概述
关于多表联合查询
在关系型数据库中，表与表之间是有联系的，所以在实际应用中，经常使用多表查询。多表查询就是同时查询两个或两个以上的表。
在 MySQL 中，多表查询主要有交叉连接、内连接、外连接、分组查询与子查询等5种。
交叉连接
语法
#SELECT <字段名> FROM <表1> CROSS JOIN <表2> [WHERE子句];
#SELECT <字段名> FROM <表1>, <表2> [WHERE子句];
字段名：需要查询的字段名称。
<表1><表2>：需要交叉连接的表名。
WHERE 子句：用来设置交叉连接的查询条件案例
创建school库 创建tb_students_info表，插入数据
mysql> create database school;
Query OK, 1 row affected (0.01 sec)
mysql> use school;
Database changed
mysql> create table tb_students_info(id int not null primary key auto_increment,name varchar(50),age tinyint,sex varchar(10),height int,course_id int);
Query OK, 0 rows affected (0.01 sec)
mysql> desc tb_students_info;
+-----------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(50) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | varchar(10) | YES | | NULL | |
| height | int(11) | YES | | NULL | |
| course_id | int(11) | YES | | NULL | |
+-----------+-------------+------+-----+---------+----------------+
6 rows in set (0.01 sec)
mysql> insert tb_students_info(name,age,sex,height,course_id) values('Dany',25,'Male',160,1),('Green',23,'Ma,le',158,2),('Henry',23,'Female',185,1),('Jane',22,'Male',162,3),('Jim',24,'Female',175,2),('John',21,'Male',)172,4),('Lily',22,'Male',165,4),('Susan',23,'Male',170,5),('Thomas',22,'Female',178,5),('Tom',23,'Female',165,5);
Query OK, 10 rows affected (0.01 sec)
Records: 10 Duplicates: 0 Warnings: 0
mysql> select * from tb_students_info;
+----+--------+------+--------+--------+-----------+
| id | name | age | sex | height | course_id |
+----+--------+------+--------+--------+-----------+
| 1 | Dany | 25 | Male | 160 | 1 |
| 2 | Green | 23 | Male | 158 | 2 |
| 3 | Henry | 23 | Female | 185 | 1 |
| 4 | Jane | 22 | Male | 162 | 3 |
| 5 | Jim | 24 | Female | 175 | 2 |
| 6 | John | 21 | Male | 172 | 4 |
| 7 | Lily | 22 | Male | 165 | 4 |
| 8 | Susan | 23 | Male | 170 | 5 |
| 9 | Thomas | 22 | Female | 178 | 5 |
| 10 | Tom | 23 | Female | 165 | 5 |
+----+--------+------+--------+--------+-----------+
10 rows in set (0.00 sec)
mysql>创建tb_course表，插入数据
mysql> create table tb_course(id int not null primary key auto_increment,course_name varchar(50));
Query OK, 0 rows affected (0.00 sec)
mysql> insert tb_course(course_name) values('Java'),('Mysql'),('Python'),('Go'),('C++');
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> select * from tb_crouse;
ERROR 1146 (42S02): Table 'school.tb_crouse' doesn't exist
mysql> select * from tb_course;
+----+-------------+
| id | course_name |
+----+-------------+
| 1 | Java |
| 2 | Mysql |
| 3 | Python |
| 4 | Go |
| 5 | C++ |
+----+-------------+
5 rows in set (0.00 sec)不使用where子句查询出两张表中的笛卡尔积 ，返回5*10=50条记录，因为无用信息过多，不常见这种使用方法
mysql> SELECT * FROM tb_course CROSS JOIN tb_students_info;
+----+-------------+----+--------+------+--------+--------+-----------+
| id | course_name | id | name | age | sex | height | course_id |
+----+-------------+----+--------+------+--------+--------+-----------+
| 1 | Java | 1 | Dany | 25 | Male | 160 | 1 |
| 2 | Mysql | 1 | Dany | 25 | Male | 160 | 1 |
| 3 | Python | 1 | Dany | 25 | Male | 160 | 1 |
| 4 | Go | 1 | Dany | 25 | Male | 160 | 1 |
| 5 | C++ | 1 | Dany | 25 | Male | 160 | 1 |
| 1 | Java | 2 | Green | 23 | Male | 158 | 2 |
| 2 | Mysql | 2 | Green | 23 | Male | 158 | 2 |
| 3 | Python | 2 | Green | 23 | Male | 158 | 2 |
| 4 | Go | 2 | Green | 23 | Male | 158 | 2 |
| 5 | C++ | 2 | Green | 23 | Male | 158 | 2 |
| 1 | Java | 3 | Henry | 23 | Female | 185 | 1 |
| 2 | Mysql | 3 | Henry | 23 | Female | 185 | 1 |
| 3 | Python | 3 | Henry | 23 | Female | 185 | 1 |
| 4 | Go | 3 | Henry | 23 | Female | 185 | 1 |
| 5 | C++ | 3 | Henry | 23 | Female | 185 | 1 |
| 1 | Java | 4 | Jane | 22 | Male | 162 | 3 |
| 2 | Mysql | 4 | Jane | 22 | Male | 162 | 3 |
| 3 | Python | 4 | Jane | 22 | Male | 162 | 3 |
| 4 | Go | 4 | Jane | 22 | Male | 162 | 3 |
| 5 | C++ | 4 | Jane | 22 | Male | 162 | 3 |
| 1 | Java | 5 | Jim | 24 | Female | 175 | 2 |
| 2 | Mysql | 5 | Jim | 24 | Female | 175 | 2 |
| 3 | Python | 5 | Jim | 24 | Female | 175 | 2 |
| 4 | Go | 5 | Jim | 24 | Female | 175 | 2 |
| 5 | C++ | 5 | Jim | 24 | Female | 175 | 2 |
| 1 | Java | 6 | John | 21 | Male | 172 | 4 |
| 2 | Mysql | 6 | John | 21 | Male | 172 | 4 |
| 3 | Python | 6 | John | 21 | Male | 172 | 4 |
| 4 | Go | 6 | John | 21 | Male | 172 | 4 |
| 5 | C++ | 6 | John | 21 | Male | 172 | 4 |
| 1 | Java | 7 | Lily | 22 | Male | 165 | 4 |
| 2 | Mysql | 7 | Lily | 22 | Male | 165 | 4 |
| 3 | Python | 7 | Lily | 22 | Male | 165 | 4 |
| 4 | Go | 7 | Lily | 22 | Male | 165 | 4 |
| 5 | C++ | 7 | Lily | 22 | Male | 165 | 4 |
| 1 | Java | 8 | Susan | 23 | Male | 170 | 5 |
| 2 | Mysql | 8 | Susan | 23 | Male | 170 | 5 |
| 3 | Python | 8 | Susan | 23 | Male | 170 | 5 |
| 4 | Go | 8 | Susan | 23 | Male | 170 | 5 |
| 5 | C++ | 8 | Susan | 23 | Male | 170 | 5 |
| 1 | Java | 9 | Thomas | 22 | Female | 178 | 5 |
| 2 | Mysql | 9 | Thomas | 22 | Female | 178 | 5 |
| 3 | Python | 9 | Thomas | 22 | Female | 178 | 5 |
| 4 | Go | 9 | Thomas | 22 | Female | 178 | 5 |
| 5 | C++ | 9 | Thomas | 22 | Female | 178 | 5 |
| 1 | Java | 10 | Tom | 23 | Female | 165 | 5 |
| 2 | Mysql | 10 | Tom | 23 | Female | 165 | 5 |
| 3 | Python | 10 | Tom | 23 | Female | 165 | 5 |
| 4 | Go | 10 | Tom | 23 | Female | 165 | 5 |
| 5 | C++ | 10 | Tom | 23 | Female | 165 | 5 |
+----+-------------+----+--------+------+--------+--------+-----------+
50 rows in set (0.01 sec)使用where子句查询 tb_course 表中的 id 字段和 tb_students_info 表中的 course_id 字段相等的内容
mysql> select * from tb_course cross join tb_students_info where tb_students_info.course_id = tb_course.id;
+----+-------------+----+--------+------+--------+--------+-----------+
| id | course_name | id | name | age | sex | height | course_id |
+----+-------------+----+--------+------+--------+--------+-----------+
| 1 | Java | 1 | Dany | 25 | Male | 160 | 1 |
| 2 | Mysql | 2 | Green | 23 | Male | 158 | 2 |
| 1 | Java | 3 | Henry | 23 | Female | 185 | 1 |
| 3 | Python | 4 | Jane | 22 | Male | 162 | 3 |
| 2 | Mysql | 5 | Jim | 24 | Female | 175 | 2 |
| 4 | Go | 6 | John | 21 | Male | 172 | 4 |
| 4 | Go | 7 | Lily | 22 | Male | 165 | 4 |
| 5 | C++ | 8 | Susan | 23 | Male | 170 | 5 |
| 5 | C++ | 9 | Thomas | 22 | Female | 178 | 5 |
| 5 | C++ | 10 | Tom | 23 | Female | 165 | 5 |
+----+-------------+----+--------+------+--------+--------+-----------+
10 rows in set (0.00 sec)使用order by以course表的id排序
mysql> select * from tb_students_info cross join tb_course where tb_course.id = tb_students_info.course_id order by tb_course.id;
+----+--------+------+--------+--------+-----------+----+-------------+
| id | name | age | sex | height | course_id | id | course_name |
+----+--------+------+--------+--------+-----------+----+-------------+
| 1 | Dany | 25 | Male | 160 | 1 | 1 | Java |
| 3 | Henry | 23 | Female | 185 | 1 | 1 | Java |
| 5 | Jim | 24 | Female | 175 | 2 | 2 | Mysql |
| 2 | Green | 23 | Male | 158 | 2 | 2 | Mysql |
| 4 | Jane | 22 | Male | 162 | 3 | 3 | Python |
| 7 | Lily | 22 | Male | 165 | 4 | 4 | Go |
| 6 | John | 21 | Male | 172 | 4 | 4 | Go |
| 9 | Thomas | 22 | Female | 178 | 5 | 5 | C++ |
| 8 | Susan | 23 | Male | 170 | 5 | 5 | C++ |
| 10 | Tom | 23 | Female | 165 | 5 | 5 | C++ |
+----+--------+------+--------+--------+-----------+----+-------------+
10 rows in set (0.00 sec)单独列出学习某种语言的人的名单
mysql> select tb_students_info.name from tb_students_info cross join tb_course where tb_course.id = tb_students_info.course_id and tb_course.course_name = 'Go';
+------+
| name |
+------+
| John |
| Lily |
+------+
2 rows in set (0.00 sec)内连接
#SELECT <字段名> FROM <表1> INNER JOIN <表2> [ON子句];
字段名：需要查询的字段名称。
<表1><表2>：需要内连接的表名。
INNER JOIN ：内连接中可以省略 INNER 关键字，只用关键字 JOIN。
ON 子句：用来设置内连接的连接条件。取tb_students_info的name和tb_course的course_name 且满足tb_students_info的course_id = tb_course的id (文章后面的s,c分别为两个表的别名)
mysql> select s.name,c.course_name from tb_students_info as s inner join tb_course as c on s.course_id = c.id;
+--------+-------------+
| name | course_name |
+--------+-------------+
| Dany | Java |
| Green | Mysql |
| Henry | Java |
| Jane | Python |
| Jim | Mysql |
| John | Go |
| Lily | Go |
| Susan | C++ |
| Thomas | C++ |
| Tom | C++ |
+--------+-------------+
10 rows in set (0.00 sec)
按照course_name来排序
mysql> select s.name,c.course_name from tb_students_info as s inner join tb_course as c on s.course_id = c.id order by c.course_name;
+--------+-------------+
| name | course_name |
+--------+-------------+
| Thomas | C++ |
| Tom | C++ |
| Susan | C++ |
| John | Go |
| Lily | Go |
| Dany | Java |
| Henry | Java |
| Green | Mysql |
| Jim | Mysql |
| Jane | Python |
+--------+-------------+
10 rows in set (0.00 sec)外连接
左连接
#SELECT <字段名> FROM <表1> LEFT OUTER JOIN <表2> <ON子句>;
字段名：需要查询的字段名称。
<表1><表2>：需要左连接的表名。
LEFT OUTER JOIN：左连接中可以省略 OUTER 关键字，只使用关键字 LEFT JOIN。
ON 子句：用来设置左连接的连接条件，不能省略。
#"表1"为基表，"表2"为参考表。在tb_course表插入Html，不赋予Html对应的course_id
mysql> insert tb_course(course_name) value('Html');
Query OK, 1 row affected (0.00 sec)
#以tb_students_info表为基础查询tb_students_info表的course_id 匹配 tb_course表的id 没有显示Html，因为Html没有对应的course_id
mysql> select * from tb_students_info left join tb_course on tb_students_info.course_id = tb_course.id;
+----+--------+------+--------+--------+-----------+------+-------------+
| id | name | age | sex | height | course_id | id | course_name |
+----+--------+------+--------+--------+-----------+------+-------------+
| 1 | Dany | 25 | Male | 160 | 1 | 1 | Java |
| 2 | Green | 23 | Male | 158 | 2 | 2 | Mysql |
| 3 | Henry | 23 | Female | 185 | 1 | 1 | Java |
| 4 | Jane | 22 | Male | 162 | 3 | 3 | Python |
| 5 | Jim | 24 | Female | 175 | 2 | 2 | Mysql |
| 6 | John | 21 | Male | 172 | 4 | 4 | Go |
| 7 | Lily | 22 | Male | 165 | 4 | 4 | Go |
| 8 | Susan | 23 | Male | 170 | 5 | 5 | C++ |
| 9 | Thomas | 22 | Female | 178 | 5 | 5 | C++ |
| 10 | Tom | 23 | Female | 165 | 5 | 5 | C++ |
+----+--------+------+--------+--------+-----------+------+-------------+
10 rows in set (0.00 sec)添加Liming，但赋予不存在的course_id和course_name
mysql> insert tb_students_info(name,age,sex,height,course_id) value('Liming',22,'Male',180,7);
Query OK, 1 row affected (0.00 sec)
再次以tb_students_info表为基础查询tb_students_info表的course_id 匹配 tb_course表的id 因为没有对应的course_id和course_name,所以会显示null
mysql> select * from tb_students_info left join tb_course on tb_students_info.course_id = tb_course.id;
+----+--------+------+--------+--------+-----------+------+-------------+
| id | name | age | sex | height | course_id | id | course_name |
+----+--------+------+--------+--------+-----------+------+-------------+
| 1 | Dany | 25 | Male | 160 | 1 | 1 | Java |
| 2 | Green | 23 | Male | 158 | 2 | 2 | Mysql |
| 3 | Henry | 23 | Female | 185 | 1 | 1 | Java |
| 4 | Jane | 22 | Male | 162 | 3 | 3 | Python |
| 5 | Jim | 24 | Female | 175 | 2 | 2 | Mysql |
| 6 | John | 21 | Male | 172 | 4 | 4 | Go |
| 7 | Lily | 22 | Male | 165 | 4 | 4 | Go |
| 8 | Susan | 23 | Male | 170 | 5 | 5 | C++ |
| 9 | Thomas | 22 | Female | 178 | 5 | 5 | C++ |
| 10 | Tom | 23 | Female | 165 | 5 | 5 | C++ |
| 11 | Liming | 22 | Male | 180 | 7 | NULL | NULL |
+----+--------+------+--------+--------+-----------+------+-------------+
11 rows in set (0.00 sec)
以s表为基础表查询course_id匹配c表的id c表没有对应值 所有为空null值
mysql> select s.name,c.course_name from tb_students_info s left join tb_course c on s.course_id=c.id;
+--------+-------------+
| name | course_name |
+--------+-------------+
| Dany | Java |
| Green | Mysql |
| Henry | Java |
| Jane | Python |
| Jim | Mysql |
| John | Go |
| Lily | Go |
| Susan | C++ |
| Thomas | C++ |
| Tom | C++ |
| Liming | NULL |
+--------+-------------+
11 rows in set (0.00 sec)右连接
#SELECT <字段名> FROM <表1> RIGHT OUTER JOIN <表2> <ON子句>;
字段名：需要查询的字段名称。
<表1><表2>：需要右连接的表名。
RIGHT OUTER JOIN：右连接中可以省略 OUTER 关键字，只使用关键字 RIGHT JOIN。
ON 子句：用来设置右连接的连接条件，不能省略。
#与左连接相反，右连接以"表2"为基表，"表1"为参考表。以c表为基础表查询id匹配s表的course_id c表没有对应值 所有为空null值
mysql> select s.name,c.course_name from tb_students_info s right join tb_course c on s.course_id=c.id;
+--------+-------------+
| name | course_name |
+--------+-------------+
| Dany | Java |
| Green | Mysql |
| Henry | Java |
| Jane | Python |
| Jim | Mysql |
| John | Go |
| Lily | Go |
| Susan | C++ |
| Thomas | C++ |
| Tom | C++ |
| NULL | Html |
+--------+-------------+
11 rows in set (0.00 sec)分组查询
GROUP BY 关键字可以根据一个或多个字段对查询结果进行分组。
语法
#GROUP BY <字段名>
<字段名>表示需要分组的字段名称，多个字段时用逗号隔开单独使用
#根据 tb_students_info 表中的 sex 字段进行分组查询 只显示两条分为男女，因为只匹配一条，所以男女各匹配一条后结束
mysql> SELECT name,sex FROM tb_students_info GROUP BY sex;
+-------+--------+
| name | sex |
+-------+--------+
| Henry | Female |
| Dany | Male |
+-------+--------+
2 rows in set (0.00 sec)GROUP BY 关键字可以和 GROUP_CONCAT() 函数一起使用。GROUP_CONCAT() 函数会把每个分组的字段值都显示出来。
#把性别sex男，女的名字都列出来
mysql> select sex, group_concat(name) from tb_students_info group by sex;
+--------+----------------------------------------+
| sex | group_concat(name) |
+--------+----------------------------------------+
| Female | Henry,Jim,Thomas,Tom |
| Male | Dany,Green,Jane,John,Lily,Susan,Liming |
+--------+----------------------------------------+
2 rows in set (0.00 sec)根据 tb_students_info 表中的 age 和 sex 字段进行分组查询
mysql> select age,sex,group_concat(name) names from tb_students_info group by age,sex;
+------+--------+------------------+
| age | sex | names |
+------+--------+------------------+
| 21 | Male | John |
| 22 | Female | Thomas |
| 22 | Male | Jane,Lily,Liming |
| 23 | Female | Henry,Tom |
| 23 | Male | Green,Susan |
| 24 | Female | Jim |
| 25 | Male | Dany |
+------+--------+------------------+
7 rows in set (0.00 sec)GROUP BY 与聚合函数
在数据统计时，GROUP BY 关键字经常和聚合函数一起使用。
聚合函数包括 COUNT()，SUM()，AVG()，MAX() 和 MIN()。其中，COUNT() 用来统计记录的条数；SUM() 用来计算字段值的总和；AVG() 用来计算字段值的平均值；MAX() 用来查询字段的最大值；MIN() 用来查询字段的最小值。
tb_students_info 表的 sex 字段进行分组查询，使用 COUNT() 函数计算每一组的记录数。
<code class="language-plaintext hljs">mysql> select sex,count(sex) from tb_students_info group by sex;
+--------+------------+
| sex | count(sex) |
+--------+------------+
| Female | 4 |
| Male | 7 |
+--------+------------+
2 rows in set (0.00 sec)
mysql> select sex 性别,count(sex) 个数 from tb_students_info group by sex;
+--------+--------+
| 性别 | 个数 |
+--------+--------+
| Female | 4 |
| Male | 7 |
+--------+--------+
2 rows in set (0.00 sec)
mysql></code>GROUP BY 与 WITH ROLLUP
WITH POLLUP 关键字用来在所有记录的最后加上一条记录，这条记录是上面所有记录的总和，即统计记录数量。
下面根据 tb_students_info 表中的 sex 字段进行分组查询，并使用 WITH ROLLUP 显示记录的总和：
<code class="language-plaintext hljs">mysql> select sex,group_concat(name) name from tb_students_info group by sex with rollup;
+--------+-------------------------------------------------------------+
| sex | name |
+--------+-------------------------------------------------------------+
| Female | Henry,Jim,Thomas,Tom |
| Male | Dany,Green,Jane,John,Lily,Susan,Liming |
| NULL | Henry,Jim,Thomas,Tom,Dany,Green,Jane,John,Lily,Susan,Liming |
+--------+-------------------------------------------------------------+
3 rows in set (0.00 sec)
mysql> select sex,group_concat(name) name from tb_students_info group by sex;
+--------+----------------------------------------+
| sex | name |
+--------+----------------------------------------+
| Female | Henry,Jim,Thomas,Tom |
| Male | Dany,Green,Jane,John,Lily,Susan,Liming |
+--------+----------------------------------------+
2 rows in set (0.00 sec)
mysql> </code>with rollup 统计所有的名单
计算平均年龄
<code class="language-plaintext hljs">mysql> select avg(age) from tb_students_info;
+----------+
| avg(age) |
+----------+
| 22.7273 |
+----------+
1 row in set (0.00 sec)
mysql></code>查询男女的平均年龄
<code class="language-plaintext hljs">mysql> select sex,avg(age) from tb_students_info group by sex;
+--------+----------+
| sex | avg(age) |
+--------+----------+
| Female | 23.0000 |
| Male | 22.5714 |
+--------+----------+
2 rows in set (0.00 sec)
mysql> </code>子查询
子查询是 MySQL 中比较常用的查询方法，通过子查询可以实现多表查询。子查询指将一个查询语句嵌套在另一个查询语句中。子查询可以在 SELECT、UPDATE 和 DELETE 语句中使用，而且可以进行多层嵌套。在实际开发时，子查询经常出现在 WHERE 子句中。
语法格式：
<codeclass="language-plaintext hljs">WHERE <表达式><操作符> (子查询)</code>操作符可以是比较运算符和 IN、NOT IN、EXISTS、NOT EXISTS 等关键字。
IN ；NOT IN
当表达式与子查询返回的结果集中的某个值相等时，返回 TRUE，否则返回 FALSE；若使用关键字 NOT，则返回值正好相反
EXISTS ；NOT EXISTS
用于判断子查询的结果集是否为空，若子查询的结果集不为空，返回 TRUE，否则返回 FALSE；若使用关键字 NOT，则返回的值正好相反
使用子查询在 tb_students_info 表和 tb_course 表中查询学习 Java 课程的学生姓名
<code class="language-plaintext hljs">mysql> select * from tb_students_info;
+----+--------+------+--------+--------+-----------+
| id | name | age | sex | height | course_id |
+----+--------+------+--------+--------+-----------+
| 1 | Dany | 25 | Male | 160 | 1 |
| 2 | Green | 23 | Male | 158 | 2 |
| 3 | Henry | 23 | Female | 185 | 1 |
| 4 | Jane | 22 | Male | 162 | 3 |
| 5 | Jim | 24 | Female | 175 | 2 |
| 6 | John | 21 | Male | 172 | 4 |
| 7 | Lily | 22 | Male | 165 | 4 |
| 8 | Susan | 23 | Male | 170 | 5 |
| 9 | Thomas | 22 | Female | 178 | 5 |
| 10 | Tom | 23 | Female | 165 | 5 |
| 11 | Liming | 22 | Male | 180 | 7 |
+----+--------+------+--------+--------+-----------+
11 rows in set (0.00 sec)
mysql> select id from tb_course where course_name = 'Java';
+----+
| id |
+----+
| 1 |
+----+
1 row in set (0.00 sec)
mysql> select name from tb_students_info where course_id in (select id from tb_course where course_name = 'Java');
+-------+
| name |
+-------+
| Dany |
| Henry |
+-------+
2 rows in set (0.00 sec)
mysql></code>括号里的值可以有多个
<code class="language-plaintext hljs">mysql> select id from tb_course where course_name = 'Java' or course_name = 'Go' ;
+----+
| id |
+----+
| 1 |
| 4 |
+----+
2 rows in set (0.00 sec)
mysql>
mysql> select name from tb_students_info where course_id in (select id from tb_course where course_name = 'Java' or course_name = 'Go');
+-------+
| name |
+-------+
| Dany |
| Henry |
| John |
| Lily |
+-------+
4 rows in set (0.00 sec)
mysql></code>在 SELECT 语句中使用 NOT IN 关键字，查询没有学习Java和Go的
<code class="language-plaintext hljs">mysql> select name from tb_students_info where course_id not in (select id from tb_course where course_name ='Java' or course_name = 'Go');
+--------+
| name |
+--------+
| Green |
| Jane |
| Jim |
| Susan |
| Thomas |
| Tom |
| Liming |
+--------+
7 rows in set (0.00 sec)
mysql> </code>外层的 SELECT 查询称为父查询，圆括号中嵌入的查询称为子查询（子查询必须放在圆括号内）。MySQL 在处理上例的 SELECT 语句时，执行流程为：先执行子查询，再执行父查询
使用=运算符，在 tb_course 表和 tb_students_info 表中查询出所有学习 Python 课程的学生姓名
<code class="language-plaintext hljs">mysql> select * from tb_course;
+----+-------------+
| id | course_name |
+----+-------------+
| 1 | Java |
| 2 | Mysql |
| 3 | Python |
| 4 | Go |
| 5 | C++ |
| 6 | Html |
+----+-------------+
6 rows in set (0.00 sec)
mysql>
mysql> select name from tb_students_info where course_id = (select id from tb_course where course_name = 'Python');
+------+
| name |
+------+
| Jane |
+------+
1 row in set (0.00 sec)
mysql> </code>使用<>运算符(不等于)，在 tb_course 表和 tb_students_info 表中查询出没有学习 Python 课程的学生姓名
（!= 也可以实现<>的效果，!表示取反!=意思是不等于）
<code class="language-plaintext hljs">mysql> select name from tb_students_info where course_id <> (select id from tb_course where course_name = 'Pythhon');
+--------+
| name |
+--------+
| Dany |
| Green |
| Henry |
| Jim |
| John |
| Lily |
| Susan |
| Thomas |
| Tom |
| Liming |
+--------+
10 rows in set (0.00 sec)
mysql></code>
<code class="language-plaintext hljs">mysql> select name from tb_students_info where course_id != (select id from tb_course where course_name = 'Pythhon');
+--------+
| name |
+--------+
| Dany |
| Green |
| Henry |
| Jim |
| John |
| Lily |
| Susan |
| Thomas |
| Tom |
| Liming |
+--------+
10 rows in set (0.00 sec)
mysql></code>查询 tb_course 表中是否存在 id=1 的课程，如果存在，就查询出 tb_students_info 表中的记录,如果不存在就显示为空
<code class="language-plaintext hljs">mysql> select * from tb_students_info where exists (select course_name from tb_course where id = 1);
+----+--------+------+--------+--------+-----------+
| id | name | age | sex | height | course_id |
+----+--------+------+--------+--------+-----------+
| 1 | Dany | 25 | Male | 160 | 1 |
| 2 | Green | 23 | Male | 158 | 2 |
| 3 | Henry | 23 | Female | 185 | 1 |
| 4 | Jane | 22 | Male | 162 | 3 |
| 5 | Jim | 24 | Female | 175 | 2 |
| 6 | John | 21 | Male | 172 | 4 |
| 7 | Lily | 22 | Male | 165 | 4 |
| 8 | Susan | 23 | Male | 170 | 5 |
| 9 | Thomas | 22 | Female | 178 | 5 |
| 10 | Tom | 23 | Female | 165 | 5 |
| 11 | Liming | 22 | Male | 180 | 7 |
+----+--------+------+--------+--------+-----------+
11 rows in set (0.00 sec)
mysql> select * from tb_students_info where exists (select course_name from tb_course where id = 8);
Empty set (0.00 sec)
mysql> </code>
tb_course 表中存在 id=1 的记录，因此 EXISTS 表达式返回 TRUE，外层查询语句接收 TRUE 之后对表 tb_students_info 进行查询，返回所有的记录EXISTS 关键字可以和其它查询条件一起使用，条件表达式与 EXISTS 关键字之间用 AND 和 OR 连接
<code class="language-plaintext hljs">mysql> select * from tb_students_info where exists (select course_name from tb_course where id = 8)or not exists (select course_name from tb_course where id = 8);
+----+--------+------+--------+--------+-----------+
| id | name | age | sex | height | course_id |
+----+--------+------+--------+--------+-----------+
| 1 | Dany | 25 | Male | 160 | 1 |
| 2 | Green | 23 | Male | 158 | 2 |
| 3 | Henry | 23 | Female | 185 | 1 |
| 4 | Jane | 22 | Male | 162 | 3 |
| 5 | Jim | 24 | Female | 175 | 2 |
| 6 | John | 21 | Male | 172 | 4 |
| 7 | Lily | 22 | Male | 165 | 4 |
| 8 | Susan | 23 | Male | 170 | 5 |
| 9 | Thomas | 22 | Female | 178 | 5 |
| 10 | Tom | 23 | Female | 165 | 5 |
| 11 | Liming | 22 | Male | 180 | 7 |
+----+--------+------+--------+--------+-----------+
11 rows in set (0.00 sec)
mysql> ##命令能执行成功是因为后面not exists (select course_name from tb_course where id = 8)是成立的</code>查询 tb_course 表中是否存在 id=1 的课程，如果存在，就查询出 tb_students_info 表中 age 字段大于 24 的记录
<code class="language-plaintext hljs">mysql> select * from tb_students_info;
+----+--------+------+--------+--------+-----------+
| id | name | age | sex | height | course_id |
+----+--------+------+--------+--------+-----------+
| 1 | Dany | 25 | Male | 160 | 1 |
| 2 | Green | 23 | Male | 158 | 2 |
| 3 | Henry | 23 | Female | 185 | 1 |
| 4 | Jane | 22 | Male | 162 | 3 |
| 5 | Jim | 24 | Female | 175 | 2 |
| 6 | John | 21 | Male | 172 | 4 |
| 7 | Lily | 22 | Male | 165 | 4 |
| 8 | Susan | 23 | Male | 170 | 5 |
| 9 | Thomas | 22 | Female | 178 | 5 |
| 10 | Tom | 23 | Female | 165 | 5 |
| 11 | Liming | 22 | Male | 180 | 7 |
+----+--------+------+--------+--------+-----------+
11 rows in set (0.00 sec)
mysql> select * from tb_students_info where age > 24 and exists (select course_name from tb_course where id =1);
+----+------+------+------+--------+-----------+
| id | name | age | sex | height | course_id |
+----+------+------+------+--------+-----------+
| 1 | Dany | 25 | Male | 160 | 1 |
+----+------+------+------+--------+-----------+
1 row in set (0.00 sec)
mysql> </code>
结果显示，从 tb_students_info 表中查询出了一条记录，这条记录的 age 字段取值为 25。内层查询语句从 tb_course 表中查询到记录，返回 TRUE。外层查询语句开始进行查询。根据查询条件，从 tb_students_info 表中查询 age 大于 24 的记录。查询 tb_course 表中是否存在 id=2 的课程，如果存在，就查询出 tb_students_info 表中 age 字段大于 24 的记录
<code class="language-plaintext hljs">mysql> select * from tb_students_info where age > 24 and exists (select course_name from tb_course where id = 2);
+----+------+------+------+--------+-----------+
| id | name | age | sex | height | course_id |
+----+------+------+------+--------+-----------+
| 1 | Dany | 25 | Male | 160 | 1 |
+----+------+------+------+--------+-----------+
1 row in set (0.00 sec)
mysql></code>
结果显示，明明没有大于24岁学习课程1的人但还是执行了出来，说明后面exists (select course_name from tb_course where id = 2)是一个条件，主要还前面是select * from tb_students_info where age > 24（查询 tb_students_info 表中 age 字段大于 24 ）查询班级身高最高的学生
<code class="language-plaintext hljs">mysql> select name,height from tb_students_info where height = (select max(height) from tb_students_info);
+-------+--------+
| name | height |
+-------+--------+
| Henry | 185 |
+-------+--------+
1 row in set (0.00 sec)
mysql></code>
<code class="language-plaintext hljs">mysql> select * from tb_students_info;
+----+--------+------+--------+--------+-----------+
| id | name | age | sex | height | course_id |
+----+--------+------+--------+--------+-----------+
| 1 | Dany | 25 | Male | 160 | 1 |
| 2 | Green | 23 | Male | 158 | 2 |
| 3 | Henry | 23 | Female | 185 | 1 |
| 4 | Jane | 22 | Male | 162 | 3 |
| 5 | Jim | 24 | Female | 175 | 2 |
| 6 | John | 21 | Male | 172 | 4 |
| 7 | Lily | 22 | Male | 165 | 4 |
| 8 | Susan | 23 | Male | 170 | 5 |
| 9 | Thomas | 22 | Female | 178 | 5 |
| 10 | Tom | 23 | Female | 165 | 5 |
| 11 | Liming | 22 | Male | 180 | 7 |
+----+--------+------+--------+--------+-----------+
11 rows in set (0.00 sec)
mysql></code>查询班级年龄最小的学生姓名和性别
<code class="language-plaintext hljs">mysql> select name,sex,age from tb_students_info where age = (select min(age)from tb_students_info);
+------+------+------+
| name | sex | age |
+------+------+------+
| John | Male | 21 |
+------+------+------+
1 row in set (0.00 sec)
mysql></code>查询 tb_students_info表的名字，人数，平均年龄，身高根据tb_students_info的姓名分组
<code class="language-plaintext hljs">mysql> select s.name,count(*),avg(s.age),s.height from tb_students_info s,tb_course c where s.course_id = c.id group by s.name;
+--------+----------+------------+--------+
| name | count(*) | avg(s.age) | height |
+--------+----------+------------+--------+
| Dany | 1 | 25.0000 | 160 |
| Green | 1 | 23.0000 | 158 |
| Henry | 2 | 24.0000 | 185 |
| Jane | 1 | 22.0000 | 162 |
| Jim | 1 | 24.0000 | 175 |
| John | 1 | 21.0000 | 172 |
| Lily | 1 | 22.0000 | 165 |
| Susan | 1 | 23.0000 | 170 |
| Thomas | 1 | 22.0000 | 178 |
| Tom | 1 | 23.0000 | 165 |
+--------+----------+------------+--------+
10 rows in set (0.00 sec)
mysql></code>查询 tb_students_info表的名字，人数，平均年龄，身高根据身高进行降序排序
<code class="language-plaintext hljs">mysql> select s.name,count(*),avg(s.age),s.height from tb_students_info s,tb_course c where s.course_id = c.id group by s.name order by s.height desc;
+--------+----------+------------+--------+
| name | count(*) | avg(s.age) | height |
+--------+----------+------------+--------+
| Henry | 2 | 24.0000 | 185 |
| Thomas | 1 | 22.0000 | 178 |
| Jim | 1 | 24.0000 | 175 |
| John | 1 | 21.0000 | 172 |
| Susan | 1 | 23.0000 | 170 |
| Lily | 1 | 22.0000 | 165 |
| Tom | 1 | 23.0000 | 165 |
| Jane | 1 | 22.0000 | 162 |
| Dany | 1 | 25.0000 | 160 |
| Green | 1 | 23.0000 | 158 |
+--------+----------+------------+--------+
10 rows in set (0.00 sec)
mysql> </code>查询班级身高最矮三个学生的名字，人数，平均年龄，身高
<code class="language-plaintext hljs">mysql> select s.name,count(*),avg(s.age),s.height from tb_students_info s,tb_course c where s.course_id = c.id group by s.name order by s.height limit 3;
+-------+----------+------------+--------+
| name | count(*) | avg(s.age) | height |
+-------+----------+------------+--------+
| Green | 1 | 23.0000 | 158 |
| Dany | 1 | 25.0000 | 160 |
| Jane | 1 | 22.0000 | 162 |
+-------+----------+------------+--------+
3 rows in set (0.00 sec)
mysql> </code>最后
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