1.示例表内容
dept表：
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
emp表：
+-------+--------+-----------+------+------------+---------+---------+--------+
| 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 |
+-------+--------+-----------+------+------------+---------+---------+--------+
salgrade表：
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
2.简单查询
mysql 数据库使用SQL SELECT语句来查询数据。
例如我们要查询一个表的全部信息，可以这样做：
当然，这种方式进行查询的效率较低，我们更推荐您使用多列查询的方式：
mysql> select * from dept;
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
4 rows in set (0.00 sec)
那么，如果想查询单个列的信息呢？
可以指定列名进行查询：
mysql> select DNAME from dept;
+------------+
| DNAME |
+------------+
| ACCOUNTING |
| RESEARCH |
| SALES |
| OPERATIONS |
+------------+
4 rows in set (0.00 sec)
查询多个列的信息，可以这样写：
mysql> select DNAME , LOC from dept;
+------------+----------+
| DNAME | LOC |
+------------+----------+
| ACCOUNTING | NEW YORK |
| RESEARCH | DALLAS |
| SALES | CHICAGO |
| OPERATIONS | BOSTON |
+------------+----------+
4 rows in set (0.00 sec)
3.给列起别名
在查询的过程中，我们还可以选择给列起一个别名：
mysql> select DNAME as NAME from dept;
+------------+
| NAME |
+------------+
| ACCOUNTING |
| RESEARCH |
| SALES |
| OPERATIONS |
+------------+
4 rows in set (0.00 sec)
也可以不加 as 关键字：
mysql> select DNAME NAME from dept;
+------------+
| NAME |
+------------+
| ACCOUNTING |
| RESEARCH |
| SALES |
| OPERATIONS |
+------------+
4 rows in set (0.00 sec)
在所有的数据库中，字符串统一使用单引号，这是一个标准
4.列运算
在查询的时候，我们也可以直接进行列运算操作：
比如，我们想计算员工的年薪：
mysql> select ename,sal*12 from emp;
+--------+----------+
| 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 |
+--------+----------+
14 rows in set (0.00 sec)
5.条件查询
示例1：查询工资大于等于3000的信息：
mysql> select empno,ename from emp where sal >= 3000;
+-------+-------+
| empno | ename |
+-------+-------+
| 7788 | SCOTT |
| 7839 | KING |
| 7902 | FORD |
+-------+-------+
3 rows in set (0.00 sec)
示例2：查询工资在2000到3000（包含2000和3000）的信息：
mysql> select empno,ename from emp where sal between 2000 and 3000;
+-------+-------+
| empno | ename |
+-------+-------+
| 7566 | JONES |
| 7698 | BLAKE |
| 7782 | CLARK |
| 7788 | SCOTT |
| 7902 | FORD |
+-------+-------+
5 rows in set (0.00 sec)
示例3：查询员工补助为空的（不为空为is not null）：
mysql> select empno,ename from emp where comm is null;
+-------+--------+
| empno | ename |
+-------+--------+
| 7369 | SMITH |
| 7566 | JONES |
| 7698 | BLAKE |
| 7782 | CLARK |
| 7788 | SCOTT |
| 7839 | KING |
| 7876 | ADAMS |
| 7900 | JAMES |
| 7902 | FORD |
| 7934 | MILLER |
+-------+--------+
10 rows in set (0.00 sec)
Mysql比较NULL值不能使用=号
示例4：查询岗位为MANAGER并且工资大于等于2500的信息：
mysql> select * from emp where JOB = "MANAGER" and SAL >= 2500;
+-------+-------+---------+------+------------+---------+------+--------+
| 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 |
+-------+-------+---------+------+------------+---------+------+--------+
2 rows in set (0.00 sec)
示例5：查询岗位为MANAGER或者SALESMAN的员工：（使用关键字in）（不在某几个值之间使用not in）
mysql> select empno,ename,job from emp where job in ('MANAGER','SALESMAN');
+-------+--------+----------+
| empno | ename | job |
+-------+--------+----------+
| 7499 | ALLEN | SALESMAN |
| 7521 | WARD | SALESMAN |
| 7566 | JONES | MANAGER |
| 7654 | MARTIN | SALESMAN |
| 7698 | BLAKE | MANAGER |
| 7782 | CLARK | MANAGER |
| 7844 | TURNER | SALESMAN |
+-------+--------+----------+
7 rows in set (0.00 sec)
示例6：模糊查询，找出名字中含有字母o的：
mysql> select ename from emp where ename like '%o%';
+-------+
| ename |
+-------+
| JONES |
| SCOTT |
| FORD |
+-------+
3 rows in set (0.00 sec)
示例7：模糊查询，找出名字以T结尾的：
mysql> select ename from emp where ename like '%T';
+-------+
| ename |
+-------+
| SCOTT |
+-------+
1 row in set (0.00 sec)
示例8：模糊查询，找出名字以K开头的：
mysql> select ename from emp where ename like 'K%';
+-------+
| ename |
+-------+
| KING |
+-------+
1 row in set (0.00 sec)
示例9：模糊查询，找出名字第二个字母是A的：
mysql> select ename from emp where ename like '_A%';
+--------+
| ename |
+--------+
| WARD |
| MARTIN |
| JAMES |
+--------+
3 rows in set (0.00 sec)
示例10：模糊查询，找出名字第三个字母是A的：
mysql> select ename from emp where ename like '__A%';
+-------+
| ename |
+-------+
| BLAKE |
| CLARK |
| ADAMS |
+-------+
3 rows in set (0.00 sec)到此这篇关于MySQL数据库学习之查询操作详解的文章就介绍到这了,更多相关MySQL数据库 查询内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！
