11.查看创建表的语句： show create table emp（表名字）；
简单的查询-DQL（必须掌握）
1. 语法格式：select 字段名1，字段名2.....from 表明；
提醒：1.分号结尾 2.不区分大小写 3.字符串用单引号括起来
查询员工年薪： select ename，sal*12 from emp；
字段可以参与数学运算
+--------+----------+
| ename | sal*12 |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 19200.00 |
| WARD | 15000.00 |
| JONES | 35700.00 |
| MARTIN | 15000.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| SCOTT | 36000.00 |
| KING | 60000.00 |
| TURNER | 18000.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| FORD | 36000.00 |
| MILLER | 15600.00 |
+--------+----------+
2.给查询结果的列重命名：select ename,sal12 as yearsal from emp; select ename,sal12 as '年薪' from emp;
mysql> select ename,sal*12 as yearsal from emp;
+--------+----------+
| ename | yearsal |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 19200.00 |
| WARD | 15000.00 |
| JONES | 35700.00 |
| MARTIN | 15000.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| SCOTT | 36000.00 |
| KING | 60000.00 |
| TURNER | 18000.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| FORD | 36000.00 |
| MILLER | 15600.00 |
+--------+----------+
mysql> select ename,sal*12 as '年薪' from emp;
+--------+----------+
| ename | 年薪 |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 19200.00 |
| WARD | 15000.00 |
| JONES | 35700.00 |
| MARTIN | 15000.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| SCOTT | 36000.00 |
| KING | 60000.00 |
| TURNER | 18000.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| FORD | 36000.00 |
| MILLER | 15600.00 |
+--------+----------+
3.as关键字可以省略；
+--------+----------+
| ename | 年薪 |
+--------+----------+
| SMITH | 9600.00 |
| ALLEN | 19200.00 |
| WARD | 15000.00 |
| JONES | 35700.00 |
| MARTIN | 15000.00 |
| BLAKE | 34200.00 |
| CLARK | 29400.00 |
| SCOTT | 36000.00 |
| KING | 60000.00 |
| TURNER | 18000.00 |
| ADAMS | 13200.00 |
| JAMES | 11400.00 |
| FORD | 36000.00 |
| MILLER | 15600.00 |
+--------+----------+
14 rows in set (0.00 sec)
4.查询全部字段：select * from emp;(实际开发中不建议使用，效率低)
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 |
+-------+--------+-----------+------+------------+---------+---------+--------+
- 条件查询
1.语法格式：select 字段，字段 from 表名 where 条件；
执行顺序：先from，然后where，最后select
2.查询工资等于5000的员工的姓名 select ename from emp where sal = 5000;
mysql> select ename from emp where sal = 5000;
+-------+
| ename |
+-------+
| KING |
+-------+
1 row in set (0.00 sec)
查询smith的工资： select sal from emp where ename = 'smith';
mysql> select sal from emp where ename = 'smith';
+--------+
| sal |
+--------+
| 800.00 |
+--------+
1 row in set (0.00 sec)
找出工资高于3000的员工： select ename,from emp where sal>3000; <>：不等于
mysql> select ename,sal from emp where sal>3000;
+-------+---------+
| ename | sal |
+-------+---------+
| KING | 5000.00 |
+-------+---------+
1 row in set (0.00 sec)
找出工资在1100-3000之间的员工：select ename,sal from emp where sal>=1100 and sal <3000; select ename,sal from emp where sal between 1100 and 3000;
mysql> select ename,sal from emp where sal between 1100 and 3000;
+--------+---------+
| ename | sal |
+--------+---------+
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| FORD | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
11 rows in set (0.01 sec)
找出那些人没有津贴：在数据库里nil代表什么都没有，不是值，必须使用is null ，is not null
select ename,sal ,comm from emp where comm is null;
mysql> select ename,sal, comm from emp where comm is null;
+--------+---------+------+
| ename | sal | comm |
+--------+---------+------+
| SMITH | 800.00 | NULL |
| JONES | 2975.00 | NULL |
| BLAKE | 2850.00 | NULL |
| CLARK | 2450.00 | NULL |
| SCOTT | 3000.00 | NULL |
| KING | 5000.00 | NULL |
| ADAMS | 1100.00 | NULL |
| JAMES | 950.00 | NULL |
| FORD | 3000.00 | NULL |
| MILLER | 1300.00 | NULL |
+--------+---------+------+
10 rows in set (0.00 sec)
找出那些人没有津贴： select ename,sal,comm from emp where comm is null or comm = 0;
mysql> select ename,sal,comm from emp where comm is null or comm = 0;
+--------+---------+------+
| ename | sal | comm |
+--------+---------+------+
| SMITH | 800.00 | NULL |
| JONES | 2975.00 | NULL |
| BLAKE | 2850.00 | NULL |
| CLARK | 2450.00 | NULL |
| SCOTT | 3000.00 | NULL |
| KING | 5000.00 | NULL |
| TURNER | 1500.00 | 0.00 |
| ADAMS | 1100.00 | NULL |
| JAMES | 950.00 | NULL |
| FORD | 3000.00 | NULL |
| MILLER | 1300.00 | NULL |
+--------+---------+------+
11 rows in set (0.00 sec)
找出工作岗位事manager和 salesman员工：
mysql> select ename,job from emp where job = 'manager'or job='salesman';
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
7 rows in set (0.00 sec)
and or 联合起来用： 找出薪资大于3000的并且本门编号是20或者30笨笨的员工 （and优先级高）
mysql> select ename,sal,deptno from emp where sal >1000 and (deptno = 20 or deptno = 30);
+--------+---------+--------+
| ename | sal | deptno |
+--------+---------+--------+
| ALLEN | 1600.00 | 30 |
| WARD | 1250.00 | 30 |
| JONES | 2975.00 | 20 |
| MARTIN | 1250.00 | 30 |
| BLAKE | 2850.00 | 30 |
| SCOTT | 3000.00 | 20 |
| TURNER | 1500.00 | 30 |
| ADAMS | 1100.00 | 20 |
| FORD | 3000.00 | 20 |
+--------+---------+--------+
9 rows in set (0.00 sec)
in 等同于 or in(a,b) = a or b
not in 不在这几个值当中，找反面
模糊查询：like
找出名字中含有o的： %:任意多个字符 _：任意一个字符
mysql> select ename from emp where ename like '%O%';
+-------+
| ename |
+-------+
| JONES |
| SCOTT |
| FORD |
+-------+
3 rows in set (0.00 sec)
找出第二个字符是a的：
mysql> select ename from emp where ename like '_a%';
+--------+
| ename |
+--------+
| WARD |
| MARTIN |
| JAMES |
+--------+
3 rows in set (0.00 sec)
找出名字中有下划线的：
mysql> select ename from emp where ename like '%_%';
最后一个字母为t:
mysql> select ename from emp where ename like '%t';
+-------+
| ename |
+-------+
| SCOTT |
+-------+
1 row in set (0.00 sec)
-**排序（升序降序）**19
