MySQL数据库（四）----查询相关操作（单表）

准备四张表：

dept(部门表),emp(员工表),salgrade(薪资等级表),bonus(奖金表)

建表语句：

create table DEPT(  DEPTNO int(2) not null,  DNAME  VARCHAR(14),  LOC    VARCHAR(13)  
);  
alter table DEPT  add constraint PK_DEPT primary key (DEPTNO); create table EMP  
(  EMPNO    int(4) primary key,  ENAME    VARCHAR(10),  JOB      VARCHAR(9),  MGR      int(4),  HIREDATE DATE,  SAL      double(7,2),  COMM     double(7,2),  DEPTNO   int(2)  
);  
alter table EMP  add constraint FK_DEPTNO foreign key (DEPTNO)  references DEPT (DEPTNO);  create table SALGRADE  
(  GRADE int primary key,  LOSAL double(7,2),  HISAL double(7,2)  
);  
create table BONUS  
(  ENAME VARCHAR(10),  JOB   VARCHAR(9),  SAL   double(7,2),  COMM  double(7,2)  
);  
insert into DEPT (DEPTNO, DNAME, LOC)  
values (10, 'ACCOUNTING', 'NEW YORK');  
insert into DEPT (DEPTNO, DNAME, LOC)  
values (20, 'RESEARCH', 'DALLAS');  
insert into DEPT (DEPTNO, DNAME, LOC)  
values (30, 'SALES', 'CHICAGO');  
insert into DEPT (DEPTNO, DNAME, LOC)  
values (40, 'OPERATIONS', 'BOSTON');  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800, null, 20);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20', 1600, 300, 30);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7521, 'WARD', 'SALESMAN', 7698, '1981-02-22', 1250, 500, 30);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7566, 'JONES', 'MANAGER', 7839, '1981-04-02', 2975, null, 20);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28', 1250, 1400, 30);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01', 2850, null, 30);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7782, 'CLARK', 'MANAGER', 7839, '1981-06-09', 2450, null, 10);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19', 3000, null, 20);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7839, 'KING', 'PRESIDENT', null, '1981-11-17', 5000, null, 10);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7844, 'TURNER', 'SALESMAN', 7698, '1981-09-08', 1500, 0, 30);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7876, 'ADAMS', 'CLERK', 7788, '1987-05-23', 1100, null, 20);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7900, 'JAMES', 'CLERK', 7698, '1981-12-03', 950, null, 30);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7902, 'FORD', 'ANALYST', 7566, '1981-12-03', 3000, null, 20);  
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)  
values (7934, 'MILLER', 'CLERK', 7782, '1982-01-23', 1300, null, 10);  
insert into SALGRADE (GRADE, LOSAL, HISAL)  
values (1, 700, 1200);  
insert into SALGRADE (GRADE, LOSAL, HISAL)  
values (2, 1201, 1400);  
insert into SALGRADE (GRADE, LOSAL, HISAL)  
values (3, 1401, 2000);  
insert into SALGRADE (GRADE, LOSAL, HISAL)  
values (4, 2001, 3000);  
insert into SALGRADE (GRADE, LOSAL, HISAL)  
values (5, 3001, 9999);

单表查询：

对emp表查询：

select * from emp; -- *代表所有数据

显示部分列：

select empno,ename,sal from emp;

显示部分行：where子句

select * from emp where sal > 2000;

显示部分列，部分行：

select empno,ename,job,mgr from emp where sal > 2000;

起别名：

select empno 员工编号,ename 姓名,sal 工资 from emp; -- as 省略，''或者""省略了
-- as alias 别名
--在别名中有特殊符号的时候，''或者""不可以省略不写

算术运算符：

select empno,ename,sal,sal+1000 as '涨薪后',deptno from emp where sal < 2500;

去重操作：

select job from emp;
select distinct job from emp;
select job,deptno from emp;
select distinct job,deptno from emp; -- 对后面的所有列组合 去重 ，而不是单独的某一列去重

排序：

select * from emp order by sal; -- 默认情况下是按照升序排列的
select * from emp order by sal asc; -- asc 升序，可以默认不写
select * from emp order by sal desc; -- desc 降序
select * from emp order by sal asc ,deptno desc; -- 在工资升序的情况下，deptno按照降序排列

指定查询条件使用where子句，可以查询符合条件的部分记录。

where子句：将过滤条件放在where子句的后面，可以筛选/过滤出我们想要的符合条件的数据

where 子句 + 关系运算符

select * from emp where deptno = 10;
select * from emp where deptno > 10;
select * from emp where deptno >= 10;
select * from emp where deptno < 10;
select * from emp where deptno <= 10;
select * from emp where deptno <> 10;
select * from emp where deptno != 10;
select * from emp where job = 'CLERK'; 
select * from emp where job = 'clerk'; -- 默认情况下不区分大小写 
select * from emp where binary job = 'clerk'; -- binary区分大小写
select * from emp where hiredate < '1981-12-25';

where 子句 + 逻辑运算符：and

select * from emp where sal > 1500 and sal < 3000;  -- (1500,3000)
select * from emp where sal > 1500 && sal < 3000; 
select * from emp where sal > 1500 and sal < 3000 order by sal;
select * from emp where sal between 1500 and 3000; -- [1500,3000]

where 子句 + 逻辑运算符：or

select * from emp where deptno = 10 or deptno = 20;
select * from emp where deptno = 10 || deptno = 20;
select * from emp where deptno in (10,20);
select * from emp where job in ('MANAGER','CLERK','ANALYST');

where子句 + 模糊查询：

-- 查询名字中带A的员工  -- %代表任意多个字符 0,1,2，.....
select * from emp where ename like '%A%' ;
-- -任意一个字符
select * from emp where ename like '__A%' ;

关于null的判断：

select * from emp where comm is null;
select * from emp where comm is not null;

小括号的使用：因为不同的运算符的优先级别不同，加括号为了可读性

select * from emp where job = 'SALESMAN' or job = 'CLERK' and sal >=1500; -- 先and再or  and > or
select * from emp where job = 'SALESMAN' or (job = 'CLERK' and sal >=1500); 
select * from emp where (job = 'SALESMAN' or job = 'CLERK') and sal >=1500;