一、 去除重复查询
去除重复记录（两行或两行以上记录中系列的上的数据都相同），例如employee表中sal字段就存在相同的记录。当只查询employee表的sal字段时，那么会出现重复记录，那么想去除重复记录，需要使用DISTINCT
SELECT DISTINCT sal FROM employee;
mysql> SELECT DISTINCT sal FROM employee;
+---------+
| sal |
+---------+
| 800.00 |
| 1600.00 |
| 1250.00 |
| 2975.00 |
| 2850.00 |
| 2450.00 |
| 3000.00 |
| 1500.00 |
| 1100.00 |
| 950.00 |
| 1300.00 |
+---------+
二、 查看雇员的月薪与佣金之和
因为sal和comm两列的类型都是数值类型，所以可以做加运算。
注意：如果sal或comm中有一个字段不是数值类型，那么会出错。
SELECT empno, ename, sal+comm FROM employee;
+-------+--------+----------+
| empno | ename | sal+comm |
+-------+--------+----------+
| 7369 | SMITH | NULL |
| 7499 | ALLEN | 1900.00 |
| 7521 | WARD | 1750.00 |
| 7566 | JONES | NULL |
| 7654 | MARTIN | 2650.00 |
| 7698 | BLAKE | NULL |
| 7782 | CLARK | NULL |
| 7788 | SCOTT | NULL |
| 7844 | TURNER | 1500.00 |
| 7876 | ADAMS | NULL |
| 7900 | JAMES | NULL |
| 7902 | FORD | NULL |
| 7934 | MILLER | NULL |
+-------+--------+----------+
出现NULL值的处理
comm列有很多记录的值为NULL，因为任何东西与NULL相加结果还是NULL，
所以结算结果可能会出现NULL。下面使用了把NULL转换成数值0的函数IFNULL：
SELECT *,sal+IFNULL(comm,0) FROM emp;
过滤 comm NULL 查询结果：
mysql> SELECT *,sal+IFNULL(comm,0) FROM employee;
+-------+--------+----------+------+------------+---------+---------+--------+--------------------+
| empno | ename | job | mgr | hiredata | sal | comm | deptno | sal+IFNULL(comm,0) |
+-------+--------+----------+------+------------+---------+---------+--------+--------------------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | 800.00 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | 1900.00 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | 1750.00 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | 2975.00 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | 2650.00 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | 2850.00 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | 2450.00 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | 3000.00 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | 1500.00 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | 1100.00 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | 950.00 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | 3000.00 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | 1300.00 |
+-------+--------+----------+------+------------+---------+---------+--------+--------------------+
二 、给列起别名 AS
查询中出现列名为sal+IFNULL(comm,0)，给这个总和金额取一个别名为 total：
给列起别名
SELECT *, sal+IFNULL(comm,0) AS total FROM employee;
+-------+--------+----------+------+------------+---------+---------+--------+---------+
| empno | ename | job | mgr | hiredata | sal | comm | deptno | total | 最后 total 计算的总和
+-------+--------+----------+------+------------+---------+---------+--------+---------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | 800.00 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | 1900.00 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | 1750.00 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | 2975.00 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | 2650.00 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | 2850.00 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | 2450.00 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | 3000.00 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | 1500.00 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | 1100.00 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | 950.00 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | 3000.00 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | 1300.00 |
+-------+--------+----------+------+------------+---------+---------+--------+---------+
