mysql笔试题大餐---2、exists加一些查询
mysql笔试题大餐---2、exists加一些查询
一、总结
一句话总结:
实践:我之前的mysql真的学的太浅了,这种情况下,依据实践(做题)才是唯一能把它学好的方式
1、MySQL中EXISTS如何使用?
1、exists是和in相提并论的东西
2、返回值bool值:EXISTS用于检查子查询是否至少会返回一行数据,该子查询实际上并不返回任何数据,而是返回值True或False
比如在Northwind数据库中有一个查询为
SELECT c.CustomerId,CompanyName FROM Customers c WHERE EXISTS(SELECT OrderID FROM Orders o WHERE o.CustomerID=c.CustomerID) 这里面的EXISTS是如何运作呢?子查询返回的是OrderId字段,可是外面的查询要找的是CustomerID和CompanyName字段,这两个字段肯定不在OrderID里面啊,这是如何匹配的呢? EXISTS用于检查子查询是否至少会返回一行数据,该子查询实际上并不返回任何数据,而是返回值True或False
EXISTS 指定一个子查询,检测 行 的存在。 语法: EXISTS subquery
参数: subquery 是一个受限的 SELECT 语句 (不允许有 COMPUTE 子句和 INTO 关键字)。
结果类型: Boolean 如果子查询包含行,则返回 TRUE ,否则返回 FLASE 。
2、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名?
注意exists:select Student.S#,Student.Sname from Student,SC where Student.S#=SC.S# and SC.C#='001'and exists( Select * from SC as SC_2 where SC_2.S#=SC.S# and SC_2.C#='002');
EXISTS()查询是将主查询的结果集放到子查询中做验证,根据验证结果是true或false来决定主查询数据结果是否得以保存。
3、mysql中的增删改查?
用脑子:站在设计者的思想上怎么才能完成任务:INSERT INTO student(id,name,grade) VALUES(1,'zhangshan',98);
删:DELETE FROM student WHERE id=7;
改:UPDATE student SET name=‘caocao’,grade=50 WHERE id=1;
4、查询所有同学的学号、姓名、选课数、总成绩?
多表查询推荐用链接查询方式:比如inner join
##多表查询推荐用链接查询方式:比如inner join
select s.s_id,s.s_name,count(ss.ss_s_id) course_num,sum(ss.ss_score) from student s inner join student_score ss on s.s_id=ss.ss_s_id group by ss.ss_s_id;
5、查询姓“鲁”的老师的个数?
like中的百分号的用法 + count用法
select count(t.t_id) num from teacher t where t_name like '鲁%'
6、查询没学过“鲁迅”老师课的同学的学号、姓名?
没学过:总的减去学过了的
去掉重复:distinct函数
总的减去学过了的如何实现:通过嵌套查询+ not in
##找出鲁迅老师课的id,学生not in 里面就好
##没学过:总的减去学过了的
##去掉重复:distinct函数
##总的减去学过了的如何实现:通过嵌套查询+ not in
select c.c_id from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅'
select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅'
select distinct(s_id) from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id where c.c_id in (select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅') select s.s_id,s.s_name from student s where s.s_id not in (select distinct(s_id) from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id where c.c_id in (select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅'))
7、查询学过“鲁迅作品集”并且也学过“狂人日记”课程的同学的学号、姓名?
思路:把学过“鲁迅作品集”的人的id找出来,把学过“狂人日记”的人的id找出来,做对比找出同时学过的人的id
特别注意:from的表示一个嵌套查询的表:from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a
##思路:把学过“鲁迅作品集”的人的id找出来,把学过“狂人日记”的人的id找出来,做对比找出同时学过的人的id
##特别注意:from的表示一个嵌套查询的表:from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a
select * from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id
#把学过“鲁迅作品集”的人的id找出来
select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集'
#把学过“狂人日记”的人的id找出来
select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记'
#做对比
select a.s_id from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a,(select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记') b where a.s_id=b.s_id
#找学生信息
select * from student where s_id in (select a.s_id from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a,(select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记') b where a.s_id=b.s_id)
二、内容在总结中
1、具体操作及截图
数据库数据的大致样子:
select * from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id
#3、查询所有同学的学号、姓名、选课数、总成绩;
#3、查询所有同学的学号、姓名、选课数、总成绩;
##多表查询推荐用链接查询方式:比如inner join
select s.s_id,s.s_name,count(ss.ss_s_id) course_num,sum(ss.ss_score) from student s inner join student_score ss on s.s_id=ss.ss_s_id group by ss.ss_s_id;
#4、查询姓“鲁”的老师的个数;
select count(t.t_id) num from teacher t where t_name like '鲁%'
#5、查询没学过“鲁迅”老师课的同学的学号、姓名;
##找出鲁迅老师课的id,学生not in 里面就好
##没学过:总的减去学过了的
##去掉重复:distinct函数
##总的减去学过了的如何实现:通过嵌套查询+ not in
select c.c_id from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅'
select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅'
select distinct(s_id) from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id where c.c_id in (select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅') select s.s_id,s.s_name from student s where s.s_id not in (select distinct(s_id) from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id where c.c_id in (select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅'))
#6、查询学过“鲁迅作品集”并且也学过“狂人日记”课程的同学的学号、姓名;
##思路:把学过“鲁迅作品集”的人的id找出来,把学过“狂人日记”的人的id找出来,做对比找出同时学过的人的id
##特别注意:from的表示一个嵌套查询的表:from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a
select * from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id
#把学过“鲁迅作品集”的人的id找出来
select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集'
#把学过“狂人日记”的人的id找出来
select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记'
#做对比
select a.s_id from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a,(select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记') b where a.s_id=b.s_id
#找学生信息
select * from student where s_id in (select a.s_id from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a,(select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记') b where a.s_id=b.s_id)
2、代码
#3、查询所有同学的学号、姓名、选课数、总成绩;
##多表查询推荐用链接查询方式:比如inner join
select s.s_id,s.s_name,count(ss.ss_s_id) course_num,sum(ss.ss_score) from student s inner join student_score ss on s.s_id=ss.ss_s_id group by ss.ss_s_id; #4、查询姓“鲁”的老师的个数;
select count(t.t_id) num from teacher t where t_name like '鲁%' #5、查询没学过“鲁迅”老师课的同学的学号、姓名;
##找出鲁迅老师课的id,学生not in 里面就好
##没学过:总的减去学过了的
##去掉重复:distinct函数
##总的减去学过了的如何实现:通过嵌套查询+ not in
select c.c_id from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅'
select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅'
select distinct(s_id) from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id where c.c_id in (select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅') select s.s_id,s.s_name from student s where s.s_id not in (select distinct(s_id) from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id where c.c_id in (select distinct(c.c_id) from course c inner join teacher t on t.t_id=c.c_t_id where t_name='鲁迅')) #6、查询学过“鲁迅作品集”并且也学过“狂人日记”课程的同学的学号、姓名;
##思路:把学过“鲁迅作品集”的人的id找出来,把学过“狂人日记”的人的id找出来,做对比找出同时学过的人的id
##特别注意:from的表示一个嵌套查询的表:from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a
select * from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id inner join teacher t on t.t_id=c.c_t_id
#把学过“鲁迅作品集”的人的id找出来
select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集'
#把学过“狂人日记”的人的id找出来
select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记'
#做对比
select a.s_id from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a,(select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记') b where a.s_id=b.s_id
#找学生信息
select * from student where s_id in (select a.s_id from (select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='鲁迅作品集') a,(select s.s_id from student s inner join student_score ss on s.s_id=ss.ss_s_id inner join course c on c.c_id=ss.ss_c_id where c.c_name='狂人日记') b where a.s_id=b.s_id)
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