1.示例表内容
dept表：
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+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+ |
emp表：
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+-------+--------+-----------+------+------------+---------+---------+--------+| 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：
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+-------+-------+-------+| 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：
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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：
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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：
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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：
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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：
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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：
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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：
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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：
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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）：
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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：
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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）
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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：
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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：
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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：
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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：
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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：
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mysql> select ename from emp where ename like '__A%';+-------+| ename |+-------+| BLAKE || CLARK || ADAMS |+-------+3 rows in set (0.00 sec) |
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原文链接：https://blog.csdn.net/Gherbirthday0916/article/details/125945946
