最近參與面試，作爲剛入門的一名後端程序員，永遠離不開增刪改查，熟練使用mybatis。數據庫成爲了必考的一個分支，想想還是做點題來鞏固一下以前的知識；
首先是三個表的的創建工作：
dept | CREATE TABLE `dept` (
`deptnu` int(11) NOT NULL COMMENT '部門編號',
`dname` varchar(50) DEFAULT NULL COMMENT '部門名稱',
`addr` varchar(50) DEFAULT NULL COMMENT '部門地址',
PRIMARY KEY (`deptnu`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
employee | CREATE TABLE `employee` (
`empno` int(11) NOT NULL COMMENT '僱員編號',
`ename` varchar(50) DEFAULT NULL COMMENT '僱員姓名',
`job` varchar(50) DEFAULT NULL COMMENT '僱員職位',
`mgr` int(11) DEFAULT NULL COMMENT '僱員上級編號',
`hiredate` date DEFAULT NULL COMMENT '僱傭日期',
`sal` decimal(7,2) DEFAULT NULL COMMENT '薪資',
`deptnu` int(11) DEFAULT NULL COMMENT '部門編號',
PRIMARY KEY (`empno`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8
salgrade | CREATE TABLE `salgrade` (
`grade` int(11) NOT NULL COMMENT '等級',
`lowsal` int(11) DEFAULT NULL COMMENT '最低薪資',
`higsal` int(11) DEFAULT NULL COMMENT '最高薪資',
PRIMARY KEY (`grade`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
然後插入數據：
/*插入dept表數據*/
INSERT INTO dept VALUES (10, '研發部', '北京');
INSERT INTO dept VALUES (20, '工程部', '上海');
INSERT INTO dept VALUES (30, '銷售部', '廣州');
INSERT INTO dept VALUES (40, '財務部', '深圳');
/*插入emp表數據*/
INSERT INTO employee VALUES (1009, '唐僧', '董事長', NULL, '2010-11-17', 50000, 10);
INSERT INTO employee VALUES (1004, '豬八戒', '經理', 1009, '2001-04-02', 29750, 20);
INSERT INTO employee VALUES (1006, '猴子', '經理', 1009, '2011-05-01', 28500, 30);
INSERT INTO employee VALUES (1007, '張飛', '經理', 1009, '2011-09-01', 24500,10);
INSERT INTO employee VALUES (1008, '諸葛亮', '分析師', 1004, '2017-04-19', 30000, 20);
INSERT INTO employee VALUES (1013, '林俊杰', '分析師', 1004, '2011-12-03', 30000, 20);
INSERT INTO employee VALUES (1002, '牛魔王', '銷售員', 1006, '2018-02-20', 16000, 30);
INSERT INTO employee VALUES (1003, '程咬金', '銷售員', 1006, '2017-02-22', 12500, 30);
INSERT INTO employee VALUES (1005, '後裔', '銷售員', 1006, '2011-09-28', 12500, 30);
INSERT INTO employee VALUES (1010, '韓信', '銷售員', 1006, '2018-09-08', 15000,30);
INSERT INTO employee VALUES (1012, '安琪拉', '文員', 1006, '2011-12-03', 9500, 30);
INSERT INTO employee VALUES (1014, '甄姬', '文員', 1007, '2019-01-23', 7500, 10);
INSERT INTO employee VALUES (1011, '妲己', '文員', 1008, '2018-05-23', 11000, 20);
INSERT INTO employee VALUES (1001, '小喬', '文員', 1013, '2018-12-17', 8000, 20);
/*插入salgrade表數據*/
INSERT INTO salgrade VALUES (1, 7000, 12000);
INSERT INTO salgrade VALUES (2, 12010, 14000);
INSERT INTO salgrade VALUES (3, 14010, 20000);
INSERT INTO salgrade VALUES (4, 20010, 30000);
INSERT INTO salgrade VALUES (5, 30010, 99990);
接着開始實戰演練（練習的題基本是常見的面試題）
1.查出至少有一個員工的部門。顯示部門編號、部門名稱、部門位置、部門人數。
涉及表： employee dept
語句：select deptnu,count(*) from employee group by deptnu
語句：select a.deptnu,a.dname,a.addr, b.zongshu from dept a,(select deptnu,count(*) as
zongshu from employee group by deptnu) b where a.deptnu=b.deptnu;
2.列出薪金比安琪拉高的所有員工。
涉及表：employee
語句：select * from employee where sal > (select sal from employee where ename='安琪
拉');
3.列出所有員工的姓名及其直接上級的姓名。
涉及表：employee
語句：select a.ename,ifnull(b.ename,'BOSS') as leader from employee a left join employee
b on a.mgr=b.empno;
4.列出受僱日期早於直接上級的所有員工的編號、姓名、部門名稱。
涉及表：employee dept
條件：a.hiredate < b.hiredate
語句：select a.empno,a.ename,c.dname from employee a left join employee b on
a.mgr=b.empno left join dept c on a.deptnu=c.deptnu where a.hiredate < b.hiredate;
5.列出部門名稱和這些部門的員工信息，同時列出那些沒有員工的部門。
涉及表：dept employee
語句：select a.dname,b.* from dept a left join employee b on a.deptnu=b.deptnu;
6.列出所有文員的姓名及其部門名稱，所在部門的總人數。
涉及表：employee dept
條件：job='文員'
語句：select deptnu,count(*) as zongshu from employee group by deptnu;
語句：select b.ename,a.dname,b.job,c.zongshu from dept a ,employee b ,(select
deptnu,count(*) as zongshu from employee group by deptnu) c where a.deptnu=b.deptnu and
b.job='文員' and b.deptnu=c.deptnu;
7.列出最低薪金大於15000的各種工作及從事此工作的員工人數。
涉及表：employee
條件：min(sal) > 15000
語句：select job,count(*) from employee group by job having min(sal) > 15000;
8.列出在銷售部工作的員工的姓名，假定不知道銷售部的部門編號。
涉及表：employee dept
select ename from employee where deptnu=(select deptnu from dept where dname='銷售部');
9.列出與諸葛亮從事相同工作的所有員工及部門名稱。
涉及表：employee dept
語句：select a.ename,b.dname from employee a,dept b where a.deptnu=b.deptnu and a.job=
(select job from employee where ename='諸葛亮');
語句：select a.ename,b.dname from employee a left join dept b on a.deptnu=b.deptnu where
a.job=(select job from employee where ename='諸葛亮');
10.列出薪金比 在部門30工作的員工的薪金 還高的員工姓名和薪金、部門名稱。
涉及表：employee dept
語句：select a.ename,a.sal,b.dname from employee a ,dept b where a.deptnu=b.deptnu and
sal > (select max(sal) from employee where deptnu=30);
11.列出每個部門的員工數量、平均工資。
涉及表：employee
語句：select deptnu , count(*) ,avg (sal) from employee group by deptnu;
12.列出薪金高於公司平均薪金的所有員工信息，所在部門名稱，上級領導，工資等級。
涉及表：employee dept salgrade
條件：select avg(sal) from employee
語句：elect a.*,c.dname,b.ename,d.grade from employee a,employee b,dept c ,salgrade d
where a.mgr=b.empno and a.deptnu =c.deptnu and a.sal > (select avg(sal) from
employee) and a.sal between d.lowsal and d.higsal;
練習到此爲止，後端初中級別程序員，能夠熟練完成以上習題，基本上面試階段就夠用了。但是長期的發展還是要繼續努力，加油吧。