一.連接查詢：
1.1、什么是連接查詢？
在實際開發中，大部分的情況下都不是從單表中查詢數據，一般都是多張表聯合查詢取出最終的結果。
在實際開發中，一般一個業務都會對應多張表，比如：學生和班級，起碼兩張表。
stuno stuname classno classname
-----------------------------------------------------------------------------------
1 zs 1 北京大興區亦庄經濟技術開發區第二中學高三1班
2 ls 1 北京大興區亦庄經濟技術開發區第二中學高三1班
...
學生和班級信息存儲到一張表中，結果就像上面一樣，數據會存在大量的重復，導致數據的冗余。
1.2、連接查詢的分類？
根據語法出現的年代來划分的話，包括：
SQL92（一些老的DBA可能還在使用這種語法,DBA：DataBase Administrator，數據庫管理員）
SQL99（比較新的語法）
根據表的連接方式來划分，包括：
內連接：
等值連接
非等值連接
自連接
外連接：
左外連接（左連接）
右外連接（右連接）
全連接（這個不講，很少用！）
1.3、在表的連接查詢方面有一種現象被稱為：笛卡爾積現象 / 笛卡爾乘積現象
笛卡爾積現象：當兩張表進行連接查詢的時候，沒有任何條件進行限制，最終的查詢結果條數是兩張表記錄條數的乘積。
案例：找出每一個員工的部門名稱，要求顯示員工名和部門名。
EMP表
+--------+--------+
| ename | deptno |
+--------+--------+
| SMITH | 20 |
| ALLEN | 30 |
| WARD | 30 |
| JONES | 20 |
| MARTIN | 30 |
| BLAKE | 30 |
| CLARK | 10 |
| SCOTT | 20 |
| KING | 10 |
| TURNER | 30 |
| ADAMS | 20 |
| JAMES | 30 |
| FORD | 20 |
| MILLER | 10 |
+--------+--------+
DEPT表
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
select ename,dname from emp,dept;
+--------+------------+
| ename | dname |
+--------+------------+
| SMITH | ACCOUNTING |
| SMITH | RESEARCH |
| SMITH | SALES |
| SMITH | OPERATIONS |
| ALLEN | ACCOUNTING |
| ALLEN | RESEARCH |
| ALLEN | SALES |
| ALLEN | OPERATIONS |
............( 4X14 )
關於表的別名：
select e.ename,d.dname from emp e,dept d;
表的別名有什么好處？
第一：執行效率高。
第二：可讀性好。
1.4、怎么避免笛卡爾積現象？當然是加條件進行過濾。
思考：避免了笛卡爾積現象，會減少記錄的匹配次數嗎？
不會，次數還是56次。只不過顯示的是有效記錄。
案例：找出每一個員工的部門名稱，要求顯示員工名和部門名。
select e.ename,d.dname from emp e,dept d where e.deptno = d.deptno;
+--------+------------+
| ename | dname |
+--------+------------+
| CLARK | ACCOUNTING |
| KING | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH | RESEARCH |
| JONES | RESEARCH |
| SCOTT | RESEARCH |
| ADAMS | RESEARCH |
| FORD | RESEARCH |
| ALLEN | SALES |
| WARD | SALES |
| MARTIN | SALES |
| BLAKE | SALES |
| TURNER | SALES |
| JAMES | SALES |
+--------+------------+
1.5、內連接之等值連接：最大特點是：條件是等量關系。
案例：查詢每個員工的部門名稱，要求顯示員工名和部門名。
SQL92:（太老，不用了）
select
e.ename,d.dname
from
emp e, dept d
where
e.deptno = d.deptno;
SQL99：（常用的）
select
e.ename,d.dname
from
emp e
inner join // inner可以省略的，帶着inner目的是可讀性好一些
dept d
on
e.deptno = d.deptno;
+--------+------------+
| ename | dname |
+--------+------------+
| CLARK | ACCOUNTING |
| KING | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH | RESEARCH |
| JONES | RESEARCH |
| SCOTT | RESEARCH |
| ADAMS | RESEARCH |
| FORD | RESEARCH |
| ALLEN | SALES |
| WARD | SALES |
| MARTIN | SALES |
| BLAKE | SALES |
| TURNER | SALES |
| JAMES | SALES |
+--------+------------+
語法：
...
A
join
B
on
連接條件
where ...
SQL99語法結構更清晰一些：表的連接條件和后來的where條件分離了。
1.6、內連接之非等值連接：最大的特點是：連接條件中的關系是非等量關系。
案例：找出每個員工的工資等級，要求顯示員工名、工資、工資等級。
mysql> select ename,sal from emp e
+--------+---------+
| ename | sal |
+--------+---------+
| SMITH | 800.00 |
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING | 5000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| JAMES | 950.00 |
| FORD | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
mysql> select * from salgrade s
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
select
e.ename,e.sal,s.grade
from
emp e
(inner) join
salgrade s
on
e.sal between s.losal and s.hisal;
+--------+---------+-------+
| ename | sal | grade |
+--------+---------+-------+
| SMITH | 800.00 | 1 |
| ALLEN | 1600.00 | 3 |
| WARD | 1250.00 | 2 |
| JONES | 2975.00 | 4 |
| MARTIN | 1250.00 | 2 |
| BLAKE | 2850.00 | 4 |
| CLARK | 2450.00 | 4 |
| SCOTT | 3000.00 | 4 |
| KING | 5000.00 | 5 |
| TURNER | 1500.00 | 3 |
| ADAMS | 1100.00 | 1 |
| JAMES | 950.00 | 1 |
| FORD | 3000.00 | 4 |
| MILLER | 1300.00 | 2 |
+--------+---------+-------+
1.7、自連接：最大的特點是：一張表看做兩張表。自己連接自己。
案例：找出每個員工的上級領導，要求顯示員工名和對應的領導名。
mysql> select empno,ename,mgr from emp;
emp a 員工表
+-------+--------+------+
| empno | ename | mgr |
+-------+--------+------+
| 7369 | SMITH | 7902 |
| 7499 | ALLEN | 7698 |
| 7521 | WARD | 7698 |
| 7566 | JONES | 7839 |
| 7654 | MARTIN | 7698 |
| 7698 | BLAKE | 7839 |
| 7782 | CLARK | 7839 |
| 7788 | SCOTT | 7566 |
| 7839 | KING | NULL |
| 7844 | TURNER | 7698 |
| 7876 | ADAMS | 7788 |
| 7900 | JAMES | 7698 |
| 7902 | FORD | 7566 |
| 7934 | MILLER | 7782 |
+-------+--------+------+
emp b 領導表
+-------+--------+
| empno | ename |
+-------+--------+
| 7566 | JONES |
| 7698 | BLAKE |
| 7782 | CLARK |
| 7788 | SCOTT |
| 7839 | KING |
| 7902 | FORD |
+-------+--------+
員工的領導編號 = 領導的員工編號
select
a.ename as '員工名',b.ename as '領導名'
from
emp a
inner join
emp b
on
a.mgr = b.empno;
+--------+--------+
| 員工名 | 領導名 |
+--------+--------+
| SMITH | FORD |
| ALLEN | BLAKE |
| WARD | BLAKE |
| JONES | KING |
| MARTIN | BLAKE |
| BLAKE | KING |
| CLARK | KING |
| SCOTT | JONES |
| TURNER | BLAKE |
| ADAMS | SCOTT |
| JAMES | BLAKE |
| FORD | JONES |
| MILLER | CLARK |
+--------+--------+
1.8、外連接:
外連接的分類？
左外連接（左連接）：表示左邊的這張表是主表。
右外連接（右連接）：表示右邊的這張表是主表。
案例：找出每個員工的上級領導？（所有員工必須全部查詢出來）
emp a 員工表
+-------+--------+------+
| empno | ename | mgr |
+-------+--------+------+
| 7369 | SMITH | 7902 |.
| 7499 | ALLEN | 7698 |
| 7521 | WARD | 7698 |
| 7566 | JONES | 7839 |
| 7654 | MARTIN | 7698 |
| 7698 | BLAKE | 7839 |
| 7782 | CLARK | 7839 |
| 7788 | SCOTT | 7566 |
| 7839 | KING | NULL |
| 7844 | TURNER | 7698 |
| 7876 | ADAMS | 7788 |
| 7900 | JAMES | 7698 |
| 7902 | FORD | 7566 |
| 7934 | MILLER | 7782 |
+-------+--------+------+
emp b 領導表
+-------+--------+
| empno | ename |
+-------+--------+
| 7566 | JONES |
| 7698 | BLAKE |
| 7782 | CLARK |
| 7788 | SCOTT |
| 7839 | KING |
| 7902 | FORD |
+-------+--------+
內連接：
select
a.ename '員工', b.ename '領導'
from
emp a
join
emp b
on
a.mgr = b.empno;
外連接：（左外連接/左連接）
select
a.ename '員工', b.ename '領導'
from
emp a
left （outer）join
emp b
on
a.mgr = b.empno;
外連接：（右外連接/右連接）
select
a.ename '員工', b.ename '領導'
from
emp b
right （outer）join
emp a
on
a.mgr = b.empno;
+--------+-------+
| 員工 | 領導 |
+--------+-------+
| SMITH | FORD |
| ALLEN | BLAKE |
| WARD | BLAKE |
| JONES | KING |
| MARTIN | BLAKE |
| BLAKE | KING |
| CLARK | KING |
| SCOTT | JONES |
| KING | NULL |
| TURNER | BLAKE |
| ADAMS | SCOTT |
| JAMES | BLAKE |
| FORD | JONES |
| MILLER | CLARK |
+--------+-------+
外連接最重要的特點是：主表的數據無條件的全部查詢出來。
1.9、三張表怎么連接查詢？
案例：找出每一個員工的部門名稱以及工資等級。
EMP e
+-------+--------+---------+--------+
| empno | ename | sal | deptno |
+-------+--------+---------+--------+
| 7369 | SMITH | 800.00 | 20 |
| 7499 | ALLEN | 1600.00 | 30 |
| 7521 | WARD | 1250.00 | 30 |
| 7566 | JONES | 2975.00 | 20 |
| 7654 | MARTIN | 1250.00 | 30 |
| 7698 | BLAKE | 2850.00 | 30 |
| 7782 | CLARK | 2450.00 | 10 |
| 7788 | SCOTT | 3000.00 | 20 |
| 7839 | KING | 5000.00 | 10 |
| 7844 | TURNER | 1500.00 | 30 |
| 7876 | ADAMS | 1100.00 | 20 |
| 7900 | JAMES | 950.00 | 30 |
| 7902 | FORD | 3000.00 | 20 |
| 7934 | MILLER | 1300.00 | 10 |
+-------+--------+---------+--------+
DEPT d
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
SALGRADE s
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
// "1"表和"2"表先進行表連接，連接之后"1"表繼續和"3"表進行連接：
select
e.ename,d.dname,s.grade
from
emp e
join
dept d
on
e.deptno = d.deptno
join
salgrade s
on
e.sal between s.losal and s.hisal;
+--------+------------+-------+
| ename | dname | grade |
+--------+------------+-------+
| SMITH | RESEARCH | 1 |
| ALLEN | SALES | 3 |
| WARD | SALES | 2 |
| JONES | RESEARCH | 4 |
| MARTIN | SALES | 2 |
| BLAKE | SALES | 4 |
| CLARK | ACCOUNTING | 4 |
| SCOTT | RESEARCH | 4 |
| KING | ACCOUNTING | 5 |
| TURNER | SALES | 3 |
| ADAMS | RESEARCH | 1 |
| JAMES | SALES | 1 |
| FORD | RESEARCH | 4 |
| MILLER | ACCOUNTING | 2 |
+--------+------------+-------+
案例：找出每一個員工的部門名稱、工資等級、以及上級領導。
select
e.ename '員工',d.dname,s.grade,e1.ename '領導'
from
emp e
join
dept d
on
e.deptno = d.deptno
join
salgrade s
on
e.sal between s.losal and s.hisal
left join
emp e1
on
e.mgr = e1.empno;
+--------+------------+-------+-------+
| 員工 | dname | grade | 領導 |
+--------+------------+-------+-------+
| SMITH | RESEARCH | 1 | FORD |
| ALLEN | SALES | 3 | BLAKE |
| WARD | SALES | 2 | BLAKE |
| JONES | RESEARCH | 4 | KING |
| MARTIN | SALES | 2 | BLAKE |
| BLAKE | SALES | 4 | KING |
| CLARK | ACCOUNTING | 4 | KING |
| SCOTT | RESEARCH | 4 | JONES |
| KING | ACCOUNTING | 5 | NULL |
| TURNER | SALES | 3 | BLAKE |
| ADAMS | RESEARCH | 1 | SCOTT |
| JAMES | SALES | 1 | BLAKE |
| FORD | RESEARCH | 4 | JONES |
| MILLER | ACCOUNTING | 2 | CLARK |
+--------+------------+-------+-------+
二.子查詢：
2.1、select語句當中嵌套select語句，被嵌套的select語句是子查詢。
子查詢可以出現在哪里？
select
..(select).
from
..(select).
where
..(select).
2.2、where子句中使用子查詢
案例：找出高於平均薪資的員工信息。
select * from emp where sal > avg(sal); //錯誤的寫法，where后面不能直接使用分組函數
第一步：找出平均薪資
select avg(sal) from emp;
+-------------+
| avg(sal) |
+-------------+
| 2073.214286 |
+-------------+
第二步：where過濾
select * from emp where sal > 2073.214286;
+-------+-------+-----------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+-------+-----------+------+------------+---------+------+--------+
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
+-------+-------+-----------+------+------------+---------+------+--------+
第一步和第二步合並：
select * from emp where sal > (select avg(sal) from emp);
2.3、from后面嵌套子查詢
案例：找出每個部門平均薪水的等級。
第一步：找出每個部門平均薪水（按照部門編號分組，求sal的平均值）
select deptno,avg(sal) as avgsal from emp group by deptno;
+--------+-------------+
| deptno | avgsal |
+--------+-------------+
| 10 | 2916.666667 |
| 20 | 2175.000000 |
| 30 | 1566.666667 |
+--------+-------------+
第二步：將以上的查詢結果當做臨時表t，讓t表和salgrade s表連接，條件是：t.avgsal between s.losal and s.hisal
select
t.*,s.grade
from
(select deptno,avg(sal) as avgsal from emp group by deptno) t
join
salgrade s
on
t.avgsal between s.losal and s.hisal;
+--------+-------------+-------+
| deptno | avgsal | grade |
+--------+-------------+-------+
| 30 | 1566.666667 | 3 |
| 10 | 2916.666667 | 4 |
| 20 | 2175.000000 | 4 |
+--------+-------------+-------+
案例：找出每個部門平均的薪水等級。
第一步：找出每個員工的薪水等級。
select e.ename,e.sal,e.deptno,s.grade from emp e join salgrade s on e.sal between s.losal and s.hisal;
+--------+---------+--------+-------+
| ename | sal | deptno | grade |
+--------+---------+--------+-------+
| SMITH | 800.00 | 20 | 1 |
| ALLEN | 1600.00 | 30 | 3 |
| WARD | 1250.00 | 30 | 2 |
| JONES | 2975.00 | 20 | 4 |
| MARTIN | 1250.00 | 30 | 2 |
| BLAKE | 2850.00 | 30 | 4 |
| CLARK | 2450.00 | 10 | 4 |
| SCOTT | 3000.00 | 20 | 4 |
| KING | 5000.00 | 10 | 5 |
| TURNER | 1500.00 | 30 | 3 |
| ADAMS | 1100.00 | 20 | 1 |
| JAMES | 950.00 | 30 | 1 |
| FORD | 3000.00 | 20 | 4 |
| MILLER | 1300.00 | 10 | 2 |
+--------+---------+--------+-------+
第二步：基於以上結果，繼續按照deptno分組，求grade平均值。
select
e.deptno,avg(s.grade)
from
emp e
join
salgrade s
on
e.sal between s.losal and s.hisal
group by
e.deptno;
+--------+--------------+
| deptno | avg(s.grade) |
+--------+--------------+
| 10 | 3.6667 |
| 20 | 2.8000 |
| 30 | 2.5000 |
+--------+--------------+
2.4、在select后面嵌套子查詢。
案例：找出每個員工所在的部門名稱，要求顯示員工名和部門名。
//方法一：
select e.ename,d.dname
from emp e join dept d on e.deptno = d.deptno;
//方法二：（select后嵌套子查詢）
select
e.ename,(select d.dname from dept d where e.deptno = d.deptno) as dname
from
emp e;
+--------+------------+
| ename | dname |
+--------+------------+
| SMITH | RESEARCH |
| ALLEN | SALES |
| WARD | SALES |
| JONES | RESEARCH |
| MARTIN | SALES |
| BLAKE | SALES |
| CLARK | ACCOUNTING |
| SCOTT | RESEARCH |
| KING | ACCOUNTING |
| TURNER | SALES |
| ADAMS | RESEARCH |
| JAMES | SALES |
| FORD | RESEARCH |
| MILLER | ACCOUNTING |
+--------+------------+
三.union：
3.1union:拼接查詢結果；
案例：找出工作崗位是SALESMAN和MANAGER的員工？
第一種：select ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';
第二種：select ename,job from emp where job in('MANAGER','SALESMAN');
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
第三種：union
select ename,job from emp where job = 'MANAGER'
union
select ename,job from emp where job = 'SALESMAN';
+--------+----------+
| ename | job |
+--------+----------+
| JONES | MANAGER |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| MARTIN | SALESMAN |
| TURNER | SALESMAN |
+--------+----------+
3.2兩張不相干的表中的數據拼接在一起顯示？
select ename from emp
union
select dname from dept;
//毫無意義：
+------------+
| ename |
+------------+
| SMITH |
| ALLEN |
| WARD |
| JONES |
| MARTIN |
| BLAKE |
| CLARK |
| SCOTT |
| KING |
| TURNER |
| ADAMS |
| JAMES |
| FORD |
| MILLER |
| ACCOUNTING |
| RESEARCH |
| SALES |
| OPERATIONS |
+------------+
3.3 mysql> select ename,sal from emp
-> union
-> select dname from dept;
ERROR 1222 (21000): The used SELECT statements have a different number of columns（列數不對應）
四.limit ：
4.1、limit是mysql特有的，其他數據庫中沒有，不通用（Oracle中有一個相同的機制，叫做rownum）；
4.2、limit取結果集中的部分數據，這是它的作用（分頁）；
4.3、語法機制：
limit startIndex, length
startIndex表示起始位置，從0開始，0表示第一條數據。
length表示取幾個
案例：取出工資前5名的員工（思路：降序取前5個）
select ename,sal from emp order by sal desc;
取前5個：
select ename,sal from emp order by sal desc limit 0, 5;
select ename,sal from emp order by sal desc limit 5;
//limit是sql語句最后執行的一個環節;
4.4、案例：找出工資排名在第4到第9名的員工？
select ename,sal from emp order by sal desc limit 3,6;
+--------+---------+
| ename | sal |
+--------+---------+
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| ALLEN | 1600.00 |
| TURNER | 1500.00 |
| MILLER | 1300.00 |
+--------+---------+
4.5、通用的標准分頁sql？
每頁顯示3條記錄：
第1頁：0, 3
第2頁：3, 3
第3頁：6, 3
......
每頁顯示pageSize條記錄： //pageSize是什么？是每頁顯示多少條記錄
第pageNo頁：(pageNo - 1) * pageSize, pageSize //pageNo是什么？顯示第幾頁
java代碼{
int pageNo = 2; // 頁碼是2
int pageSize = 10; // 每頁顯示10條
limit (pageNo - 1) * pageSize, pageSize
}
資料出處：https://www.bilibili.com/video/BV1fx411X7BD