操作的表
mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 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 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
其每個欄位的欄位名分別為員工編號、員工姓名、員工職位、員工上級領導編號、員工雇傭日期，員工薪水、員工津貼、員工部門編號，
mysql> select * from dept;
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
其每個欄位的欄位名分別部門編號、部門名稱、位置，
mysql> select * from salgrade;
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
其中每個欄位的欄位名分別為等級、該等級薪水下限、該等級薪水上限
where后面嵌套子查詢
什么是子查詢？子查詢都可以出現在哪里？
select陳述句當中嵌套select陳述句，被嵌套的select陳述句是子查詢，
子查詢可以出現在哪里？
??select
???? …(select).
??from
???? …(select).
??where
???? …(select).
案例：找出高于平均薪資的員工資訊，
select * from emp where sal > avg(sal); //錯誤的寫法，where后面不能直接使用分組函式，
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 |
+-------+-------+-----------+------+------------+---------+------+--------+
第一步和第二步合并：
??select * from emp where sal > (select avg(sal) from emp);
+-------+-------+-----------+------+------------+---------+------+--------+
| 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 |
+-------+-------+-----------+------+------------+---------+------+--------
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
t
JOIN
salgrade s
on
t.avgsal BETWEEN s.LOSAL and s.HISAL;
而t暫時未知，所以我們需要把第一步的查詢結果當成表t，改成如下所示
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 |
+--------+-------------+-------+
| 10 | 2916.666667 | 4 |
| 20 | 2175.000000 | 4 |
| 30 | 1566.666667 | 3 |
+--------+-------------+-------+
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.deptno,e.ename,e.sal,s.grade from emp e join salgrade s on e.SAL BETWEEN s.LOSAL and s.HISAL
+--------+--------+---------+-------+
| deptno | ename | sal | grade |
+--------+--------+---------+-------+
| 20 | SMITH | 800.00 | 1 |
| 30 | ALLEN | 1600.00 | 3 |
| 30 | WARD | 1250.00 | 2 |
| 20 | JONES | 2975.00 | 4 |
| 30 | MARTIN | 1250.00 | 2 |
| 30 | BLAKE | 2850.00 | 4 |
| 10 | CLARK | 2450.00 | 4 |
| 20 | SCOTT | 3000.00 | 4 |
| 10 | KING | 5000.00 | 5 |
| 30 | TURNER | 1500.00 | 3 |
| 20 | ADAMS | 1100.00 | 1 |
| 30 | JAMES | 950.00 | 1 |
| 20 | FORD | 3000.00 | 4 |
| 10 | MILLER | 1300.00 | 2 |
+--------+--------+---------+-------+
第二步：基于以上結果，繼續按照deptno分組，求grade平均值，
select
t.deptno,avg(t.grade)
from
(select e.deptno,e.ename,e.sal,s.grade from emp e join salgrade s on e.SAL BETWEEN s.LOSAL and s.HISAL) t
group BY
t.deptno;
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，
select
e.ename,d.dname
from
emp e
join
dept d
on
e.deptno = d.deptno;
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 |
+--------+------------+
union的用法
union （可以將查詢結果集相加）
案例：找出作業崗位是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’);
第三種：
select ename,job from emp where job=‘MANAGER’
UNION
select ename,job from emp where job=‘SALESMAN’;
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
兩張不相干的表中的資料拼接在一起顯示？
select ename from emp
union
select dname from dept;
第一個查詢結果得數量要和第二個查詢結果得列數一致
mysql> select ename,sal from emp
-> union
-> select dname from dept;
ERROR 1222 (21000): The used SELECT statements have a different number of columns
limit以及通用分頁SQL
limit (重點中的重點，以后分頁查詢全靠它了，)
limit是mysql特有的，其他資料庫中沒有，不通用，（Oracle中有一個相同的機制，叫做rownum）
limit取結果集中的部分資料，這是它的作用，
語法機制：
limit startIndex, length
startIndex表示起始位置，從0開始，0表示第一條資料，
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 limit 0,5
??select ename,sal from emp ORDER BY sal desc limit 5（取前5個）
limit是sql陳述句最后執行的一個環節：
select 5
...
from 1
...
where 2
...
group by 3
...
having 4
...
order by 6
...
limit 7
案例：找出工資排名在第4到第9名的員工？
+---------+
| sal |
+---------+
| 2975.00 |
| 2850.00 |
| 2450.00 |
| 1600.00 |
| 1500.00 |
| 1300.00 |
+---------+
通用的標準分頁sql？
每頁顯示3條記錄：
第1頁：0, 3
第2頁：3, 3
第3頁：6, 3
第4頁：9, 3
第5頁：12, 3
每頁顯示pageSize條記錄：
第pageNo頁：(pageNo - 1) * pageSize, pageSize
pageSize是什么？是每頁顯示多少條記錄
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
}
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