MySQL學習-子查詢及limit分頁

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 |
+-------+--------+-----------+------+------------+---------+---------+--------+

其每個字段的字段名分別爲員工編號、員工姓名、員工職位、員工上級領導編號、員工僱傭日期，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   |
+--------+------------+----------+

其每個字段的字段名分別部門編號、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 |
+-------+-------+-------+

其中每個字段的字段名分別爲等級、該等級薪水下限、該等級薪水上限

where後面嵌套子查詢

什麼是子查詢？子查詢都可以出現在哪裏？
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裏？
  select
     …(select).
  from
     …(select).
  where
     …(select).








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後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 |
+-------+-------+-----------+------+------------+---------+------+--------+

第一步和第二步合併：
  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後面嵌套子查詢

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：基於以上結果，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後面嵌套子查詢

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’);
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數據拼接在一起顯示？

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，以後分頁查詢全靠它了。)
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）
limit取結果集中的部分數據，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 limit 0,5
  select ename,sal from emp ORDER BY sal desc limit 5（取前5個）

limit是sql語句最後執行的一個環節：

	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是什麼？是每頁顯示多少條記錄
pageNo是什麼？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
}