MySQL操作之条件,排序,分页,聚合函数,分组,连接,子查询,自连接查询总结

时间:2022-03-07 09:55:10
-- 查询练习
	-- 查询所有字段
	-- select * from 表名;
	select * from students;

	-- 查询指定字段
	-- select 列1,列2,... from 表名;
	select name,gender from students;
	
	-- 使用 as 给字段起别名
	-- select 字段 as 名字.... from 表名;
	select name as "姓名",gender as "性别" from students;

	-- select 表名.字段 .... from 表名;
	select students.name,students.gender from students;

	
	-- 可以通过 as 给表起别名
	-- select 别名.字段 .... from 表名 as 别名;
	select s.name,s.gender from students as s;
	
	失败的select students.name, students.age from students as s;
	

	-- 消除重复行(查性别)
	
	-- distinct 字段 
	select distinct gender from students;
	

-- 条件查询
	-- 比较运算符
		-- select .... from 表名 where .....
		-- >
		-- 查询大于18岁的信息
		select * from students where age > 18;

		-- <
		-- 查询小于18岁的信息
		select * from students where age < 18;
		

		-- >=
		-- <=
		-- 查询小于或者等于18岁的信息
		select * from students where age <= 18;
		-- =
		-- 查询年龄为18岁的所有学生的名字
		select name,age from students where age = 18;


		-- != 或者 <>
		select * from students where age != 18;
		select * from students where age <> 18;

	-- 逻辑运算符
		-- and
		-- 18和28之间的所以学生信息
		select * from students where age > 18 and age < 28;
		
		失败select * from students where age>18 and <28;
		select * from students where 18<age<28;

		-- 18岁以上的女性
		select * from students where age > 18 and gender=2;


		-- or
		-- 18以上或者身高高过180(包含)以上
		select * from students where age > 18 or height >= 180;


		-- not
		-- 不在 18岁以上的女性 这个范围内的信息
		select * from students where not (age > 18 and gender = 2);

		select * from students where not (age > 18 and gender = 2);(注意)

			


	-- 模糊查询(where name like 要查询的数据)
		-- like 
		-- % 替换任意个
		-- _ 替换1个
		-- 查询姓名中 以 "小" 开始的名字
		select * from students where name like "小%";
		

		-- 查询姓名中 有 "小" 所有的名字
	    select * from students where name like "%小%";
		

		-- 查询有2个字的名字
		select * from students where name like "__";
		

		-- 查询有3个字的名字
		select * from students where name like "___";

		-- 查询至少有2个字的名字
		select * from students where name like "__%";



	-- 范围查询
		-- in (1, 3, 8)表示在一个非连续的范围内
		-- 查询 年龄为18、34的姓名
		select name,age from students where age in (18,34);

		-- not in 不非连续的范围之内
		-- 年龄不是 18、34岁的信息
		select name,age from students where age not in (18,34);

		(注意)select name from students where not age in (18,34);


		-- between ... and ...表示在一个连续的范围内
		-- 查询 年龄在18到34之间的的信息
		select * from students where age between 18 and 34;
		
		-- not between ... and ...表示不在一个连续的范围内
		-- 查询 年龄不在在18到34之间的的信息
		select * from students where age not between 18 and 34;
		
		失败的select * from students where age not (between 18 and 34);
		

	-- 空判断
		-- 判空is null
		-- 查询身高为空的信息
		select * from students where height is null;
		
		-- 判非空is not null
		select * from students where height is not null;
		
		失败select * from students where height not is  null;
		

-- 排序
	-- order by 字段
	-- asc从小到大排列,即升序
	-- desc从大到小排序,即降序
	
	-- 查询年龄在18到34岁之间的男性,按照年龄从小到大到排序(默认是asc升序)
	select * from students where (age between 18 and 34) and gender = 1 order by age asc; 
	
	-- 查询年龄在18到34岁之间的女性,身高从高到矮排序
	select * from students where (age between 18 and 34) and gender = 2 order by height desc;
	

	-- order by 多个字段
	-- 查询年龄在18到34岁之间的女性,身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序
	select * from students where (age between 18 and 34) and gender = 2 order by height desc,age;

	-- 查询年龄在18到34岁之间的女性,身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序,
	-- 如果年龄也相同那么按照id从大到小排序
	select * from students where (age between 18 and 34) and gender = 2 order by height desc,age,id desc;


-- 聚合函数
	-- 总数
	-- count
	-- 查询男性有多少人,女性有多少人
	select count(*) from students where gender=1;
	select count(*) from students where gender=2;

	-- 最大值
	-- max
	-- 查询最大的年龄
	select max(age) from students;

	-- 查询女性的最高 身高
	select max(height) from students where gender=2;

	-- 最小值
	-- min
	select min(height) from students;
	
	-- 求和
	-- sum
	-- 计算所有人的年龄总和
	select sum(age) from students;

	
	-- 平均值
	-- avg
	-- 计算平均年龄
	select avg(age) from students;


	-- 计算平均年龄 sum(age)/count(*)
	select sum(age)/count(*) from students;


	-- 四舍五入 round(123.23 , 1) 保留1位小数
	-- 计算所有人的平均年龄,保留2位小数
	select round(avg(age),2) from students;

	-- 计算男性的平均身高 保留2位小数
	select round(avg(height),2) from students where gender=1;

-- 分组(重点)

	-- group by
	-- 按照性别分组,查询所有的性别
	select gender from students group by gender;
	
	select name,gender from students group by gender;错误
	-- select name,gender from students group by gender;
	-- 失败select * from students group by gender;

	-- 计算每种性别中的人数
	select count(*),gender from students group by gender;

	-- group_concat(...)
	-- 查询同种性别中的姓名
 	select gender,group_concat(name) from students group by gender;
	
	-- 查询每组性别的平均年龄
	select avg(age),gender from students group by gender;
	
	-- 查询平均年龄超过30岁的性别,以及姓名 having avg(age) > 30(重点)
	select gender,group_concat(name) from students  group by gender having avg(age) > 30;
	
	-- 查询每种性别的平均年龄和名字
	select avg(age),group_concat(name) from students group by gender;
	
	-- 查询每种性别中的人数多于2个的性别和姓名(重点)
	select gender,group_concat(name) from students group by gender having count(*) > 2;

	-- with rollup 汇总的作用(了解)
	select gender,count(*) from students group by gender with rollup;

-- 分页
	-- limit start, count
	
	-- 限制查询出来的数据个数
	-- 查询前5个数据
	select * from students limit 5;
	
	-- 每页显示2个,第1个页面
	select * from students limit 0,2;

	-- 每页显示2个,第2个页面
	select * from students limit 2,2;

	-- 每页显示2个,第3个页面
	select * from students limit 4,2;

	-- 每页显示2个,第4个页面
	select * from students limit 6,2;
	

	-- 每页显示2个,显示第6页的信息, 按照年龄从小到大排序
	select * from students order by age asc limit 10,2;
	
	错误1 select * from students limit 10,2 order by age asc;
	
	-- 错误的写法
	错误2 select * from students limit 2*(6-1),2;
	
	-- limit 放在最后面(注意)
	 


-- 连接查询(重点)
	-- inner join ... on
	-- select ... from 表A inner join 表B;
	select * from students inner join classes;
	-- 查询 有能够对应班级的学生以及班级信息
	select * from students inner join classes on students.cls_id=classes.id;

	-- 按照要求显示姓名、班级
	select students.name,classes.name from students inner join classes on students.cls_id=classes.id;

	-- 给数据表起名字
	select s.name,c.name from students as s inner join classes as c on s.cls_id=c.id;

	-- 查询 有能够对应班级的学生以及班级信息,显示学生的所有信息 students.*,只显示班级名称 classes.name.
	select students.*,classes.name from students inner join classes on students.cls_id=classes.id;
		
	-- 在以上的查询中,将班级姓名显示在第1列
	select classes.name,students.* from students inner join classes on students.cls_id=classes.id;
	

	-- 查询 有能够对应班级的学生以及班级信息, 按照班级进行排序
	-- select c.xxx s.xxx from students as s inner join clssses as c on .... order by ....;
	select classes.name,students.* from students inner join classes on students.cls_id=classes.id order by classes.name;
	
	-- 当时同一个班级的时候,按照学生的id进行从小到大排序
	select classes.name,students.* from students inner join classes on students.cls_id=classes.id order by classes.name,students.id;
	
	

	-- left join
	-- 查询每位学生对应的班级信息
	select * from students inner join classes on students.cls_id=classes.id;
	select * from students left join classes on students.cls_id=classes.id;
	
	-- select * from students right join classes on students.cls_id = classes.id;

	-- 查询没有对应班级信息的学生
	-- select ... from xxx as s left join xxx as c on..... where .....
	-- select ... from xxx as s left join xxx as c on..... having .....
	select * from students left join classes on students.cls_id=classes.id where classes.id is null;
	
	(注意)不建议使用 select * from students left join classes on students.cls_id=classes.id having classes.id is null;
	
	-- right join   on
	-- 将数据表名字互换位置,用left join完成


	

-- 子查询
	-- 标量子查询: 子查询返回的结果是一个数据(一行一列)
	-- 列子查询: 返回的结果是一列(一列多行)
	-- 行子查询: 返回的结果是一行(一行多列)
	
	-- 查询出高于平均身高的信息(height)
	-- 1 查出平均身高
	select avg(height) from students;
	-- 2 查出高于平均身高的信息
	select * from students where height > (select avg(height) from students);
	
	-- 查询学生的班级号能够对应的 学生名字
	-- select name from students where cls_id in (select id from classes);
	-- 1 查出所有的班级id
	select id from classes;
	(1,2)
	-- 2 查出能够对应上班级号的学生信息
	select * from students where cls_id in (select id from classes);