create database day16;
use day16;
//部門表
create table dept(
did int primary key auto_increment,
dname varchar(30)
);
//職員表
create table emp(
eid int primary key auto_increment,
ename varchar(20),
salaly double,
dno int
);
//查看所有編碼
show variables like 'char%';
insert into dept values(null,'研發(fā)部');
insert into dept values(null,'銷售部');
insert into dept values(null,'人事部');
insert into emp(ename,salaly,dno) values('班長',10000,1);
insert into emp(ename,salaly,dno) values('美美',40000,1);
insert into emp(ename,salaly,dno) values('小鳳',50000,1);
update emp set dno = 2 where ename='美美';
update emp set dno = 3 where ename='小鳳';
mysql> select * from dept;
+-----+--------+
| did | dname |
+-----+--------+
| 1 | 研發(fā)部 |
| 2 | 銷售部 |
+-----+--------+
2 rows in set (0.00 sec)
mysql> select * from emp;
+-----+-------+--------+------+
| eid | ename | salaly | dno |
+-----+-------+--------+------+
| 1 | 班長 | 10000 | 1 |
| 2 | 美美 | 40000 | 2 |
| 3 | 小鳳 | 50000 | 3 |
+-----+-------+--------+------+
3 rows in set (0.00 sec)
像這種情況明顯不合理 第一張表是部門表 而第二張員工表 員工表中小鳳的這條數(shù)據(jù)dno對應(yīng)的部門被
刪掉明顯不合理，隨便刪掉明顯不合理
這中時候就需要多表設(shè)置外鍵來約束不能隨便的刪除一表的數(shù)據(jù)
多表設(shè)計——外鍵約束
作用：保證數(shù)據(jù)的完整性。
定義外鍵約束
可以直接在create語句中定義外鍵
foreign key 當(dāng)前表名 (字段名) references 目標(biāo)表名 (目標(biāo)表的主
鍵)
create table emp(
eid int primary key auto_increment,
ename varchar(20),
salaly double,
dno int,
foreign key emp('dno') references dept('did');
);
create table renyuan
(
id int primary key ,
name varchar(20) ,
age int NOT NULL,
sex varchar(2) ,
address varchar(20) default '重慶'
) ;
insert into renyuan (id,name,age,sex) values(1,'小李',12,'男');
insert into renyuan (id,name,age,sex) values(2,'老王',32,'男');
修改人員表的默認(rèn) address默認(rèn)約束字段為null；
ALTER TABLE renyuan CHANGE ename ename VARCHAR(20) NULL;
alter table renyuan change address address varchar(20) null;
create table score(
id int primary key,
sid int,
zh int,
en int,
math int,
constraint fk_sid foreign key(sid) references user(id)
);
insert into score values(1,2,45,56,34);
insert into score values(2,1,88,90,120);
alter table score drop foreign key fk_sid;
mysql> update emp set dno =3 where eid=3;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from emp;
+-----+-------+--------+-----+
| eid | ename | salaly | dno |
+-----+-------+--------+-----+
| 1 | 班長 | 10000 | 2 |
| 2 | 美美 | 40000 | 1 |
| 3 | 小鳳 | 50000 | 3 |
+-----+-------+--------+-----+
3 rows in set (0.00 sec)
mysql> desc emp;
+--------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+----------------+
| eid | int(11) | NO | PRI | NULL | auto_increment |
| ename | varchar(20) | YES | | NULL | |
| salaly | double | YES | | NULL | |
| dno | int(11) | NO | MUL | NULL | |
+--------+-------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)
mysql> delete from dept where did = 1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constrai
nt fails (day16.emp, CONSTRAINT emp_ibfk_1 FOREIGN KEY (dno) REFERENCES
dept (did))
mysql>
多表的查詢
笛卡爾積的概念：（了解）
舉個栗子
表a 表b
aid aname bid bname
a1 aa1 b1 bb1
a2 aa2 b2 bb2
b3 bb3
查詢的語法
select * from a,b;
這么寫返回的結(jié)果就是笛卡爾積
a1 aa1 b1 bb1
a1 aa1 b2 bb2
a1 aa1 b3 bb3
a2 aa2 b1 bb1
a2 aa2 b2 bb2
a2 aa2 b3 bb3
這記過就是笛卡爾積 2*3 = 6條結(jié)果
這就是多表查詢啊哈哈哈哈
select * from dept ,emp where dept.did=emp.eid;
select * from dept inner join emp on dept.did=emp.eid && salaly = 10000;
*多表查詢
* 內(nèi)連接（用的比較多）
* 普通內(nèi)連接
前提條件:需要有外鍵的
關(guān)鍵字 inner join...on
select * from dept inner join emp on dept.did=emp.eid;
* 隱式內(nèi)連接（用的是最多的）
select * from dept,emp where dept.did=emp.eid;
*外連接
左外連接
select * from dept left outer join emp on dept.did=emp.eid;
select * from dept left join emp on dept.did=emp.eid;
*右外連接
insert into dept values(null,'呵呵部');
insert into dept values(null,'衛(wèi)生部');
select * from dept;
select * from dept left join emp on dept.did=emp.eid;
select * from dept right join emp on dept.did=emp.eid;
+--------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+----------------+
| eid | int(11) | NO | PRI | NULL | auto_increment |
| ename | varchar(20) | YES | | NULL | |
| salaly | double | YES | | NULL | |
| dno | int(11) | NO | MUL | NULL | |
+--------+-------------+------+-----+---------+----------------+
insert into emp values(null,'呵呵部');
insert into emp values(null,'馬云',2565000000,null);
insert into emp values(null,'馬化騰',20000000000,null);
insert into emp values(null,'馬克龍',1000000000,null);
insert into emp values(null,'馬蓉',40000000,null);
5 rows in set (0.00 sec)
mysql> select * from dept left join emp on dept.did=emp.dno;
+-----+--------+------+-------+------------+------+
| did | dname | eid | ename | salaly | dno |
+-----+--------+------+-------+------------+------+
| 1 | 研發(fā)部 | 6 | 阿冷 | 40000 | 1 |
| 2 | 銷售部 | 1 | 馬云 | 2565000000 | 2 |
| 3 | 人事部 | 5 | 小鳳 | 50000 | 3 |
| 3 | 人事部 | 7 | 提莫 | 100000 | 3 |
| 6 | 呵呵部 | NULL | NULL | NULL | NULL |
| 7 | 衛(wèi)生部 | NULL | NULL | NULL | NULL |
+-----+--------+------+-------+------------+------+
6 rows in set (0.00 sec)
mysql> select * from dept right join emp on dept.did=emp.dno;
+------+--------+-----+--------+-------------+------+
| did | dname | eid | ename | salaly | dno |
+------+--------+-----+--------+-------------+------+
| 1 | 研發(fā)部 | 6 | 阿冷 | 40000 | 1 |
| 2 | 銷售部 | 1 | 馬云 | 2565000000 | 2 |
| 3 | 人事部 | 5 | 小鳳 | 50000 | 3 |
| 3 | 人事部 | 7 | 提莫 | 100000 | 3 |
| NULL | NULL | 2 | 馬化騰 | 20000000000 | NULL |
| NULL | NULL | 3 | 馬克龍 | 1000000000 | NULL |
| NULL | NULL | 4 | 馬蓉 | 40000000 | NULL |
+------+--------+-----+--------+-------------+------+
7 rows in set (0.00 sec)
mysql>
select * from dept,emp where dept.did = emp.dno;
select d.dname,count(*) from dept d ,emp e where d.did = e.dno group by d.dname;