MySQL多表数据记录查询（二）

（1）子查询概念

子查询也称嵌套查询，就是指一个查询之中嵌套了其他的若干查询，即在一个select语句中的where或者from字句中包含一个select查询语句。在查询语句中，外层select查询语句称为主查询，where字句中的select查询语句被称为子查询。

（2）联合查询概念

使用union关键字可以把来自多个select语句的结果组合到一个结果集中，这种查询方式称为并(union)运算或联合查询。合并时，多个select字句中对应的字段数和数据类型必须相同。

1.1、带in关键字的子查询：

带in关键字的子查询是最常用的一类子查询，1)用于判定一个给定值是否存在于子查询的结果集中。2)使用in关键字进行子查询时，内层查询语句仅仅返回一个数据列，其值将提供给外层查询进行比较操作；

1：查询选修了课程的学生姓名；(in查询)

select a.studentName

    -> from tb_student as a

    -> where a.studentNo in

-> (select distinct tb_score.studentNo from tb_score);

先查内层：

select distinct tb_score.studentNo from tb_score;

再外层查询；

练习：

例12:P96

查询选修了"计算机基础"和"管理学"的学生学号;

select tb_score.studentNo

     from tb_score,tb_course

     where tb_score.courseNo=tb_course.courseNo and tb_course.courseName="计算机基础"

     and tb_score.studentNo in

     (select studentNo from tb_score,tb_course

     where tb_score.courseNo=tb_course.courseNo and tb_course.courseName="管理学");

2:P89(not in查询)

查询没有选修过课程的学生学号和学生姓名；

 select a.studentNo,a.studentName

    -> from tb_student as a

    -> where a.studentNo not in

    -> (select distinct tb_score.studentNo from tb_score);

结果：林海

练习;

查询选修了"计算机基础"但是没有选修"管理学"的学生学号；

select tb_score.studentNo

     from tb_score,tb_course

     where tb_score.courseNo=tb_course.courseNo and tb_course.courseName="计算机基础"

     and tb_score.studentNo not in

     (select studentNo from tb_score,tb_course

     where tb_score.courseNo=tb_course.courseNo and tb_course.courseName="管理学");

1.2、带比较运算符的子查询：

带比较运算符的子查询是指1)父查询与子查询之间用比较运算符进行连接。2)当用户能确切知道内层返回的是单值时，3)可以用<,<=,>,>=,=,<>,!=等比较运算符构造子查询；

3:查询班级"计算机14-1班"所有学生的学号、姓名；

select a.studentNo,a.studentName

    -> from tb_student as a

    -> where a.classNo=

-> (select tb_class.ClassNo from tb_class where ClassName="计算机14-1班");

先进行内层查询：

select tb_class.ClassNo from tb_class where ClassName="计算机14-1班";

结果：CS1401

再外层查询：

select a.studentNo,a.studentName

    -> from tb_student as a

-> where a.classNo="CS1401";

思考：

使用连接查询怎么查？

select a.studentNo,a.studentName

    -> from tb_student as a,tb_class as b

    -> where a.classNo=b.ClassNo

-> and b.ClassName="计算机14-1班";

4:查询与"李明"在同一个班学习的学生学号、姓名、班号；(tb_student)

select a.studentNo,a.studentName,a.classNo

    -> from tb_student as a

    -> where a.classNo=

    -> (select b.classNo from tb_student as b where b.studentName="李明") and a.studentName!="李明";

思考：该查询也是自连接查询，连接语句怎么写？

select a.studentNo,a.studentName,a.classNo

    -> from tb_student as a,tb_student as b

    -> where a.classNo=b.classNo

-> and b.studentName="李明" and a.studentName!="李明";

any比较：

例5:P91,//any比较

查询男生中比某个女生出生晚的学生姓名和出生年份：

select a.studentName,year(birthday)

-> from tb_student as a

-> where a.sex="男" and year(birthday)>any

-> (select year(birthday) from tb_student where sex="女");

用聚合函数语句：min

 select a.studentName,year(birthday)

-> from tb_student as a

-> where a.sex="男" and year(birthday)>

-> (select min(year(birthday)) from tb_student where sex="女" );

6:,//all比较：

查询男生中比所有女生出生晚的学生姓名和出生年份：

select a.studentName,year(a.birthday)

-> from tb_student as a

-> where a.sex="男" and year(a.birthday)>all

-> (select year(birthday) from tb_student where sex="女");

用聚合函数写：max

 select a.studentName,year(birthday)

-> from tb_student as a

-> where a.sex="男" and year(birthday)>
-> (select max(year(birthday)) from tb_student where sex="女" );

1.3、带exists关键字的子查询：

使用关键字exists构造子查询时，1)系统对子查询进行运算判断它是否返回结果集。2)由于带exists的子查询只返回true或false，内层查询的select子句给出的字段名没有实际意义，所以目标列表达式通常用星号*。

7:P92,关键字exists查询

查询选修了课程号为"31002"的学生姓名；

 select a.studentName

    -> from tb_student as a

    -> where exists

    -> (select * from tb_score as b where a.studentNo=b.studentNo and b.courseNo="31002");

用in查询：

 select a.studentName

-> from tb_student as a

-> where a.studentNo in

-> (select studentNo from tb_score where courseNo="31002");

8:P93,//关键字not exists查询

查询没有选修课程号为"31002"的学生姓名；

 select a.studentName

    -> from tb_student as a

    -> where not exists

    -> (select * from tb_score as b where a.studentNo=b.studentNo and b.courseNo="31002");

用in查询：

 select a.studentName

    -> from tb_student as a

    -> where a.studentNo not in

    -> (select studentNo from tb_score where courseNo="31002");

9：exists成对使用，双重否定表示肯定

查询选修了全部课程的学生姓名；

思路：

查询这样一个学生，没有一门课是他没有选修的；

select a.studentName

    -> from tb_student as a

    -> where not exists

    -> (select * from tb_course as b

    -> where not exists

    -> (select * from tb_score as c

-> where c.courseNo=b.courseNo and a.studentNo=c.studentNo));

结果：没有这样的学生；empty set；

看一共有几门课：(以课程分组)

select tb_score.courseNo,count(*)

    -> from tb_score

    -> group by courseNo;

+----------+----------+

| courseNo | count(*) |

+----------+----------+

|    11003 |        4 |

|    21001 |        6 |

|    21002 |        2 |

|    21004 |        6 |

|    31002 |        2 |

|    44444 |        1 |

+----------+----------+

6 rows in set (0.06 sec)

 select tb_score.studentNo,count(courseNo)

    -> from tb_score

    -> group by studentNo;

+------------+-----------------+

| studentNo  | count(courseNo) |

+------------+-----------------+

| 2013110101 |               2 |

| 2013110103 |               2 |

| 2013110201 |               2 |

| 2013110202 |               2 |

| 2013310101 |               2 |

| 2013310103 |               2 |

| 2014210101 |               2 |

| 2014210102 |               2 |

| 2014310101 |               2 |

| 2014310102 |               2 |

+------------+-----------------+

10 rows in set (0.00 sec)

联合查询：

1)使用union关键字可以把多个来自select语句的结果组合到一个结果集中，这种查询方式称为并(union)运算或联合查询。

2)合并时，多个select字句中对应的字段数和数据类型必须相同。

3)语法：

Select-from-where

Union[all]

Select-from-where

[...union[all]]

Select-from-where

Union查询：

10:union用法,unoin去重了；

使用union查询选修了"管理学"或"计算机基础"的学生学号；

 select tb_score.studentNo

    -> from tb_score,tb_course

    -> where tb_score.courseNo=tb_course.courseNo and tb_course.courseName="计

算机基础"

    -> union

    -> select tb_score.studentNo

    -> from tb_score,tb_course

    -> where tb_score.courseNo=tb_course.courseNo and tb_course.courseName="管

理学";

用连接语句查询：使用distinct去重

 select distinct tb_score.studentNo

    -> from tb_score,tb_course

    -> where tb_score.courseNo=tb_course.courseNo

-> and (tb_course.courseName="管理学" or tb_course.courseName="计算机基础");

Union all查询：

11:union all

使用union all查询选修了"管理学"或"计算机基础"的学生学号；

 select tb_score.studentNo

    -> from tb_score,tb_course

    -> where tb_score.courseNo=tb_course.courseNo and tb_course.courseName="计

算机基础"

    -> union all

    -> select tb_score.studentNo

    -> from tb_score,tb_course

    -> where tb_score.courseNo=tb_course.courseNo and tb_course.courseName="管

理学";

用连接语句查询://不用distinct去重；

 select tb_score.studentNo

    -> from tb_score,tb_course

    -> where tb_score.courseNo=tb_course.courseNo

    -> and (tb_course.courseName="管理学" or tb_course.courseName="计算机基础");