491 lines
17 KiB
Markdown
491 lines
17 KiB
Markdown
## 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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-- 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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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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-- 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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-- 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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-- 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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-- 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 and stu_addr = '四川成都';
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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 or stu_addr = '四川成都';
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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 '1980-1-1' <= stu_birth and stu_birth <= '1989-12-31';
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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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-- 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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-- 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 mod 2 <> 0;
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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 and score > 90;
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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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-- 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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-- 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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-- 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%' or stu_name like '%嫣%';
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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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from tb_student
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where stu_name like '%嫣%';
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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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select stu_name
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from tb_student
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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_name like '%嫣%';
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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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-- 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 is not null and trim(stu_addr) <> '';
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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 stu_sex = 1
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order by stu_birth asc;
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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 stu_sex = 1
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order by stu_age desc;
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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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-- 24. 查询年龄最小的学生的出生日期(聚合函数)
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select max(stu_birth)
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from tb_student;
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-- 25. 查询编号为1111的课程考试成绩的最高分(聚合函数)
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select max(score)
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from tb_record
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where cou_id = 1111;
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-- 26. 查询学号为1001的学生考试成绩的最低分(聚合函数)
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select min(score)
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from tb_record
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where stu_id = 1001;
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-- 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)
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-- 四舍五入函数：round(num, n)
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select round(avg(score), 1) as avg_score
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, round(std(score), 4) as std_score
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from tb_record
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where stu_id = 1001;
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-- 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)
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select sum(score) / count(*)
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from tb_record
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where stu_id = 1001;
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-- 29. 查询男女学生的人数(分组和聚合函数)
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select case stu_sex when 1 then '男' else '女' end as stu_sex
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, count(*) as total
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from tb_student
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group by stu_sex;
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-- 30. 查询每个学院学生人数(分组和聚合函数)
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select col_id
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, count(*) as total
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from tb_student
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group by col_id
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with rollup;
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-- 31. 查询每个学院男女学生人数(分组和聚合函数)
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select col_id
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, case stu_sex when 1 then '男' else '女' end as stu_sex
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, count(*) as total
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from tb_student
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group by col_id, stu_sex;
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-- 32. 查询选课学生的学号和平均成绩(分组和聚合函数)
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select stu_id
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, round(avg(score), 1) as avg_score
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from tb_record
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group by stu_id;
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-- 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)
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-- 结论：分组前的筛选使用where子句，分组后的筛选使用having子句
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select stu_id
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, round(avg(score), 1) as avg_score
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from tb_record
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group by stu_id
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having avg(score) >= 90;
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-- 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)
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select stu_id
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from tb_record
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group by stu_id
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having min(score) > 80;
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-- Error Code: 1242. Subquery returns more than 1 row
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select stu_id
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, stu_name
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from tb_student
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where stu_id in (select stu_id
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from tb_record
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group by stu_id
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having min(score) > 80);
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-- 35. 查询年龄最大的学生的姓名(嵌套查询)
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-- 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
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select @a := min(stu_birth)
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from tb_student;
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select stu_name
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from tb_student
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where stu_birth = @a;
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select stu_name
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from tb_student
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where stu_birth = (select min(stu_birth)
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from tb_student);
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-- 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)
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select stu_name
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from tb_student
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where stu_id in (select stu_id
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from tb_record
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group by stu_id
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having count(*) > 2);
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-- 37. 查询学生的姓名、生日和所在学院名称(连接查询)
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select stu_name
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, stu_birth
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, col_name
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from tb_student, tb_college
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where tb_student.col_id = tb_college.col_id;
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select stu_name
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, stu_birth
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, col_name
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from tb_student inner join tb_college
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on tb_student.col_id = tb_college.col_id;
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select stu_name
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, stu_birth
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, col_name
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from tb_student natural join tb_college;
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-- 38. 查询学生姓名、课程名称以及成绩(连接查询)
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select stu_name
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, cou_name
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, score
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from tb_student, tb_course, tb_record
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where tb_student.stu_id = tb_record.stu_id
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and tb_course.cou_id = tb_record.cou_id
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and score is not null;
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select stu_name
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, cou_name
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, score
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from tb_student
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inner join tb_record
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on tb_student.stu_id = tb_record.stu_id
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inner join tb_course
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on tb_course.cou_id = tb_record.cou_id
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where score is not null;
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select stu_name
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, cou_name
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, score
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from tb_student
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natural join tb_record
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natural join tb_course
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where score is not null;
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-- 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)
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select stu_name
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, cou_name
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, score
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from tb_student
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natural join tb_record
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natural join tb_course
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where score is not null
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order by cou_id asc, score desc
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limit 5;
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-- 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)
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select stu_name
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, cou_name
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, score
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from tb_student
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natural join tb_record
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natural join tb_course
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where score is not null
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order by cou_id asc, score desc
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limit 5
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offset 5;
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-- 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)
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select stu_name
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, cou_name
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, score
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from tb_student
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natural join tb_record
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natural join tb_course
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where score is not null
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order by cou_id asc, score desc
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limit 5
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offset 10;
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-- 42. 查询选课学生的姓名和平均成绩(嵌套查询和连接查询)
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select stu_name
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, avg_score
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from tb_student
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natural join (select stu_id
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, avg(score) as avg_score
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from tb_record
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group by stu_id) as tmp;
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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) 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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left 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) 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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1. MySQL目前的版本不支持全外连接，上面我们通过`union`操作，将左外连接和右外连接的结果求并集实现全外连接的效果。大家可以通过下面的图来加深对连表操作的认识。
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<img src="http://localhost/mypic/20211121135117.png" style="zoom:50%">
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2. MySQL 中支持多种类型的运算符，包括：算术运算符（`+`、`-`、`*`、`/`、`%`）、比较运算符（`=`、`<>`、`<=>`、`<`、`<=`、`>`、`>=`、`BETWEEN...AND..`.、`IN`、`IS NULL`、`IS NOT NULL`、`LIKE`、`RLIKE`、`REGEXP`）、逻辑运算符（`NOT`、`AND`、`OR`、`XOR`）和位运算符（`&`、`|`、`^`、`~`、`>>`、`<<`），我们可以在 DML 中使用这些运算符处理数据。
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3. 在查询数据时，可以在`SELECT`语句及其子句（如`WHERE`子句、`ORDER BY`子句、`HAVING`子句等）中使用函数，这些函数包括字符串函数、数值函数、时间日期函数、流程函数等，如下面的表格所示。
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常用字符串函数。
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| 函数 | 功能 |
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| --------------------------- | ----------------------------------------------------- |
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| `CONCAT` | 将多个字符串连接成一个字符串 |
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| `FORMAT` | 将数值格式化成字符串并指定保留几位小数 |
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| `FROM_BASE64` / `TO_BASE64` | BASE64解码/编码 |
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| `BIN` / `OCT` / `HEX` | 将数值转换成二进制/八进制/十六进制字符串 |
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| `LOCATE` | 在字符串中查找一个子串的位置 |
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| `LEFT` / `RIGHT` | 返回一个字符串左边/右边指定长度的字符 |
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| `LENGTH` / `CHAR_LENGTH` | 返回字符串的长度以字节/字符为单位 |
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| `LOWER` / `UPPER` | 返回字符串的小写/大写形式 |
|
||
| `LPAD` / `RPAD` | 如果字符串的长度不足，在字符串左边/右边填充指定的字符 |
|
||
| `LTRIM` / `RTRIM` | 去掉字符串前面/后面的空格 |
|
||
| `ORD` / `CHAR` | 返回字符对应的编码/返回编码对应的字符 |
|
||
| `STRCMP` | 比较字符串，返回-1、0、1分别表示小于、等于、大于 |
|
||
| `SUBSTRING` | 返回字符串指定范围的子串 |
|
||
|
||
常用数值函数。
|
||
|
||
| 函数 | 功能 |
|
||
| -------------------------------------------------------- | ---------------------------------- |
|
||
| `ABS` | 返回一个数的绝度值 |
|
||
| `CEILING` / `FLOOR` | 返回一个数上取整/下取整的结果 |
|
||
| `CONV` | 将一个数从一种进制转换成另一种进制 |
|
||
| `CRC32` | 计算循环冗余校验码 |
|
||
| `EXP` / `LOG` / `LOG2` / `LOG10` | 计算指数/对数 |
|
||
| `POW` | 求幂 |
|
||
| `RAND` | 返回[0,1)范围的随机数 |
|
||
| `ROUND` | 返回一个数四舍五入后的结果 |
|
||
| `SQRT` | 返回一个数的平方根 |
|
||
| `TRUNCATE` | 截断一个数到指定的精度 |
|
||
| `SIN` / `COS` / `TAN` / `COT` / `ASIN` / `ACOS` / `ATAN` | 三角函数 |
|
||
|
||
常用时间日期函数。
|
||
|
||
| 函数 | 功能 |
|
||
| ----------------------------- | ------------------------------------- |
|
||
| `CURDATE` / `CURTIME` / `NOW` | 获取当前日期/时间/日期和时间 |
|
||
| `ADDDATE` / `SUBDATE` | 将两个日期表达式相加/相减并返回结果 |
|
||
| `DATE` / `TIME` | 从字符串中获取日期/时间 |
|
||
| `YEAR` / `MONTH` / `DAY` | 从日期中获取年/月/日 |
|
||
| `HOUR` / `MINUTE` / `SECOND` | 从时间中获取时/分/秒 |
|
||
| `DATEDIFF` / `TIMEDIFF` | 返回两个时间日期表达式相差多少天/小时 |
|
||
| `MAKEDATE` / `MAKETIME` | 制造一个日期/时间 |
|
||
|
||
常用流程函数。
|
||
|
||
| 函数 | 功能 |
|
||
| -------- | ------------------------------------------------ |
|
||
| `IF` | 根据条件是否成立返回不同的值 |
|
||
| `IFNULL` | 如果为NULL则返回指定的值否则就返回本身 |
|
||
| `NULLIF` | 两个表达式相等就返回NULL否则返回第一个表达式的值 |
|
||
|
||
其他常用函数。
|
||
|
||
| 函数 | 功能 |
|
||
| -------------------------- | ----------------------------- |
|
||
| `MD5` / `SHA1` / `SHA2` | 返回字符串对应的哈希摘要 |
|
||
| `CHARSET` / `COLLATION` | 返回字符集/校对规则 |
|
||
| `USER` / `CURRENT_USER` | 返回当前用户 |
|
||
| `DATABASE` | 返回当前数据库名 |
|
||
| `VERSION` | 返回当前数据库版本 |
|
||
| `FOUND_ROWS` / `ROW_COUNT` | 返回查询到的行数/受影响的行数 |
|
||
| `LAST_INSERT_ID` | 返回最后一个自增主键的值 |
|
||
| `UUID` / `UUID_SHORT` | 返回全局唯一标识符 |
|