MySQL札记9_DQL(数据查询语言)

DQL（data query language）数据查询语言，主要的作用是对数据库中的数据进行查询的操作，也是最重要的功能，查询的方法也是多种多样：联合查询、分组查询、内连查询、子查询等，还可以限制查询的条数等，下面一一介绍。

复习创建表

先创建两个表：student和mark表，其中student表中存储的学生姓名、年纪等信息，mark表中存储的是分数和学生学号等信息

student表

+----+-----------+-------------------+----------+--------------------------------+--------+
| id | user_name | user_email        | user_age | password                       | fee    |
+----+-----------+-------------------+----------+--------------------------------+--------+
|  1 | peter     | [email protected]   |       27 | *C3BC3E91915DCAE22014892F9827D |  28.87 |
|  2 | jim       | [email protected]   |       22 | *3013993DF5AB62B1310BFB1A4257D |  90.18 |
|  3 | Tom       | [email protected]  |       32 | *438030B7A94B726F59997202525B5 |  77.18 |
|  4 | Rose      | [email protected] |       23 | *3DB09C49A1B49D6AF3B3EF807B91F |  55.45 |
|  5 | zhuliye   | [email protected]   |       27 | *B688A0F40FDA5181039DACA63C33B |  56.18 |
|  6 | John      | [email protected] |       26 | *024D34153E31E0CFBCD619D701232 | 100.18 |
+----+-----------+-------------------+----------+--------------------------------+--------+

mark表

# 建表
mysql> create table mark(
         > id int unsigned not null auto_increment primary key, 
         > mark int(20) not null comment "score", 
         > stu_id varchar(20) not null comment "student_id" 
         > );

# 查看表结构
mysql> desc mark;
+--------+------------------+------+-----+---------+----------------+
| Field  | Type             | Null | Key | Default | Extra          |
+--------+------------------+------+-----+---------+----------------+
| id     | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| mark   | int(20)          | NO   |     | NULL    |                |
| stu_id | varchar(20)      | NO   |     | NULL    |                |
+--------+------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

# 插入数据
mysql> insert into mark values(1, 66, "2");
Query OK, 1 row affected (0.00 sec)

mysql> insert into mark values(2, 75, "3");
Query OK, 1 row affected (0.01 sec)

mysql> insert into mark values(3, 88, "4");
Query OK, 1 row affected (0.01 sec)

mysql> insert into mark values(4, 90, "5");
Query OK, 1 row affected (0.00 sec)

mysql> insert into mark values(5, 82, "6");
Query OK, 1 row affected (0.00 sec)

mysql> select * from mark;
+----+------+--------+
| id | mark | stu_id |
+----+------+--------+
|  1 |   66 | 2      |
|  2 |   75 | 3      |
|  3 |   88 | 4      |
|  4 |   90 | 5      |
|  5 |   82 | 6      |
+----+------+--------+
5 rows in set (0.00 sec)

普通查询

where普通查询

带上where条件进行查询，条件可以是指定id或者其他字段。

mysql> select password, fee from student where id=3;   
+--------------------------------+-------+
| password                       | fee   |
+--------------------------------+-------+
| *438030B7A94B726F59997202525B5 | 77.18 |
+--------------------------------+-------+
1 row in set (0.02 sec)

mysql> select password from student where user_name="Tom";
+--------------------------------+
| password                       |
+--------------------------------+
| *438030B7A94B726F59997202525B5 |
+--------------------------------+
1 row in set (0.00 sec)

过滤查询

.如果某个表中，某些字段的值有重复，可以进行过滤查询。过滤重复字段的关键词是distinct。student表中的age字段有重复值：

image.png

mysql> select distinct user_age from student;   # 括号有没有均可
mysql> select distinct(user_age) from student;

image.png

连接查询

连接查询的关键词是concat，有三种不同的使用情况：

直接使用concat
使用concat....as....；显示出来的时候用as后面的别名
带上连接符号的查询concat_ws("==", 列名1，列名2)；其中"=="就是指定连接符

image.png

image.png

image.png

模糊查询

模糊查询的关键字是like，中文翻译成像：

mysql> select user_name from student where user_name like "peter";    # 像peter
mysql> select user_name from student where user_name like "%e";      # %表示任意，表示名字以e结尾
mysql> select user_name from student where user_name like "%e%";    # 表示名字中含有e

image.png

排列

对表中的记录进行升序asc或者降序desc的排列，默认的是升序asc：

mysql> select * from student order by user_age asc;   # 年龄的升序
mysql> select * from student order by user_age desc;  # 年龄的降序

image.png

聚合函数

select count(*) from student;    # 总记录
select sum(列名) from student;  # 总和
select avg(列名) from student;   # 平均值
select max/min(列名) from student;  # 最大/小值

高级查询

分组查询

分组查询中主要使用的是group by，通过by后面的条件来筛选，同时会有一个排序（默认是升序）和去重的功能存在，从而进行查询：

mysql> select * from student group by user_age;

image.png

年纪中有重复的值，对比上图，进行去重显示：

image.png
根据年龄统计个数，并且显示user_name 和别名totals：

image.png
通过group by ...having...联合进行查询

mysql> select user_name, fee from student group by user_age having count(*)=1;

mysql> select user_name, fee from student group by user_age having count(*)=2;

image.png

内连查询

仔细看看笔者之前创建的两个表，student表中有学生的各种信息，mark表中有个分数和学生编号，请问如何通过这两个表，找到student表中对应mark表里面stu_id的成绩呢？内连查询解决的就是这个问题

image.png

mysql> select user_name, mark from student, mark where mark.stu_id = student.id;      #   通过指定mark.stu_id = student.id 作为限定条件 
+-----------+------+
| user_name | mark |
+-----------+------+
| jim       |   66 |
| Tom       |   75 |
| Rose      |   88 |
| zhuliye   |   90 |
| John      |   82 |
+-----------+------+
5 rows in set (0.00 sec)

使用别名

# 适合两个表中没有想用字段
mysql> select user_name, mark from student as s, mark as m where m.stu_id = s.id;   

# 适合两个表中具有相同字段使用：一定要指明哪个字段属于那个表
mysql> select s.user_name, m.mark from student as s, mark as m where m.stu_id = s.id;

image.png

inner join查询
将上面两个表之间的逗号换成inner join

mysql> select s.user_name, m.mark from student as s inner join mark as m where m.stu_id = s.id;

image.png

where换成on

where可以换成on
on不能和逗号连用
on只能和inner join连用

image.png

左右连接

左连接

mysql> select s.user_name, m.mark from student as s left join mark as m on m.stu_id = s.id;

右连接

mysql> select s.user_name, m.mark from student as s right join mark as m on m.stu_id = s.id;

联合查询

联合查询的关键字是union all，需要注意的是查询的时候字段需要保持一致。

image.png

where条件的拆分

mysql> select * from student where id in (2,4);
mysql> select * from student where id=2 union all select * from student where id=4;

image.png

子查询

mysql> select user_name, id from student where id in (select stu_id from mark);

# 结果
+-----------+----+
| user_name | id |
+-----------+----+
| jim       |  2 |
| Tom       |  3 |
| Rose      |  4 |
| zhuliye   |  5 |
| John      |  6 |
+-----------+----+
5 rows in set (0.00 sec)