1.行列互换，记录A,B,C三个仓库三个月的库存情况的表，但是我们想看每个仓库每一个月的库存情况，这个时候需要使用行列互换，
主要就是分组列＋分组函数，列进行分组，函数显示每个月的库存
一旦使用group by分组条件
可以select显示的列 只有两种 (分组条件 分组函数)
select
仓库名称,
max(判断 月份=1 显示库存), if(表达式,v1,v2)
max(判断 月份=2 显示库存),
max(判断 月份=3 显示库存)
from warehouse
group by 仓库名称;
分组函数不一定是max，min或者avg都行，因为仓库名跟月份的组合是唯一的。
仓库---->warehouse
库存---->inventory
wname winventory wmonth
仓库名称 仓库库存 月份
A 100 一月份
B 1000 一月份
C 10 一月份
A 200 二月份
B 2000 二月份
C 20 二月份
A 300 三月份
B 3000 三月份
C 30 三月份
仓库名称 一月份 二月份 三月份 。。。。
A 100 200 300 A组 三个值 每一个月份对应一个库存
B 1000 2000 3000 B组 三个值 每一个月份对应一个库存
C 10 20 30 C组 三个值 每一个月份对应一个库存
2.分页查询，在mysql中使用limit进行分页查询，limit 5，表示查询前5行，limit 5,10 表示查询第5到10行，总共5行记录。一般需要联合排序
+-------+--------+-----------+------+------------+---------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+-----------+------+------------+---------+---------+--------+
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 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 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
+-------+--------+-----------+------+------------+---------+---------+--------+
select * from emp order by sal desc limit 0,5;//从0行开始5行记录
+-------+-------+-----------+------+------------+---------+------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
+-------+-------+-----------+------+------------+---------+------+--------+
select * from emp order by sal desc limit 5,5;//从5行开始(不算第5行，从6行开始)，查询5行记录，即6,7,8,9,10
+-------+--------+----------+------+------------+---------+--------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+----------+------+------------+---------+--------+--------+
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
+-------+--------+----------+------+------------+---------+--------+--------+
select * from emp order by sal desc limit 10,20;//如果行数不够，显示剩余实际行数
+-------+--------+----------+------+------------+---------+---------+--------+
| empno | ename | job | mgr | hiredate | sal | comm | deptno |
+-------+--------+----------+------+------------+---------+---------+--------+
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
+-------+--------+----------+------+------------+---------+---------+--------+