-- DQL操作，数据基本查询语言使用----------------------------------------------------------------------------------------------- -- 创建数据表-- 注释：员工编号，员工姓名，领导姓名，领导编号，入职时间，工资，奖金，部门编号CREATE TABLE `employee` ( `empid` int(11) NOT NULL, `ename` varchar(30) DEFAULT NULL, `job` varchar(30) DEFAULT NULL, `leaderid` int(11) DEFAULT NULL, `hiredate` datetime DEFAULT NULL, `wage` decimal(10,2) DEFAULT NULL, `prize` decimal(10,2) DEFAULT NULL, `deptid` int(11) DEFAULT NULL, PRIMARY KEY (`empid`)) ENGINE=InnoDB DEFAULT CHARSET=utf8; -- 添加数据INSERT INTO `employee` VALUES ('2069', 'JALEN', 'CLERK', '7902', '2009-12-17 00:00:00', '18000.00', null, '20'),('3099', 'WANRE', 'SALESMAN', '7698', '2010-02-20 00:00:00', '18000.00', '300.00', '30'),('3021', 'FIKEN', 'SALESMAN', '7698', '2010-02-22 00:00:00', '16500.00', '500.00', '30'),('3066', 'JONES', 'MANAGER', '7839', '2011-04-02 00:00:00', '16000.00', null, '20'),('3054', 'RANKEE', 'SALESMAN', '7698', '2012-09-28 00:00:00', '16500.00', '1400.00', '30'),('3098', 'BLAKE', 'MANAGER', '7839', '2013-05-01 00:00:00', '16000.00', null, '30'),('1082', 'CALAN', 'MANAGER', '7839', '2014-06-09 00:00:00', '16000.00', null, '10'),('2088', 'SCOTT', 'ANALYST', '7566', '2015-04-19 00:00:00', '16000.00', null, '20'),('3039', 'DIVE', 'PRESIDENT', null, '2016-11-17 00:00:00', '15000.00', null, '10'),('3044', 'TURNER', 'SALESMAN', '7698', '2016-09-08 00:00:00', '15000.00', '0.00', '30'),('2076', 'JULI', 'CLERK', '7788', '2017-05-23 00:00:00', '11000.00', null, '20'),('3000', 'JAMES', 'CLERK', '7698', '2017-12-03 00:00:00', '9500.00', null, '30'),('2002', 'FAXI', 'ANALYST', '7566', '2017-12-03 00:00:00', '9000.00', null, '20'),('1034', 'MOKA', 'CLERK', '7782', '2018-01-23 00:00:00', '8800.00', null, '10'); -- 查询语句语法： select cols #查询并展示的数据(字段,表达式等) from tablename #查询的数据来源(表,结果集,视图等) where condition #条件语句 group by #分组 having #分组之后的条件判断 order by #排序(asc升序 desc降序) limit #限制结果查询（仅限于mysql）  -- 1、查询所有数据select * from employee; -- 2、查询部分字段select ename,job from employee; -- 3、单一条件查询select * from employee where ename = "FAXI";select * from employee where wage < 10000; -- 4、组合条件查询select * from employee where wage >= 6000 and wage <= 10000;select * from employee where wage <10000 or hiredate > '2016-01-01 00:00:00'; -- 5、范围查询 between ... and ... | andselect * from employee where hiredate between '2015-01-01 00:00:00' and '2019-01-01 00:00:00';select * from employee where hiredate >= '2015-01-01 00:00:00' and hiredate <= '2019-01-01 00:00:00'; -- 6、集合查询 in | orselect * from employee where deptid in (10,20);select * from employee where deptid = 10 or deptid = 20; -- 7、别名 [as]可以省略select e.ename,e.job,e.wage from employee as e;select e.ename,e.job,e.wage from employee e;select e.ename as '姓名',e.job as '岗位',e.wage '薪资' from employee as e; -- 8、去重distinctselect distinct e.deptid from employee as e;select distinct deptid from employee; -- 9 、模糊查找select * from employee where ename like "B%"; #以B开头的,%表示通配符select * from employee where ename like "%A%"; #包含A的select * from employee where ename like "A%"; #以A结尾的select * from employee where ename like "_A%"; #第2个字母是A的，_表示占位字符select * from employee where ename like "__A%"; #第3个字母是A的，_表示占位字符 -- 10、排序order by ... desc | [asc]select * from employee order by deptid;select * from employee order by deptid asc;select * from employee order by deptid desc;select * from employee order by deptid desc ,hiredate asc ;select * from employee order by hiredate,wage; -- 11、限制结果查询（limit:mysql专用）select * from employee limit 5; #查看前5行select * from employee limit 0,5; #查看前5行select * from employee limit 2,5; #查看从第3行开始往后数5行，即(2,2+5] -- 12、查询并排名select @a,@b; #输出申明的a变量和b变量，默认值都为nullselect @a := 1,@b; #select命令赋值用 := 给申明的a变量赋值，注意 := 中间不能有空格set @b = 2; #set命令给申明的变量赋值用 = select @a,@b; #此时输出的结果应该为@a = 1 ,@b = 2, 两个变量都已经被赋值select @a := "a" ,@b := 12.35; #声明的变量可以赋予任意值（整数，小数，字符串，...） #案例  set @rank = 0; #申明一个rank变量，并赋予初始值0 -- 注释：查询表emp,并按照sal字段降序,同时输出@rank := @rank + 1字段（表示申明的变量rank每一次都等于上一次的结果加1） select *, @rank := @rank + 1 from emp order by sal desc; -- 13、分组查询 # 分组函数  # group by 分组字段 (先分组再聚合,每组一个结果) # having子句: 和where类似,分组之后过滤 # 分组之后,select能够出现聚合函数,分组的字段 select deptno,avg(sal) avg from emp group by deptno; # 查询平均工资>2000的部门的编号和平均工资 # 1.每个部门的平均工资 # 2.平均工资中再筛选>2000的 select deptno,avg(sal) avg from emp group by deptno having avg > 2000;