MySQL 查詢（條件、分組、聚合、分頁、連接、子查詢）

MySQL查詢 - as & distinct

-- 使用 as 給字段起別名
-- select 字段 as 字段別名 from 表名;
select name as 姓名, age as 年齡 from students;
-- 使用 as 給表起別名
-- select 表別名.字段 from 表名 as 表別名;
select students.name, students.age from students;
select s.name, s.age from students as s;
-- 使用 distinct 消除重復行
select distinct gender from students;

一、條件查詢

1. 比較運算符
   • >, <=, =, !=

select * from students where age>18;
select * from students where age=18;

2. 邏輯運算符
   • and & or & not: 與或非

-- and & or
select * from students where age>18 and age<28;
select * from students where age>18 or height>=180;
-- not
select * from students where not (age>18 and gender=2);
select * from students where (not age<=18) and gender=2;

3. 模糊查詢
   • like: % 匹配1個或者多個，_ 匹配1個
   • rlike: 正則匹配

-- like 
-- 
-- 查詢姓名中 有 "小" 所有的名字
select name from students where name like "%小%";
-- 查詢有2個字的名字
select name from students where name like "__";
-- 查詢至少有2個字的名字
select name from students where name like "__%";
-- rlike 正則
select name from students where name rlike "^周.*倫$";

4. 范圍查詢
   • in (a, b): 集合匹配
   • between and: 范圍匹配

-- in (1, 3, 8) 非連續范圍
select name,age from students where age in (12, 18, 34);
-- not in 非連續范圍
select name,age from students where age not in (12, 18, 34);
-- between and 連續范圍
select name, age from students where age between 18 and 34;
-- not between and 表示不在一個連續的范圍內
select * from students where age not between 18 and 34;
select * from students where not age between 18 and 34;
-- 錯誤：select * from students where age not (between 18 and 34);

5. 空判斷
   • is null & is not null: 空 & 非空

-- null 或 Null 或 NULL 都行
select * from students where height is null
select * from students where height is not null;

二、排序 - order by [asc/desc]

-- select 默認以主鍵從小到大排序
-- order by 字段 [asc/desc]
select * from students where (age between 18 and 34) and gender=1;
select * from students where (age between 18 and 34) and gender=1 order by age;
select * from students where (age between 18 and 34) and gender=1 order by age asc;
-- order by 多個字段
-- 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 gender=2 order by height desc,age asc,id desc;

三、分組與聚合函數

聚合函數 - count()/max()/min()/sum()/avg()

-- 總數-count()
select count(*) as 男性人數 from students where gender=1;
-- 最大最小值-max()/min()
select max(age) from students;
-- 求和-sum
select sum(age) from students;
-- 平均值-avg
select avg(age) from students;

• 綜合使用：

-- 計算平均年齡 sum(age)/count(*)
select sum(age)/count(*) from students;
-- 四舍五入-round(x,n) 四舍五入x保留n位小數
-- 計算平均年齡，保留2位小數
select round(sum(age)/count(*), 2) from students;
select round(sum(age)/count(*), 3) 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;
-- 錯誤：select name, round(avg(height), 2) from students where gender=1;

分組 - group by [having]

• 分組和聚合函數一起用，語法：

select 字段a, 聚合函數()
from 表名
where 字段n 符合 條件
group by 字段a;

• 使用舉例：

-- 顯示一列所有的性別
select gender from students group by gender;
-- 計算男性的人數
select gender,count(*) from students where gender=1 group by gender;
-- 錯誤：select name from students group by gender;
-- 錯誤：select * from students group by gender;

• group_concat()

-- 姓名按性別分組
select gender,group_concat(name) from students group by gender;
select gender,group_concat(name, " ", age, " ", id) from students where gender=1 group by gender;

• having
  • where和having區別：where 是篩選數據來分組，having 是對分組后的數據進行篩選

-- 查詢平均年齡超過30歲的姓名，並按性別分組
select gender, group_concat(name),avg(age) from students group by gender having avg(age)>30;
-- 查詢人數多於2個的性別分組
select gender, group_concat(name) from students group by gender having count(*)>2;

四、分頁 - limit

• 語法：limit 開始, 個數

-- 查詢前5個數據
select * from students limit 5;
select * from students limit 0, 5;
-- 查詢從第6個開始之后的總共10個數據(6-15)
select * from students limit 5, 10;
-- 不支持運算符，錯誤：select * from students limit 2*(6-1),2;

• limit 必須要寫在 sql 語句的最后

-- 錯誤：select * from students limit 10,2 order by age asc;
select * from students order by age asc limit 10,2;

五、連接查詢

• 內連接：inner join on
  • 若沒有寫 on 條件，classes 表的內容將順序重復連接到 students 表

select * from students inner join classes on students.cls_id=classes.id;

-- 按照要求顯示姓名、班級
select students.*, classes.name from students inner join classes on students.cls_id=classes.id;
-- 查詢有能夠對應班級的學生的所有信息，並顯示班級名稱，並給數據表起別名
select s.*, c.name from students as s inner join classes as c on s.cls_id=c.id;
-- 在以上的查詢中，將班級姓名顯示在第1列
select c.name, s.* 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, 按照班級進行排序
select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;

• 外連接：left join (& right join)

-- 查詢每位學生對應的班級信息
select * from students as s left join classes as c on s.cls_id=c.id;
-- 查詢沒有對應班級信息的學生
select * from students as s left join classes as c on s.cls_id=c.id having c.id is null;

自關聯&子查詢

1. 自關聯

-- 省份和城市的信息都存在同一張表，查山東省有哪些市
select province.name, city.name 
from areas_tb as province 
inner join areas_tb as city 
on city.pid=province.id 
having province.name="山東省";
-- 上面等價於下面這條子查詢語句
select * from areas_tb where pid=(select id from areas_tb where name="山東省")

2. 子查詢

select * from students where height = (select max(height) from students);