 1 | zhangsan | 80.00 |
| 3 | wangwu | 60.00 |
| 7 | lilei | 50.00 |
| 6 | hanmeimei | 40.00 |
+------+-----------+-------+
7 rows in set (0.00 sec)
结合判断语句排序
mysql> select id,name,score from test where address='nanjing' order by score desc; //指定address=南京的score以降序排列
+------+-----------+-------+
| id | name | score |
+------+-----------+-------+
| 7 | lilei | 50.00 |
| 6 | hanmeimei | 40.00 |
+------+-----------+-------+
2 rows in set (0.00 sec)
多字段的排序
- order by后面可以跟多个字段排序，优先级按照先后顺序，必须要前一个字段有相同的记录才能是第二个字段排序有意义
mysql> select id,name,hobbid from test order by hobbid desc,id desc; //指定hobbid降序排列，相同的hobbid中id降序排列
+------+-----------+--------+
| id | name | hobbid |
+------+-----------+--------+
| 7 | lilei | 5 |
| 4 | tianqi | 5 |
| 3 | wangwu | 4 |
| 6 | hanmeimei | 3 |
| 5 | wangdi | 3 |
| 2 | lisi | 2 |
| 1 | zhangsan | 2 |
+------+-----------+--------+
7 rows in set (0.00 sec)
mysql> select id,name,hobbid from test order by hobbid desc,id; //默认不加desc就是升序排列
+------+-----------+--------+
| id | name | hobbid |
+------+-----------+--------+
| 4 | tianqi | 5 |
| 7 | lilei | 5 |
| 3 | wangwu | 4 |
| 5 | wangdi | 3 |
| 6 | hanmeimei | 3 |
| 1 | zhangsan | 2 |
| 2 | lisi | 2 |
+------+-----------+--------+
7 rows in set (0.00 sec)
区间判断查询
mysql> select * from test where score >50 and score <=90; //查询score在大于50和小于等于90中间
+------+----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+----------+-------+------------+--------+
| 2 | lisi | 90.00 | shengzheng | 2 |
| 3 | wangwu | 60.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+----------+-------+------------+--------+
3 rows in set (0.00 sec)
mysql> select * from test where score >50 or score <=90; //查询score在大于50或者小于等于90的记录，所以都满足
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 60.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+------------+--------+
7 rows in set (0.00 sec)
mysql> select * from test where score >50 or (score >40 and score <90); //先执行括号的句子，括号取值为大于40和小于等于90的score记录，然后再跟大于50取或，最小值为50，所以最终为大于50
+------+----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+----------+-------+------------+--------+
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 60.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+----------+-------+------------+--------+
6 rows in set (0.00 sec)
去重查询
mysql> select distinct hobbid from test; //将重复的去掉不显示
+--------+
| hobbid |
+--------+
| 3 |
| 5 |
| 2 |
| 4 |
+--------+
4 rows in set (0.00 sec)
对结果进行分组
- 使用 GROUP BY 语句来实现 ，GROUP BY 通常都是结合聚合函数一起使用的，常用的聚合函数包括：计数（COUNT）、 求和（SUM）、求平均数（AVG）、最大值（MAX）、最小值（MIN），GROUP BY 分组的时候可以按一个或多个字段对结果进行分组处理
结合函数分组
mysql> select count(name),hobbid from test group by hobbid; //将hobbid分组并计数
+-------------+--------+
| count(name) | hobbid |
+-------------+--------+
| 2 | 2 |
| 2 | 3 |
| 1 | 4 |
| 2 | 5 |
+-------------+--------+
4 rows in set (0.00 sec)
结合排序分组
mysql> select count(name),score,hobbid from test where score>=80 group by hobbid order by count(name); //将hobbid分组并计数然后计数默认升序排列
+-------------+-------+--------+
| count(name) | score | hobbid |
+-------------+-------+--------+
| 1 | 99.00 | 5 |
| 1 | 98.00 | 3 |
| 1 | 80.00 | 4 |
| 2 | 90.00 | 2 |
+-------------+-------+--------+
4 rows in set (0.00 sec)
限制结果的条目
- LIMIT 的第一个参数是位置偏移量（可选参数），是设置 MySQL 从哪一行开始显示，如果是1就是第二行，0为第一行，第一行是字段名。
mysql> select * from test limit 4; //显示五行，第一行是字段名，实际显示是四行记录
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
+------+-----------+-------+------------+--------+
4 rows in set (0.00 sec)
mysql> select * from test limit 4,2; //从第五行起，查询后面两行
+------+--------+-------+----------+--------+
| id | name | score | address | hobbid |
+------+--------+-------+----------+--------+
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
+------+--------+-------+----------+--------+
2 rows in set (0.00 sec)
结合排序限制
mysql> select id,name from test order by id limit 3; //查询id，name并以升序排列id并限制只查询四行
+------+----------+
| id | name |
+------+----------+
| 1 | zhangsan |
| 2 | lisi |
| 3 | wangwu |
+------+----------+
3 rows in set (0.00 sec)
mysql> select id,name from test order by id desc limit 3; //将查询的记录倒序，并只查四行，也就是查记录的最后三行
+------+-----------+
| id | name |
+------+-----------+
| 7 | lilei |
| 6 | hanmeimei |
| 5 | wangdi |
+------+-----------+
3 rows in set (0.00 sec)
设置别名查询
as设置别名
- 在 MySQL 查询时，当表的名字比较长或者表内某些字段比较长时，为了方便书写或者多次使用相同的表，可以给字段列或表设置别名
mysql> select count(id) as member from test; //统计id总数并设置字段别名为member
+--------+
| member |
+--------+
| 7 |
+--------+
1 row in set (0.00 sec)
mysql> select count(id) member from test; //中间的as可以省略
+--------+
| member |
+--------+
| 7 |
+--------+
1 row in set (0.00 sec)
as作为克隆作用
mysql> create table test1 as select * from test; 用as相当于克隆test表，但是约束并不会被复制
Query OK, 7 rows affected (0.09 sec)
Records: 7 Duplicates: 0 Warnings: 0
mysql> select * from test1;
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+------------+--------+
7 rows in set (0.00 sec)
mysql> desc test;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(10) | NO | PRI | NULL | |
| score | decimal(5,2) | YES | | NULL | |
| address | varchar(20) | YES | | NULL | |
| hobbid | int(5) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> desc test1;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(10) | NO | | NULL | |
| score | decimal(5,2) | YES | | NULL | |
| address | varchar(20) | YES | | NULL | |
| hobbid | int(5) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> create table test2 as select * from test where score >=60; //也可以加上where来判断复制的内容
Query OK, 5 rows affected (0.01 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> select * from test2;
+------+----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+----------+-------+------------+--------+
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+----------+-------+------------+--------+
5 rows in set (0.00 sec)
通配符
-
通配符主要用于替换字符串中的部分字符，通过部分字符的匹配将相关结果查询出来，通常是跟like一起使用，并跟where判断来完成查询
-
常用的通配符
%：百分号表示零个、一个或多个字符类似于正则的“*”
_：下划线表示单个字符，类似正则的“.”
mysql> select * from test;
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+------------+--------+
7 rows in set (0.00 sec)
mysql> select id,name from test where name like 'l%'; //查询名字是l开头的记录
+------+-------+
| id | name |
+------+-------+
| 7 | lilei |
| 2 | lisi |
+------+-------+
2 rows in set (0.00 sec)
mysql> select id,name from test where name like 'l_s_'; //查询ls中间任一字符，s后任一字符的名字
+------+------+
| id | name |
+------+------+
| 2 | lisi |
+------+------+
1 row in set (0.00 sec)
mysql> select id,name from test where name like '%s%'; //查询中间有s的名字记录
+------+----------+
| id | name |
+------+----------+
| 2 | lisi |
| 1 | zhangsan |
+------+----------+
2 rows in set (0.00 sec)
mysql> select id,name from test where name like 'l___'; //查询l后面有三个任意字符的名字记录
+------+------+
| id | name |
+------+------+
| 2 | lisi |
+------+------+
1 row in set (0.00 sec)
mysql> select id,name from test where name like 'l_%'; //查询l后面跟任意字符的名字记录
+------+-------+
| id | name |
+------+-------+
| 7 | lilei |
| 2 | lisi |
+------+-------+
2 rows in set (0.01 sec)
子查询
- 子查询也被称作内查询或者嵌套查询，在一个查询语句中嵌套另一个查询语句，子查询语句是先于主查询语句被执行，其结果作为外层的条件返回给主查询进行下一 步的查询
相同表子查询
mysql> select name,score from test where id in (select id from test where score >80); //子语句查询出来的id给外面的主语句来查询，in是将主语句跟子语句中的表关联
+--------+-------+
| name | score |
+--------+-------+
| lisi | 90.00 |
| tianqi | 99.00 |
| wangdi | 98.00 |
+--------+-------+
3 rows in set (0.00 sec)
mysql> select id from test where score >80
-> ;
+------+
| id |
+------+
| 2 |
| 4 |
| 5 |
+------+
3 rows in set (0.00 sec)
不同表查询
mysql> select id,name,score from test where id in (select id from qq); //先查询qq表的id然后赋值给主语句查询
+------+----------+-------+
| id | name | score |
+------+----------+-------+
| 2 | lisi | 90.00 |
| 1 | zhangsan | 80.00 |
+------+----------+-------+
2 rows in set (0.00 sec)
mysql> select id from qq;
+----+
| id |
+----+
| 1 |
| 2 |
+----+
2 rows in set (0.00 sec)
不同表插入
- 子查询还可以用在 INSERT 语句中
mysql> select * from test2;
+------+----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+----------+-------+------------+--------+
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+----------+-------+------------+--------+
5 rows in set (0.00 sec)
mysql> insert into test2 select * from test where id in (select id from test); //将test的记录全部插入test2中
Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0
mysql> select * from test2;
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+------------+--------+
12 rows in set (0.00 sec)
修改时使用子查询
mysql> update test set score=50 where id in (select id from test2 where id=2); //test对应id为2的score修改为50
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from test;
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 50.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+------------+--------+
7 rows in set (0.00 sec)
删除时使用子查询
mysql> select * from test;
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 50.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+------------+--------+
7 rows in set (0.00 sec)
mysql> delete from test where id in (select id from test2 where score>80); //查询test2表中的score大于80的id然后对应test中删除
Query OK, 3 rows affected (0.00 sec)
mysql> select * from test;
+------+-----------+-------+----------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+----------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+----------+--------+
4 rows in set (0.00 sec)
mysql> select * from test;
+------+-----------+-------+----------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+----------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+----------+--------+
4 rows in set (0.00 sec)
mysql> delete from test where id not in (select id where score>=80); //查询score大于等于80的id然后排除并删除不是这些id的记录
Query OK, 2 rows affected (0.00 sec)
mysql> select * from test;
+------+----------+-------+----------+--------+
| id | name | score | address | hobbid |
+------+----------+-------+----------+--------+
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+----------+-------+----------+--------+
2 rows in set (0.00 sec)
existe结合子查询
mysql> select count(*) from test1 where exists(select id from test1 where score=80); //查询score等于80的id，如果有就执行主语句
+----------+
| count(*) |
+----------+
| 7 |
+----------+
1 row in set (0.00 sec)
mysql> select count(*) from test1 where exists(select id from test1 where score<40); //查询scorexiaoyu40的id，没有就不执行前面的主语句
+----------+
| count(*) |
+----------+
| 0 |
+----------+
1 row in set (0.00 sec)
mysql> select * from test1;
+------+-----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+-------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+-------+------------+--------+
7 rows in set (0.00 sec)
别名的子查询
- 将结果集做为一张表进行查询的时候，我们需要用到别名，不然会报错
mysql> select id from (select id,name from test);
ERROR 1248 (42000): Every derived table must have its own alias
mysql> select a.id,name from (select id,name from test) a; //将a作为别名
+------+----------+
| id | name |
+------+----------+
| 3 | wangwu |
| 1 | zhangsan |
+------+----------+
2 rows in set (0.00 sec)
数据库的视图
- 视图只是数据库中的虚拟表，不包含真实数据，只是做了一个映射
单表视图
- 创建视图
mysql> create view v_score as select * from test1 where score>=80;
Query OK, 0 rows affected (0.00 sec)
- 查看视图
mysql> select * from v_score; //只有大于等于80的score记录
+------+----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+----------+-------+------------+--------+
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 80.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+----------+-------+------------+--------+
5 rows in set (0.00 sec)
- 修改原表数据视图会变
mysql> update test1 set score='50' where name='wangwu';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from v_score; //视图中没有wangwu的记录，因为改成了小于80的score了
+------+----------+-------+------------+--------+
| id | name | score | address | hobbid |
+------+----------+-------+------------+--------+
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 99.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+----------+-------+------------+--------+
4 rows in set (0.00 sec)
- 通过视图修改原表
mysql> update v_score set score='100' where name='tianqi';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from v_score;
+------+----------+--------+------------+--------+
| id | name | score | address | hobbid |
+------+----------+--------+------------+--------+
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 100.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+----------+--------+------------+--------+
4 rows in set (0.00 sec)
mysql> select * from test1;
+------+-----------+--------+------------+--------+
| id | name | score | address | hobbid |
+------+-----------+--------+------------+--------+
| 6 | hanmeimei | 40.00 | nanjing | 3 |
| 7 | lilei | 50.00 | nanjing | 5 |
| 2 | lisi | 90.00 | shengzheng | 2 |
| 4 | tianqi | 100.00 | hangzhou | 5 |
| 5 | wangdi | 98.00 | laowo | 3 |
| 3 | wangwu | 50.00 | shanghai | 4 |
| 1 | zhangsan | 80.00 | beijing | 2 |
+------+-----------+--------+------------+--------+
7 rows in set (0.00 sec)
注：修改表不能修改以函数、复合函数方式计算出来的字段
多表视图
- 先创建一个表，插入数据
mysql> create table test3 (id int,name varchar(10),age char(10));
Query OK, 0 rows affected (0.03 sec)
mysql> insert into test3 values(1,'zhangsan',20);
Query OK, 1 row affected (0.00 sec)
mysql> insert into test3 values(2,'lisi',30);
Query OK, 1 row affected (0.00 sec)
mysql> insert into test3 values(3,'wangwu',29);
Query OK, 1 row affected (0.00 sec)
mysql> select * from test3;
+------+----------+------+
| id | name | age |
+------+----------+------+
| 1 | zhangsan | 20 |
| 2 | lisi | 30 |
| 3 | wangwu | 29 |
+------+----------+------+
3 rows in set (0.00 sec)
- 创建一个视图，需要输出id、学生姓名、分数以及年龄
mysql> create view v_test1(id,name,score,age) as select test1.id,test1.name,test1.score,test3.age from test1,test3 where test1.name=test3.name; //将各自表中需要的字段拿出来然后判断name相同，查询出来的结果就是name相同的
Query OK, 0 rows affected (0.00 sec)
mysql> select * from v_test1;
+------+----------+-------+------+
| id | name | score | age |
+------+----------+-------+------+
| 2 | lisi | 90.00 | 30 |
| 3 | wangwu | 50.00 | 29 |
| 1 | zhangsan | 80.00 | 20 |
+------+----------+-------+------+
3 rows in set (0.00 sec)
null值
null值与空值的区别
- NULL值的长度为null，占用空间
- 空值长度为0，不占空间
- 而函数计数会算入空值，忽略null值
mysql> alter table test1 add column addr varchar(50); //增加一个字段
Query OK, 0 rows affected (0.14 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> update test1 set addr='shanghai' where score >80; //score大于80的addr定义为shanghai
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3 Changed: 3 Warnings: 0
mysql> select * from test1;
+------+-----------+--------+------------+--------+----------+
| id | name | score | address | hobbid | addr |
+------+-----------+--------+------------+--------+----------+
| 6 | hanmeimei | 40.00 | nanjing | 3 | NULL |
| 7 | lilei | 50.00 | nanjing | 5 | NULL |
| 2 | lisi | 90.00 | shengzheng | 2 | shanghai |
| 4 | tianqi | 100.00 | hangzhou | 5 | shanghai |
| 5 | wangdi | 98.00 | laowo | 3 | shanghai |
| 3 | wangwu | 50.00 | shanghai | 4 | NULL |
| 1 | zhangsan | 80.00 | beijing | 2 | NULL |
+------+-----------+--------+------------+--------+----------+
7 rows in set (0.00 sec)
mysql> select count(addr) from test1; //函数只统计出三个值，也就是说null不算
+-------------+
| count(addr) |
+-------------+
| 3 |
+-------------+
1 row in set (0.00 sec)
mysql> update test1 set addr='' where name='zhangsan'; //将zhangsanaddr改为空值
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from test1;
+------+-----------+--------+------------+--------+----------+
| id | name | score | address | hobbid | addr |
+------+-----------+--------+------------+--------+----------+
| 6 | hanmeimei | 40.00 | nanjing | 3 | NULL |
| 7 | lilei | 50.00 | nanjing | 5 | NULL |
| 2 | lisi | 90.00 | shengzheng | 2 | shanghai |
| 4 | tianqi | 100.00 | hangzhou | 5 | shanghai |
| 5 | wangdi | 98.00 | laowo | 3 | shanghai |
| 3 | wangwu | 50.00 | shanghai | 4 | NULL |
| 1 | zhangsan | 80.00 | beijing | 2 | |
+------+-----------+--------+------------+--------+----------+
7 rows in set (0.00 sec)
mysql> select count(addr) from test1; //发现统计多一个，说明空值被算入统计中
+-------------+
| count(addr) |
+-------------+
| 4 |
+-------------+
1 row in set (0.01 sec)
- 查询null值
mysql> select * from test1 where addr is null;
+------+-----------+-------+----------+--------+------+
| id | name | score | address | hobbid | addr |
+------+-----------+-------+----------+--------+------+
| 6 | hanmeimei | 40.00 | nanjing | 3 | NULL |
| 7 | lilei | 50.00 | nanjing | 5 | NULL |
| 3 | wangwu | 50.00 | shanghai | 4 | NULL |
+------+-----------+-------+----------+--------+------+
3 rows in set (0.00 sec)
- 查询不是null值的记录
mysql> select * from test1 where addr is not null;
+------+----------+--------+------------+--------+----------+
| id | name | score | address | hobbid | addr |
+------+----------+--------+------------+--------+----------+
| 2 | lisi | 90.00 | shengzheng | 2 | shanghai |
| 4 | tianqi | 100.00 | hangzhou | 5 | shanghai |
| 5 | wangdi | 98.00 | laowo | 3 | shanghai |
| 1 | zhangsan | 80.00 | beijing | 2 | |
+------+----------+--------+------------+--------+----------+
4 rows in set (0.00 sec)