13 排序（升序、降序）
按照工资升序，找出员工名和薪资
select empno,ename,sal,job,comm from emp order by sal asc; // 默认升序排列 asc可以不写 desc降序
+-------+--------+---------+-----------+---------+
| empno | ename | sal | job | comm |
+-------+--------+---------+-----------+---------+
| 7369 | SMITH | 800.00 | CLERK | NULL |
| 7900 | JAMES | 950.00 | CLERK | NULL |
| 7876 | ADAMS | 1100.00 | CLERK | NULL |
| 7521 | WARD | 1250.00 | SALESMAN | 500.00 |
| 7654 | MARTIN | 1250.00 | SALESMAN | 1400.00 |
| 7934 | MILLER | 1300.00 | CLERK | NULL |
| 7844 | TURNER | 1500.00 | SALESMAN | 0.00 |
| 7499 | ALLEN | 1600.00 | SALESMAN | 300.00 |
| 7782 | CLARK | 2450.00 | MANAGER | NULL |
| 7698 | BLAKE | 2850.00 | MANAGER | NULL |
| 7566 | JONES | 2975.00 | MANAGER | NULL |
| 7902 | FORD | 3000.00 | ANALYST | NULL |
| 7788 | SCOTT | 3000.00 | ANALYST | NULL |
| 7839 | KING | 5000.00 | PRESIDENT | NULL |
+-------+--------+---------+-----------+---------+
注意：默认是升序，怎么指定升序或者降序呢？asc表示升序，desc表示降序。
select empno,ename,sal,job,comm from emp order by sal desc;
+-------+--------+---------+-----------+---------+
| empno | ename | sal | job | comm |
+-------+--------+---------+-----------+---------+
| 7839 | KING | 5000.00 | PRESIDENT | NULL |
| 7788 | SCOTT | 3000.00 | ANALYST | NULL |
| 7902 | FORD | 3000.00 | ANALYST | NULL |
| 7566 | JONES | 2975.00 | MANAGER | NULL |
| 7698 | BLAKE | 2850.00 | MANAGER | NULL |
| 7782 | CLARK | 2450.00 | MANAGER | NULL |
| 7499 | ALLEN | 1600.00 | SALESMAN | 300.00 |
| 7844 | TURNER | 1500.00 | SALESMAN | 0.00 |
| 7934 | MILLER | 1300.00 | CLERK | NULL |
| 7654 | MARTIN | 1250.00 | SALESMAN | 1400.00 |
| 7521 | WARD | 1250.00 | SALESMAN | 500.00 |
| 7876 | ADAMS | 1100.00 | CLERK | NULL |
| 7900 | JAMES | 950.00 | CLERK | NULL |
| 7369 | SMITH | 800.00 | CLERK | NULL |
+-------+--------+---------+-----------+---------+
按照工资的降序排列，当工资相同的时候在按照名字的升序排列。
select empno,ename,sal,job,comm from emp order by sal desc, ename asc;
+-------+--------+---------+-----------+---------+
| empno | ename | sal | job | comm |
+-------+--------+---------+-----------+---------+
| 7839 | KING | 5000.00 | PRESIDENT | NULL |
| 7902 | FORD | 3000.00 | ANALYST | NULL |
| 7788 | SCOTT | 3000.00 | ANALYST | NULL |
| 7566 | JONES | 2975.00 | MANAGER | NULL |
| 7698 | BLAKE | 2850.00 | MANAGER | NULL |
| 7782 | CLARK | 2450.00 | MANAGER | NULL |
| 7499 | ALLEN | 1600.00 | SALESMAN | 300.00 |
| 7844 | TURNER | 1500.00 | SALESMAN | 0.00 |
| 7934 | MILLER | 1300.00 | CLERK | NULL |
| 7654 | MARTIN | 1250.00 | SALESMAN | 1400.00 |
| 7521 | WARD | 1250.00 | SALESMAN | 500.00 |
| 7876 | ADAMS | 1100.00 | CLERK | NULL |
| 7900 | JAMES | 950.00 | CLERK | NULL |
| 7369 | SMITH | 800.00 | CLERK | NULL |
+-------+--------+---------+-----------+---------+
注意：越靠前的字段越能起到主导作用。只有当前面的字段无法完成排序的时候，才会启用后面的字段。
select empno,ename,sal,job,comm from emp order by 2; // 2就表示第二列 不建议使用这种方式，因为不健壮，只要查询的列名更改，那么排序将会发生改变。
+-------+--------+---------+-----------+---------+
| empno | ename | sal | job | comm |
+-------+--------+---------+-----------+---------+
| 7876 | ADAMS | 1100.00 | CLERK | NULL |
| 7499 | ALLEN | 1600.00 | SALESMAN | 300.00 |
| 7698 | BLAKE | 2850.00 | MANAGER | NULL |
| 7782 | CLARK | 2450.00 | MANAGER | NULL |
| 7902 | FORD | 3000.00 | ANALYST | NULL |
| 7900 | JAMES | 950.00 | CLERK | NULL |
| 7566 | JONES | 2975.00 | MANAGER | NULL |
| 7839 | KING | 5000.00 | PRESIDENT | NULL |
| 7654 | MARTIN | 1250.00 | SALESMAN | 1400.00 |
| 7934 | MILLER | 1300.00 | CLERK | NULL |
| 7788 | SCOTT | 3000.00 | ANALYST | NULL |
| 7369 | SMITH | 800.00 | CLERK | NULL |
| 7844 | TURNER | 1500.00 | SALESMAN | 0.00 |
| 7521 | WARD | 1250.00 | SALESMAN | 500.00 |
+-------+--------+---------+-----------+---------+
找出工作岗位是salesman的员工，并且要求按照薪资的降序排列？
select empno,ename,sal,job,comm,deptno from emp where job = 'salesman' order by sal desc;
+-------+--------+---------+----------+---------+--------+
| empno | ename | sal | job | comm | deptno |
+-------+--------+---------+----------+---------+--------+
| 7499 | ALLEN | 1600.00 | SALESMAN | 300.00 | 30 |
| 7844 | TURNER | 1500.00 | SALESMAN | 0.00 | 30 |
| 7521 | WARD | 1250.00 | SALESMAN | 500.00 | 30 |
| 7654 | MARTIN | 1250.00 | SALESMAN | 1400.00 | 30 |
+-------+--------+---------+----------+---------+--------+
sql执行顺序
select
* 3
from
tablename 1
where
条件 2
order by
... 4
order by 是最后执行的。