MySQL操作之條件，排序，分頁，聚合函數，分組，連接，子查詢，自連接查詢總結

-- 查詢練習
	-- 查詢所有字段
	-- select * from 表名;
	select * from students;

	-- 查詢指定字段
	-- select 列1,列2,... from 表名;
	select name,gender from students;
	
	-- 使用 as 給字段起別名
	-- select 字段 as 名字.... from 表名;
	select name as "姓名",gender as "性別" from students;

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

	
	-- 可以通過 as 給表起別名
	-- select 別名.字段 .... from 表名 as 別名;
	select s.name,s.gender from students as s;
	
	失敗的select students.name, students.age from students as s;
	

	-- 消除重復行(查性別)
	
	-- distinct 字段 
	select distinct gender from students;
	

-- 條件查詢
	-- 比較運算符
		-- select .... from 表名 where .....
		-- >
		-- 查詢大於18歲的信息
		select * from students where age > 18;

		-- <
		-- 查詢小於18歲的信息
		select * from students where age < 18;
		

		-- >=
		-- <=
		-- 查詢小於或者等於18歲的信息
		select * from students where age <= 18;
		-- =
		-- 查詢年齡為18歲的所有學生的名字
		select name,age from students where age = 18;


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

	-- 邏輯運算符
		-- and
		-- 18和28之間的所以學生信息
		select * from students where age > 18 and age < 28;
		
		失敗select * from students where age>18 and <28;
		select * from students where 18<age<28;

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


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


		-- not
		-- 不在 18歲以上的女性 這個范圍內的信息
		select * from students where not (age > 18 and gender = 2);

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

			


	-- 模糊查詢(where name like 要查詢的數據)
		-- like 
		-- % 替換任意個
		-- _ 替換1個
		-- 查詢姓名中 以 "小" 開始的名字
		select * from students where name like "小%";
		

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

		-- 查詢有2個字的名字
		select * from students where name like "__";
		

		-- 查詢有3個字的名字
		select * from students where name like "___";

		-- 查詢至少有2個字的名字
		select * from students where name like "__%";



	-- 范圍查詢
		-- in (1, 3, 8)表示在一個非連續的范圍內
		-- 查詢 年齡為18、34的姓名
		select name,age from students where age in (18,34);

		-- not in 不非連續的范圍之內
		-- 年齡不是 18、34歲的信息
		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 ...表示在一個連續的范圍內
		-- 查詢 年齡在18到34之間的的信息
		select * from students where age between 18 and 34;
		
		-- not between ... and ...表示不在一個連續的范圍內
		-- 查詢 年齡不在在18到34之間的的信息
		select * from students where age not between 18 and 34;
		
		失敗的select * from students where age not (between 18 and 34);
		

	-- 空判斷
		-- 判空is null
		-- 查詢身高為空的信息
		select * from students where height is null;
		
		-- 判非空is not null
		select * from students where height is not null;
		
		失敗select * from students where height not is  null;
		

-- 排序
	-- order by 字段
	-- asc從小到大排列，即升序
	-- desc從大到小排序，即降序
	
	-- 查詢年齡在18到34歲之間的男性，按照年齡從小到大到排序(默認是asc升序)
	select * from students where (age between 18 and 34) and gender = 1 order by age asc; 
	
	-- 查詢年齡在18到34歲之間的女性，身高從高到矮排序
	select * from students where (age between 18 and 34) and gender = 2 order by height desc;
	

	-- order by 多個字段
	-- 查詢年齡在18到34歲之間的女性，身高從高到矮排序, 如果身高相同的情況下按照年齡從小到大排序
	select * from students where (age between 18 and 34) and gender = 2 order by height desc,age;

	-- 查詢年齡在18到34歲之間的女性，身高從高到矮排序, 如果身高相同的情況下按照年齡從小到大排序,
	-- 如果年齡也相同那么按照id從大到小排序
	select * from students where (age between 18 and 34) and gender = 2 order by height desc,age,id desc;


-- 聚合函數
	-- 總數
	-- count
	-- 查詢男性有多少人，女性有多少人
	select count(*) from students where gender=1;
	select count(*) from students where gender=2;

	-- 最大值
	-- max
	-- 查詢最大的年齡
	select max(age) from students;

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

	-- 最小值
	-- min
	select min(height) from students;
	
	-- 求和
	-- sum
	-- 計算所有人的年齡總和
	select sum(age) from students;

	
	-- 平均值
	-- avg
	-- 計算平均年齡
	select avg(age) from students;


	-- 計算平均年齡 sum(age)/count(*)
	select sum(age)/count(*) from students;


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

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

-- 分組(重點)

	-- group by
	-- 按照性別分組,查詢所有的性別
	select gender from students group by gender;
	
	select name,gender from students group by gender;錯誤
	-- select name,gender from students group by gender;
	-- 失敗select * from students group by gender;

	-- 計算每種性別中的人數
	select count(*),gender from students group by gender;

	-- group_concat(...)
	-- 查詢同種性別中的姓名
 	select gender,group_concat(name) from students group by gender;
	
	-- 查詢每組性別的平均年齡
	select avg(age),gender from students group by gender;
	
	-- 查詢平均年齡超過30歲的性別，以及姓名 having avg(age) > 30(重點)
	select gender,group_concat(name) from students  group by gender having avg(age) > 30;
	
	-- 查詢每種性別的平均年齡和名字
	select avg(age),group_concat(name) from students group by gender;
	
	-- 查詢每種性別中的人數多於2個的性別和姓名（重點）
	select gender,group_concat(name) from students group by gender having count(*) > 2;

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

-- 分頁
	-- limit start, count
	
	-- 限制查詢出來的數據個數
	-- 查詢前5個數據
	select * from students limit 5;
	
	-- 每頁顯示2個，第1個頁面
	select * from students limit 0,2;

	-- 每頁顯示2個，第2個頁面
	select * from students limit 2,2;

	-- 每頁顯示2個，第3個頁面
	select * from students limit 4,2;

	-- 每頁顯示2個，第4個頁面
	select * from students limit 6,2;
	

	-- 每頁顯示2個，顯示第6頁的信息, 按照年齡從小到大排序
	select * from students order by age asc limit 10,2;
	
	錯誤1 select * from students limit 10,2 order by age asc;
	
	-- 錯誤的寫法
	錯誤2 select * from students limit 2*(6-1),2;
	
	-- limit 放在最后面(注意)
	 


-- 連接查詢(重點)
	-- inner join ... on
	-- select ... from 表A inner join 表B;
	select * from students inner join classes;
	-- 查詢 有能夠對應班級的學生以及班級信息
	select * from students inner join classes on students.cls_id=classes.id;

	-- 按照要求顯示姓名、班級
	select students.name,classes.name from students inner join classes on students.cls_id=classes.id;

	-- 給數據表起名字
	select s.name,c.name from students as s inner join classes as c on s.cls_id=c.id;

	-- 查詢 有能夠對應班級的學生以及班級信息，顯示學生的所有信息 students.*，只顯示班級名稱 classes.name.
	select students.*,classes.name from students inner join classes on students.cls_id=classes.id;
		
	-- 在以上的查詢中，將班級姓名顯示在第1列
	select classes.name,students.* from students inner join classes on students.cls_id=classes.id;
	

	-- 查詢 有能夠對應班級的學生以及班級信息, 按照班級進行排序
	-- 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;
	
	-- 當時同一個班級的時候，按照學生的id進行從小到大排序
	select classes.name,students.* from students inner join classes on students.cls_id=classes.id order by classes.name,students.id;
	
	

	-- left join
	-- 查詢每位學生對應的班級信息
	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;

	-- 查詢沒有對應班級信息的學生
	-- 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)不建議使用 select * from students left join classes on students.cls_id=classes.id having classes.id is null;
	
	-- right join   on
	-- 將數據表名字互換位置，用left join完成


	

-- 子查詢
	-- 標量子查詢: 子查詢返回的結果是一個數據(一行一列)
	-- 列子查詢: 返回的結果是一列(一列多行)
	-- 行子查詢: 返回的結果是一行(一行多列)
	
	-- 查詢出高於平均身高的信息(height)
	-- 1 查出平均身高
	select avg(height) from students;
	-- 2 查出高於平均身高的信息
	select * from students where height > (select avg(height) from students);
	
	-- 查詢學生的班級號能夠對應的 學生名字
	-- select name from students where cls_id in (select id from classes);
	-- 1 查出所有的班級id
	select id from classes;
	(1,2)
	-- 2 查出能夠對應上班級號的學生信息
	select * from students where cls_id in (select id from classes);