MySQL数据库学习之查询操作详解
作者：世界尽头与你
这篇文章主要为大家详细介绍一下MySQL数据库中一些查询操作，文中的示例代码讲解详细，对我们学习MySQL有一定帮助，需要的可以参考一下
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)
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