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@ -1,370 +1,415 @@
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## SQL详解之DQL
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## 第43课:SQL详解之DQL
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接下来,我们利用之前创建的学校选课系统数据库,为大家讲解 DQL 的应用。无论对于开发人员还是数据分析师,DQL 都是非常重要的,它关系着我们能否从关系数据库中获取我们需要的数据。建议大家把上上一节课中建库建表的 DDL 以及 上一节课中插入数据的 DML 重新执行一次,确保表和数据跟没有问题再执行下面的操作。
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```SQL
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use `school`;
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-- 查询所有学生的所有信息
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-- 说明:实际工作中不建议使用 select * 的方式进行查询
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-- 01. 查询所有学生的所有信息
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select *
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from tb_student;
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-- 查询学生的学号、姓名和籍贯
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select stu_id,
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stu_name,
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stu_addr
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select stu_id
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, stu_name
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, stu_sex
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, stu_birth
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, stu_addr
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, col_id
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from tb_student;
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select stu_id as 学号,
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stu_name as 姓名,
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stu_addr as 籍贯
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-- 02. 查询学生的学号、姓名和籍贯(投影和别名)
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select stu_id as 学号
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, stu_name as 姓名
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, stu_addr as 籍贯
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from tb_student;
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-- 查询所有课程的名称及学分
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select cou_name as 课程名称,
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cou_credit as 学分
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-- 03. 查询所有课程的名称及学分(投影和别名)
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select cou_name as 课程名称
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, cou_credit as 学分
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from tb_course;
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-- 查询所有女学生的姓名和出生日期
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select stu_name,
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stu_birth
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-- 04. 查询所有女学生的姓名和出生日期(数据筛选)
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select stu_name
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, stu_birth
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from tb_student
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where stu_sex=0;
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where stu_sex = 0;
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-- 查询籍贯为“四川成都”的女学生的姓名和出生日期
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select stu_name,
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stu_birth
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-- 05. 查询籍贯为“四川成都”的女学生的姓名和出生日期(数据筛选)
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select stu_name
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, stu_birth
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from tb_student
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where stu_sex=0
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and stu_addr='四川成都';
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where stu_sex = 0 and stu_addr = '四川成都';
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-- 查询籍贯为“四川成都”或者性别是女的学生
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select stu_name,
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stu_birth
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-- 06. 查询籍贯为“四川成都”或者性别是女的学生(数据筛选)
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select stu_name
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, stu_birth
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from tb_student
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where stu_sex=0
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or stu_addr='四川成都';
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where stu_sex = 0 or stu_addr = '四川成都';
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-- 查询所有80后学生的姓名、性别和出生日期
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-- 方法一:
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select stu_name,
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stu_sex,
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stu_birth
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-- 07. 查询所有80后学生的姓名、性别和出生日期(数据筛选)
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select stu_name
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, stu_sex
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, stu_birth
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from tb_student
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where stu_birth >= '1980-1-1'
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and stu_birth <= '1989-12-31';
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where '1980-1-1' <= stu_birth and stu_birth <= '1989-12-31';
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-- 方法二:
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select stu_name,
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stu_sex,
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stu_birth
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select stu_name
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, stu_sex
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, stu_birth
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from tb_student
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where stu_birth between '1980-1-1' and '1989-12-31';
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-- 查询学分大于2的课程的名称和学分
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select cou_name,
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cou_credit
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-- MySQL方言
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select stu_name
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, if(stu_sex, '男', '女') as stu_sex
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, stu_birth
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from tb_student
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where stu_birth between '1980-1-1' and '1989-12-31';
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select stu_name
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, case stu_sex
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when 1 then '男'
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else '女'
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end as stu_sex
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, stu_birth
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from tb_student
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where stu_birth between '1980-1-1' and '1989-12-31';
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-- 08. 查询学分大于2分的课程名称和学分(数据筛选)
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select cou_name
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, cou_credit
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from tb_course
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where cou_credit > 2;
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-- 查询学分是奇数的课程的名称和学分
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select cou_name,
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cou_credit
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-- 09. 查询学分是奇数的课程的名称和学分(数据筛选)
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select cou_name
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, cou_credit
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from tb_course
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where cou_credit % 2 <> 0;
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where cou_credit mod 2 <> 0;
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-- 查询选择选了1111的课程考试成绩在90分以上的学生学号
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-- 10. 查询选择选了1111的课程考试成绩在90分以上的学生学号(数据筛选)
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select stu_id
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from tb_record
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where cou_id = 1111
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and score > 90;
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where cou_id = 1111 and score > 90;
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-- 查询姓“杨”的学生姓名和性别(模糊)
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-- % 可以匹配零个或任意多个字符
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select stu_name,
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stu_sex
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-- 11. 查询名字叫“杨过”的学生的姓名和性别
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select stu_name
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, stu_sex
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from tb_student
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where stu_name = '杨过';
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-- 12. 查询姓“杨”的学生姓名和性别(模糊查询)
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-- wild card - 通配符 - % - 代表零个或任意多个字符
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select stu_name
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, stu_sex
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from tb_student
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where stu_name like '杨%';
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-- 查询姓“杨”名字两个字的学生姓名和性别(模糊)
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-- _ 可以匹配一个字符
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select stu_name,
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stu_sex
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-- 13. 查询姓“杨”名字两个字的学生姓名和性别(模糊查询)
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-- wild card - 通配符 - _ - 精确匹配一个字符
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select stu_name
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, stu_sex
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from tb_student
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where stu_name like '杨_';
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-- 查询姓“杨”名字三个字的学生姓名和性别(模糊)
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select stu_name,
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stu_sex
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-- 14. 查询姓“杨”名字三个字的学生姓名和性别(模糊查询)
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select stu_name
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, stu_sex
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from tb_student
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where stu_name like '杨__';
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-- 查询名字中有“不”字或“嫣”字的学生的姓名(模糊)
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-- 方法一:
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select stu_name,
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stu_sex
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-- 15. 查询名字中有“不”字或“嫣”字的学生的姓名(模糊查询)
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select stu_name
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from tb_student
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where stu_name like '%不%'
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or stu_name like '%嫣%';
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where stu_name like '%不%' or stu_name like '%嫣%';
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-- 方法二:
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select stu_name,
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stu_sex
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select stu_name
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from tb_student
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where stu_name like '%不%'
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union
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select stu_name,
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stu_sex
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select stu_name
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from tb_student
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where stu_name like '%嫣%';
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-- 查询姓“杨”或姓“林”名字三个字的学生的姓名
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select stu_name,
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stu_sex
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from tb_student
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where stu_name regexp '^[杨林][\\u4e00-\\u9fa5]{2}$';
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update tb_student
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set stu_name = '岳不嫣'
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where stu_id = 1572;
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-- 查询没有录入籍贯的学生姓名
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select stu_name
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from tb_student
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where stu_addr = ''
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or stu_addr is null;
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-- 查询录入了籍贯的学生姓名
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where stu_name like '%不%'
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union all
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select stu_name
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from tb_student
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where stu_addr <> ''
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and stu_addr is not null;
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where stu_name like '%嫣%';
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-- 查询学生选课的所有日期
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-- 16. 查询姓“杨”或姓“林”名字三个字的学生的姓名(正则表达式模糊查询)
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-- regular expression
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select stu_name
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from tb_student
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where stu_name regexp '[杨林][\\u4e00-\\u9fa5]{2}';
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-- 17. 查询没有录入籍贯的学生姓名(空值处理)
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select stu_name
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from tb_student
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where stu_addr is null or trim(stu_addr) = '';
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update tb_student
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set stu_addr = ' '
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where stu_id = 1572;
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-- 18. 查询录入了籍贯的学生姓名(空值处理)
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select stu_name
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from tb_student
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where stu_addr is not null and trim(stu_addr) <> '';
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-- 19. 查询学生选课的所有日期(去重)
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select distinct sel_date
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from tb_record;
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-- 查询学生的籍贯
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-- 20. 查询学生的籍贯(空值处理和去重)
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select distinct stu_addr
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from tb_student
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where stu_addr <> ''
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and stu_addr is not null;
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where stu_addr is not null and trim(stu_addr) <> '';
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-- 查询学院编号为1的学生姓名、性别和生日按年龄从大到小排列
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-- asc - 升序(从小到大,默认),desc - 降序(从大到小)
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select stu_name,
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stu_sex,
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stu_birth
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-- 21. 查询男学生的姓名和生日按年龄从大到小排列(排序)
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-- ascending / descending
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select stu_name
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, stu_birth
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from tb_student
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where col_id = 1
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order by stu_sex asc,
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stu_birth asc;
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where stu_sex = 1
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order by stu_birth asc;
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-- 补充:将上面的性别处理成“男”或“女”,将生日换算成年龄
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select stu_name as 姓名,
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if(stu_sex, '男', '女') as 性别,
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floor(datediff(curdate(), stu_birth) / 365) as 年龄
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-- 22. 将上面查询中的生日换算成年龄(日期函数、数值函数)
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-- 获取当前日期:curdate()
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-- 计算时间差:timestampdiff(unit, date1, date2)
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select stu_name
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, timestampdiff(year, stu_birth, curdate()) as stu_age
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from tb_student
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where col_id = 1
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order by stu_sex asc,
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年龄 desc;
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where stu_sex = 1
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order by stu_age desc;
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-- 查询年龄最大的学生的出生日期
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-- 聚合函数:max / min / avg / sum / count / std / variance
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-- 聚合函数会自动忽略掉null
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-- 23. 查询年龄最大的学生的出生日期(聚合函数)
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select min(stu_birth)
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from tb_student;
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|
|
|
-- 查询年龄最小的学生的出生日期
|
|
|
|
|
-- 24. 查询年龄最小的学生的出生日期(聚合函数)
|
|
|
|
|
select max(stu_birth)
|
|
|
|
|
from tb_student;
|
|
|
|
|
|
|
|
|
|
-- 查询学号为1001的学生一共选了几门课
|
|
|
|
|
select count(*)
|
|
|
|
|
-- 25. 查询编号为1111的课程考试成绩的最高分(聚合函数)
|
|
|
|
|
select max(score)
|
|
|
|
|
from tb_record
|
|
|
|
|
where cou_id = 1111;
|
|
|
|
|
|
|
|
|
|
-- 26. 查询学号为1001的学生考试成绩的最低分(聚合函数)
|
|
|
|
|
select min(score)
|
|
|
|
|
from tb_record
|
|
|
|
|
where stu_id = 1001;
|
|
|
|
|
|
|
|
|
|
-- 查询学号为1001的学生考试成绩的平均分
|
|
|
|
|
select round(avg(score), 1) as 平均分
|
|
|
|
|
-- 27. 查询学号为1001的学生考试成绩的平均分和标准差(聚合函数)
|
|
|
|
|
-- 四舍五入函数:round(num, n)
|
|
|
|
|
select round(avg(score), 1) as avg_score
|
|
|
|
|
, round(std(score), 4) as std_score
|
|
|
|
|
from tb_record
|
|
|
|
|
where stu_id = 1001;
|
|
|
|
|
|
|
|
|
|
-- 查询学号为1001的学生考试成绩的平均分,如果有null值,null值算0分
|
|
|
|
|
-- 方法一:
|
|
|
|
|
-- 28. 查询学号为1001的学生考试成绩的平均分,如果有null值,null值算0分(聚合函数)
|
|
|
|
|
select sum(score) / count(*)
|
|
|
|
|
from tb_record
|
|
|
|
|
where stu_id = 1001;
|
|
|
|
|
|
|
|
|
|
-- 方法二:
|
|
|
|
|
select avg(coalesce(score, 0))
|
|
|
|
|
from tb_record
|
|
|
|
|
where stu_id = 1001;
|
|
|
|
|
|
|
|
|
|
-- 查询学号为1001的学生考试成绩的标准差(聚合函数)
|
|
|
|
|
select stddev_samp(score)
|
|
|
|
|
from tb_record
|
|
|
|
|
where stu_id = 1001;
|
|
|
|
|
|
|
|
|
|
-- 查询男女学生的人数
|
|
|
|
|
select case stu_sex when 1 then '男' else '女' end as 性别,
|
|
|
|
|
count(*) as 人数
|
|
|
|
|
-- 29. 查询男女学生的人数(分组和聚合函数)
|
|
|
|
|
select case stu_sex when 1 then '男' else '女' end as stu_sex
|
|
|
|
|
, count(*) as total
|
|
|
|
|
from tb_student
|
|
|
|
|
group by stu_sex;
|
|
|
|
|
group by stu_sex;
|
|
|
|
|
|
|
|
|
|
-- 查询每个学院学生人数
|
|
|
|
|
select col_id as 学院编号,
|
|
|
|
|
count(*) as 人数
|
|
|
|
|
-- 30. 查询每个学院学生人数(分组和聚合函数)
|
|
|
|
|
select col_id
|
|
|
|
|
, count(*) as total
|
|
|
|
|
from tb_student
|
|
|
|
|
group by col_id;
|
|
|
|
|
group by col_id
|
|
|
|
|
with rollup;
|
|
|
|
|
|
|
|
|
|
-- 查询每个学院男女学生人数
|
|
|
|
|
select col_id as 学院编号,
|
|
|
|
|
case stu_sex when 1 then '男' else '女' end as 性别,
|
|
|
|
|
count(*) as 人数
|
|
|
|
|
-- 31. 查询每个学院男女学生人数(分组和聚合函数)
|
|
|
|
|
select col_id
|
|
|
|
|
, case stu_sex when 1 then '男' else '女' end as stu_sex
|
|
|
|
|
, count(*) as total
|
|
|
|
|
from tb_student
|
|
|
|
|
group by col_id, stu_sex;
|
|
|
|
|
group by col_id, stu_sex;
|
|
|
|
|
|
|
|
|
|
-- 查询每个学生的学号和平均成绩
|
|
|
|
|
select stu_id as 学号,
|
|
|
|
|
round(avg(score), 2) as 平均成绩
|
|
|
|
|
-- 32. 查询选课学生的学号和平均成绩(分组和聚合函数)
|
|
|
|
|
select stu_id
|
|
|
|
|
, round(avg(score), 1) as avg_score
|
|
|
|
|
from tb_record
|
|
|
|
|
group by stu_id;
|
|
|
|
|
group by stu_id;
|
|
|
|
|
|
|
|
|
|
-- 查询平均成绩大于等于90分的学生的学号和平均成绩
|
|
|
|
|
-- 方法一:
|
|
|
|
|
select stu_id as 学号,
|
|
|
|
|
round(avg(score), 2) as 平均成绩
|
|
|
|
|
-- 33. 查询平均成绩大于等于90分的学生的学号和平均成绩(分组和聚合函数)
|
|
|
|
|
-- 结论:分组前的筛选使用where子句,分组后的筛选使用having子句
|
|
|
|
|
select stu_id
|
|
|
|
|
, round(avg(score), 1) as avg_score
|
|
|
|
|
from tb_record
|
|
|
|
|
group by stu_id
|
|
|
|
|
having 平均成绩 >= 90;
|
|
|
|
|
group by stu_id
|
|
|
|
|
having avg(score) >= 90;
|
|
|
|
|
|
|
|
|
|
-- 方法二:
|
|
|
|
|
select *
|
|
|
|
|
from ( select stu_id as 学号,
|
|
|
|
|
round(avg(score), 2) as 平均成绩
|
|
|
|
|
-- 34. 查询所有课程成绩大于80分的同学的学号(分组和聚合函数)
|
|
|
|
|
select stu_id
|
|
|
|
|
from tb_record
|
|
|
|
|
group by stu_id) as t
|
|
|
|
|
where 平均成绩 >= 90;
|
|
|
|
|
group by stu_id
|
|
|
|
|
having min(score) > 80;
|
|
|
|
|
|
|
|
|
|
-- 查询1111、2222、3333三门课程平均成绩大于等于90分的学生的学号和平均成绩
|
|
|
|
|
select stu_id as 学号,
|
|
|
|
|
round(avg(score), 2) as 平均成绩
|
|
|
|
|
-- Error Code: 1242. Subquery returns more than 1 row
|
|
|
|
|
select stu_id
|
|
|
|
|
, stu_name
|
|
|
|
|
from tb_student
|
|
|
|
|
where stu_id in (select stu_id
|
|
|
|
|
from tb_record
|
|
|
|
|
where cou_id in (1111, 2222, 3333)
|
|
|
|
|
group by stu_id
|
|
|
|
|
having avg(score) >= 90;
|
|
|
|
|
group by stu_id
|
|
|
|
|
having min(score) > 80);
|
|
|
|
|
|
|
|
|
|
-- 35. 查询年龄最大的学生的姓名(嵌套查询)
|
|
|
|
|
-- 嵌套查询/子查询:把一个查询的结果作为另外一个查询的一部分来使用
|
|
|
|
|
select @a := min(stu_birth)
|
|
|
|
|
from tb_student;
|
|
|
|
|
|
|
|
|
|
select stu_name
|
|
|
|
|
from tb_student
|
|
|
|
|
where stu_birth = @a;
|
|
|
|
|
|
|
|
|
|
-- 嵌套查询:把一个查询的结果作为另外一个查询的一部分来使用
|
|
|
|
|
select stu_name
|
|
|
|
|
from tb_student
|
|
|
|
|
where stu_birth = (select min(stu_birth)
|
|
|
|
|
from tb_student);
|
|
|
|
|
|
|
|
|
|
-- 查询选了两门以上的课程的学生姓名
|
|
|
|
|
-- 36. 查询选了两门以上的课程的学生姓名(嵌套查询/分组/数据筛选)
|
|
|
|
|
select stu_name
|
|
|
|
|
from tb_student
|
|
|
|
|
where stu_id in ( select stu_id
|
|
|
|
|
where stu_id in (select stu_id
|
|
|
|
|
from tb_record
|
|
|
|
|
group by stu_id
|
|
|
|
|
having count(*) > 2);
|
|
|
|
|
|
|
|
|
|
-- 查询学生的姓名、生日和所在学院名称
|
|
|
|
|
-- 方法一:
|
|
|
|
|
select stu_name,
|
|
|
|
|
stu_birth,
|
|
|
|
|
col_name
|
|
|
|
|
-- 37. 查询学生的姓名、生日和所在学院名称(连接查询)
|
|
|
|
|
select stu_name
|
|
|
|
|
, stu_birth
|
|
|
|
|
, col_name
|
|
|
|
|
from tb_student, tb_college
|
|
|
|
|
where tb_student.col_id = tb_college.col_id;
|
|
|
|
|
|
|
|
|
|
-- 方法二:
|
|
|
|
|
select stu_name,
|
|
|
|
|
stu_birth,
|
|
|
|
|
col_name
|
|
|
|
|
select stu_name
|
|
|
|
|
, stu_birth
|
|
|
|
|
, col_name
|
|
|
|
|
from tb_student inner join tb_college
|
|
|
|
|
on tb_student.col_id = tb_college.col_id;
|
|
|
|
|
|
|
|
|
|
-- 方法三:
|
|
|
|
|
select stu_name,
|
|
|
|
|
stu_birth,
|
|
|
|
|
col_name
|
|
|
|
|
select stu_name
|
|
|
|
|
, stu_birth
|
|
|
|
|
, col_name
|
|
|
|
|
from tb_student natural join tb_college;
|
|
|
|
|
|
|
|
|
|
-- 查询学生姓名、课程名称以及成绩
|
|
|
|
|
-- 方法一:
|
|
|
|
|
select stu_name,
|
|
|
|
|
cou_name,
|
|
|
|
|
score
|
|
|
|
|
-- 38. 查询学生姓名、课程名称以及成绩(连接查询)
|
|
|
|
|
select stu_name
|
|
|
|
|
, cou_name
|
|
|
|
|
, score
|
|
|
|
|
from tb_student, tb_course, tb_record
|
|
|
|
|
where tb_student.stu_id = tb_record.stu_id
|
|
|
|
|
and tb_course.cou_id = tb_record.cou_id
|
|
|
|
|
and score is not null;
|
|
|
|
|
|
|
|
|
|
-- 方法二:
|
|
|
|
|
select stu_name,
|
|
|
|
|
cou_name,
|
|
|
|
|
score
|
|
|
|
|
from tb_student inner join tb_record inner join tb_course
|
|
|
|
|
select stu_name
|
|
|
|
|
, cou_name
|
|
|
|
|
, score
|
|
|
|
|
from tb_student
|
|
|
|
|
inner join tb_record
|
|
|
|
|
on tb_student.stu_id = tb_record.stu_id
|
|
|
|
|
and tb_course.cou_id = tb_record.cou_id
|
|
|
|
|
inner join tb_course
|
|
|
|
|
on tb_course.cou_id = tb_record.cou_id
|
|
|
|
|
where score is not null;
|
|
|
|
|
|
|
|
|
|
-- 方法三:
|
|
|
|
|
select stu_name,
|
|
|
|
|
cou_name,
|
|
|
|
|
score
|
|
|
|
|
from tb_student natural join tb_record natural join tb_course
|
|
|
|
|
select stu_name
|
|
|
|
|
, cou_name
|
|
|
|
|
, score
|
|
|
|
|
from tb_student
|
|
|
|
|
natural join tb_record
|
|
|
|
|
natural join tb_course
|
|
|
|
|
where score is not null;
|
|
|
|
|
|
|
|
|
|
-- 补充:上面的查询结果取前5条数据
|
|
|
|
|
select stu_name,
|
|
|
|
|
cou_name,
|
|
|
|
|
score
|
|
|
|
|
from tb_student natural join tb_record natural join tb_course
|
|
|
|
|
-- 39. 上面的查询结果按课程和成绩排序取前5条数据(分页查询)
|
|
|
|
|
select stu_name
|
|
|
|
|
, cou_name
|
|
|
|
|
, score
|
|
|
|
|
from tb_student
|
|
|
|
|
natural join tb_record
|
|
|
|
|
natural join tb_course
|
|
|
|
|
where score is not null
|
|
|
|
|
order by stu_id asc, score desc
|
|
|
|
|
order by cou_id asc, score desc
|
|
|
|
|
limit 5;
|
|
|
|
|
|
|
|
|
|
-- 补充:上面的查询结果取第6-10条数据
|
|
|
|
|
select stu_name,
|
|
|
|
|
cou_name,
|
|
|
|
|
score
|
|
|
|
|
from tb_student inner join tb_record inner join tb_course
|
|
|
|
|
on tb_student.stu_id = tb_record.stu_id
|
|
|
|
|
and tb_course.cou_id = tb_record.cou_id
|
|
|
|
|
order by stu_id asc, score desc
|
|
|
|
|
limit 5
|
|
|
|
|
offset 5;
|
|
|
|
|
|
|
|
|
|
-- 补充:上面的查询结果取第11-15条数据
|
|
|
|
|
select stu_name,
|
|
|
|
|
cou_name,
|
|
|
|
|
score
|
|
|
|
|
from tb_student natural join tb_record natural join tb_course
|
|
|
|
|
-- 40. 上面的查询结果按课程和成绩排序取第6-10条数据(分页查询)
|
|
|
|
|
select stu_name
|
|
|
|
|
, cou_name
|
|
|
|
|
, score
|
|
|
|
|
from tb_student
|
|
|
|
|
natural join tb_record
|
|
|
|
|
natural join tb_course
|
|
|
|
|
where score is not null
|
|
|
|
|
order by stu_id asc, score desc
|
|
|
|
|
order by cou_id asc, score desc
|
|
|
|
|
limit 5
|
|
|
|
|
offset 10;
|
|
|
|
|
offset 5;
|
|
|
|
|
|
|
|
|
|
-- 查询选课学生的姓名和平均成绩
|
|
|
|
|
select stu_name,
|
|
|
|
|
avg_score
|
|
|
|
|
from tb_student t1 inner join ( select stu_id,
|
|
|
|
|
round(avg(score), 2) as avg_score
|
|
|
|
|
from tb_record
|
|
|
|
|
group by stu_id) t2
|
|
|
|
|
on t1.stu_id = t2.stu_id;
|
|
|
|
|
-- 41. 上面的查询结果按课程和成绩排序取第11-15条数据(分页查询)
|
|
|
|
|
select stu_name
|
|
|
|
|
, cou_name
|
|
|
|
|
, score
|
|
|
|
|
from tb_student
|
|
|
|
|
natural join tb_record
|
|
|
|
|
natural join tb_course
|
|
|
|
|
where score is not null
|
|
|
|
|
order by cou_id asc, score desc
|
|
|
|
|
limit 5
|
|
|
|
|
offset 10;
|
|
|
|
|
|
|
|
|
|
-- 查询学生的姓名和选课的数量
|
|
|
|
|
select stu_name,
|
|
|
|
|
total
|
|
|
|
|
from tb_student t1 natural join ( select stu_id,
|
|
|
|
|
count(*) as total
|
|
|
|
|
-- 42. 查询选课学生的姓名和平均成绩(嵌套查询和连接查询)
|
|
|
|
|
select stu_name
|
|
|
|
|
, avg_score
|
|
|
|
|
from tb_student
|
|
|
|
|
natural join (select stu_id
|
|
|
|
|
, avg(score) as avg_score
|
|
|
|
|
from tb_record
|
|
|
|
|
group by stu_id) t2;
|
|
|
|
|
group by stu_id) as tmp;
|
|
|
|
|
|
|
|
|
|
-- 查询每个学生的姓名和选课数量(左外连接和子查询)
|
|
|
|
|
select stu_name as 姓名,
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coalesce (total, 0) as 选课数量
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from tb_student t1 left join ( select stu_id,
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count(*) as total
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-- 43. 查询学生的姓名和选课的数量(嵌套查询和连接查询)
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select stu_name
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, total
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from tb_student
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inner join (select stu_id
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, count(*) as total
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from tb_record
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group by stu_id) t2
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on t1.stu_id = t2.stu_id;
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group by stu_id) as tmp
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on tb_student.stu_id = tmp.stu_id;
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-- 44. 查询所有学生的姓名和选课数量(左外连接和嵌套查询)
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-- 左外连接:把左表(写在join左边的表)所有的数据都拿到,不满足连表条件的地方填充null - left outer join
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-- 右外连接:把右表(写在join右边的表)所有的数据都拿到,不满足连表条件的地方填充null - right outer join
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-- 全外连接:把左表和右表的数据全部拿到即便它们不满足连表条件,MySQL不支持全外连接 - full outer join
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select stu_name
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, coalesce(total, 0) as total
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from tb_student
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|
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left join (select stu_id
|
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|
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, count(*) as total
|
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from tb_record
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group by stu_id) as tmp
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on tb_student.stu_id = tmp.stu_id;
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-- 45. 查询没有选课的学生的姓名(左外连接和数据筛选)
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select stu_name
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from tb_student
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left join tb_record
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on tb_student.stu_id = tb_record.stu_id
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where tb_record.stu_id is null;
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```
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上面的 DQL 有几个地方需要加以说明:
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