mysql多表查询练习题_mysql多表查询练习
多对多关系建⽴多对多关系的时候 实际上就是建⽴外键
明确: 1. 在哪张表建⽴外键(中间表) 2. 两表字段之间建⽴联系
建⽴三张表
⽼师 tid tname
学⽣ sid sname
requirement什么意思中间表 tid sid
create table teacher (tid int primary key,name varchar(10));
create table stud (stid int primary key,name varchar(10));
create table temp (stid int,tid int);
alter table temp add constraint fk_temp_tid foreign key(tid) references teacher(tid);
alter table temp add constraint fk_temp_stid foreign key(stid) references stud(stid);
⼀对⼀关系例如 QQ 和 QQ的详细信息
建⽴外键的时候如何明确 主从关系
就看谁引⽤了谁 被引⽤的表⽰主表 外键在从表中建⽴ 关联主表
reate table qq(qqid int primary key, password varchar(10));
create table qqdateil(qqid int primary key, name varchar(10) not null,qgender varchar(10) default '男');
alter table qqdateil add constraint fk_qqdateil_qqid foreign key (qqid) references qq(qqid);
create ta
4000
ble stud1(sid int primary key,sname varchar(10) not null);
网站前台页面的设计与实现create table scores ()
fopen函数r合并查询要求返回的结果集 字段类型 和 数量⼀致 单独使⽤union 会把两张表中的数据合并 并且过滤掉相同的数据 不想过滤相同数据使⽤all
create table A(
name varchar(10),
score int
);
create table B(
name varchar(10),
score int
);
insert into A values('a',10),('b',20),('c',30);
insert into B values('a',10),('b',20),('d',40);
select name,score from A
union
select * from B;
多表查询create table stu(sid int primary key,sname varchar(10) not null);
create table score (sid int, score int , cid int);
create table subjects (cid int primary key,cname varchar(10));
数据格式化alter table score add constraint fk_score_sid foreign key(sid) references stu(sid);
alter table score add constraint fk_score_cid foreign key(cid) references subjects(cid);
插⼊数据
查询 学⽣成绩 和科⽬名称
select * from stu,score where stu.sid = score.sid;
mysql语句多表查询
查询学⽣的成绩 要求70以上的
select s.sid, s.sname, c.score from stu s,score c where s.sid = c.sid and c.score>70;
查询所有学⽣的成绩
select * from stu s inner join score c on s.sid = c.sid;
内连接相当于把两张表链接成⼀张表查询 使⽤inner join 关键词 条件 使⽤on 并且Inner 可以省略
外链接查询 左外连接查询 以左边的表为主 左边表中的数据都会查询出来 有可能会产⽣⽆⽤的数据
select s.sid, s.sname, c.score from score c left join stu s on s.sid = c.sid;
右外链接查询
select s.sid, s.sname, c.score from score c right join stu s on s.sid = c.sid;
⾃然链接 ：会⾃动匹配表中相同的字段 没有外键也能⾃然查询
select * from stu natural join score;
查询所有学⽣的分数和考试科⽬
select s.sid, s.sname, c.score,uame from stu s left join score c on s.sid = c.sid join subjects u on c.cid = u.cid；三张表查询时2条件 n张表n-1条件
CREATE TABLE emp(
empno INT,
ename VARCHAR(50),
job VARCHAR(50),
mgr INT,
hiredate DATE,
sal DECIMAL(7,2),
comm decimal(7,2),
);
INSERT INTO emp values(7369,'SMITH','CLERK',7902,'1980-12-17',800,NULL,20);
INSERT INTO emp values(7499,'ALLEN','SALESMAN',7698,'1981-02-20',1600,300,30); INSERT INTO emp values(7521,'WARD','SALESMAN',7698,'1981-02-22',1250,500,30); INSERT INTO emp values(7566,'JONES','MANAGER',7839,'1981-04-02',2975,NULL,20); INSERT INTO emp values(7654,'MARTIN','SALESMAN',7698,'1981-09-28',1250,1400,30); INSERT INTO emp values(7698,'BLAKE','MANAGER',7839,'1981-05-01',2850,NULL,30); INSERT INTO emp values(7782,'CLARK','MANAGER',7839,'1981-06-09',2450,NULL,10); INSERT INTO emp values(7788,'SCOTT','ANALYST',7566,'1987-04-19',3000,NULL,20); INSERT INTO emp values(7839,'KING','PRESIDENT',NULL,'1981-11-17',5000,NULL,10); INSERT INTO emp values(7844,'TURNER','SALESMAN',7698,'1981-09-08',1500,0,30); INSERT INTO emp values(7876,'ADAMS','CLERK',7788,'1987-05-23',1100,NULL,20);
INSERT INTO emp values(7900,'JAMES','CLERK',7698,'1981-12-03',950,NULL,30);
INSERT INTO emp values(7902,'FORD','ANALYST',7566,'1981-12-03',3000,NULL,20);
INSERT INTO emp values(7934,'MILLER','CLERK',7782,'1982-01-23',1300,NULL,10);
-- 部门表：dept
CREATE TABLE dept(
deptno INT,
dname varchar(14),
loc varchar(13)
);
INSERT INTO dept values(10, 'ACCOUNTING', 'NEW YORK');
INSERT INTO dept values(20, 'RESEARCH', 'DALLAS');
INSERT INTO dept values(30, 'SALES', 'CHICAGO');
什么是mvc 三层设计模式INSERT INTO dept values(40, 'OPERATIONS', 'BOSTON');
查询与 SCOTT 同⼀个部门的员⼯
select ename from emp where deptno = (select deptno from emp where ename=’SCOTT’);
⼯资⾼于30号部门所有⼈的员⼯信息
select * from emp where sal>(select max(sal) from emp where deptno = 30);
查询⼯作和⼯资与MARTIN 完全相同的员⼯信息
select * from emp where (job,sal) in (select job,sal from emp where ename=’MARTIN’);
有两个以上有直接下属的员⼯信息
select * from emp where empno in (select mgr from emp group by mgr having count(mgr)>=2);
查询员⼯编号为7788的员⼯名称 员⼯⼯资 部门名称 部门地址
ame,e.sal,d.dname,d.loc from emp e ,dept d where e.deptno = d.deptno and empno = 7788;
求7369员⼯编号、姓名、经理编号和经理姓名
select ename,empno from emp where empno in (7369, (select mgr from emp where empno = 7369));
pno from emp e1,emp e2 = e2.empno pno = 7369;
求各个部门薪⽔最⾼的员⼯所有信息
select * from emp where (deptno ,sal) in (select deptno,max(sal) from emp group by deptno);
查询过程中出现问题 查询时没有把部门的条件带进去 只是按最⾼⼯资处理的 可以把查询结果返回的结果集当做⼀个新表来使⽤
select * from emp e1 ,(select deptno,max(sal) msal from emp group by deptno) e2 where e1.sal = e2.msal and e1.deptno = e2.deptno;
900c
版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。
发表评论