http://blog.csdn.net/zuosixiaonengshou/article/details/53011452
※多表操作 (凡是多表，都要用到关联技术(把多表合并成一个新表): 左关联、右关联、内关联。还有一个外(全)关联，MySQL不支持，为考虑软件兼容，我们开发一般不用。)
※表与表之间的关系:1对1，1对多，多对多 一、1对1 ※第三范式: 1方建主表(id为主键字段), 多方建外键字段(husband--参考主表的主键id，加unique)
CREATE TABLE man( id VARCHAR(32) PRIMARY KEY, NAME VARCHAR(30) );
CREATE TABLE woman( id VARCHAR(32) PRIMARY KEY, NAME VARCHAR(30), husband VARCHAR(32) UNIQUE, CONSTRAINT wm_fk FOREIGN KEY(husband) REFERENCES man(id) );
注：husband这里要加unique约束，不加则是一对多关系
加unique才是1对1关系
//DROP TABLE woman;
INSERT INTO man VALUES('1','小明'); INSERT INTO man VALUES('2','小聪'); INSERT INTO man VALUES('3','老王');
INSERT INTO woman VALUES('1','小花','2'); INSERT INTO woman VALUES('2','小静','1'); INSERT INTO woman VALUES('3','小红','1');//Error:违反1对1 INSERT INTO woman VALUES('3','小红','10');//Error:违反外键--主表必须存在该外键值
INSERT INTO man VALUES('10','王六'); INSERT INTO woman VALUES('3','小玉','10');//OK
受unique约束，husband不能重复不然
husband是外键，同时受man中的id约束
//◇查询夫妻信息(内联) SELECT man.name AS 丈夫, woman.name AS 妻子 FROM man INNER JOIN woman ON man.id=woman.husband;
SELECT m.name AS 丈夫, w.name AS 妻子 FROM man AS m INNER JOIN woman AS w ON m.id=w.husband;
二、1对多 ※第三范式: 1方建主表(id为主键字段), 多方建外键字段(pid--参考主表的主键id，不加unique) CREATE TABLE person2( id VARCHAR(32) PRIMARY KEY, NAME VARCHAR(30), sex CHAR(1) );
DROP TABLE car2; CREATE TABLE car( id VARCHAR(32) PRIMARY KEY, NAME VARCHAR(30), price NUMERIC(10,2), pid VARCHAR(32), CONSTRAINT car_fk FOREIGN KEY(pid) REFERENCES person2(id) );
INSERT INTO person2 VALUES('P01','Jack','1'); INSERT INTO person2 VALUES('P02','Tom','1'); INSERT INTO person2 VALUES('P03','Rose','0');
INSERT INTO car VALUES('C001','BMW',30,'P01'); INSERT INTO car VALUES('C002','BEnZ',40,'P01'); INSERT INTO car VALUES('C003','Audi',40,'P01');
INSERT INTO car VALUES('C004','QQ',5.5,'P02'); //外键字段值可以为“NULL”表示该车还未卖出 INSERT INTO car(id,NAME,price) VALUES('C005','ABC',10); INSERT INTO car(id,NAME,price) VALUES('C006','BCD',10);
//◇查询哪些人有哪些车 SELECT * FROM car INNER JOIN person2 ON car.pid=person2.id; SELECT person.NAME,car.name FROM person INNER JOIN car ON person.id=car.pid;
//◇查询Jack有哪些车 SELECT car.NAME,car.price,person2.name FROM car INNER JOIN person2 ON car.pid=person2.id where person2.name='Jack';
//◇查询哪些人有两辆或两辆以上的车 //失败品:--每组只显示第一条 SELECT car.NAME,car.price,person2.name FROM car INNER JOIN person2 ON car.pid=person2.id GROUP BY person2.id HAVING COUNT(car.pid)>=2;
//过渡版 SELECT car.NAME,car.price,person2.name FROM car INNER JOIN person2 ON car.pid=person2.id WHERE person2.id IN('P01','P02'); SELECT pid FROM car GROUP BY pid HAVING COUNT(pid)>=2; //把上面写死的pid写成活的
//OK版 SELECT car.NAME,car.price,person2.name FROM car INNER JOIN person2 ON car.pid=person2.id WHERE person2.id IN( SELECT pid FROM car GROUP BY pid HAVING COUNT(pid)>=2 );
//演示左关联: SELECT * FROM person2 LEFT JOIN car ON car.pid=person2.id;//由全表可知只需条件是car.id或NAME或price或pid为空即可查出谁没有车
//◇查询哪些人没有车 SELECT person2.name FROM person2 LEFT JOIN car ON car.pid=person2.id WHERE car.id IS NULL;
其实右关联跟左关联一样，只需要把左关联的表调换一下位置便成了右关联的结果，所以只要会了左关联，右关联也是一样的。
//◇补一个外键的概念（默认是约束): 删除主键信息时，当该主键字段值在外键表中存在时，该记录是不能删除的。---要把外键表是的相关信息删除之后，才能删除。 ---更新同理 DELETE FROM person2 WHERE id='P01';//car表中存在pid='P01'的车，所以主表不能删除'P01'这条记录
三、多对多（ 3个表= 2个实体表 + 1个关系表 )
※第三范式: 两个实体都建成独立的主表, 另外再单独建一个关系表（采用联合主键） 1、分别建议两个实体表(没有外键，但有自己的主键, 没有冗余信息) //DROP TABLE stud; //学生表 CREATE TABLE stud( id VARCHAR(32) PRIMARY KEY, NAME VARCHAR(30) ); //课程表 CREATE TABLE ject( id VARCHAR(32) PRIMARY KEY, NAME VARCHAR(30) );
2、另外补建一个关系表 CREATE TABLE sj( studid VARCHAR(32) NOT NULL, jectid VARCHAR(32) ); //注意，要先建联合主键，再添加外键。顺序不能反了。 ALTER TABLE sj ADD CONSTRAINT sj_pk PRIMARY KEY(studid,jectid); ALTER TABLE sj ADD CONSTRAINT sj_fk1 FOREIGN KEY(studid) REFERENCES stud(id); ALTER TABLE sj ADD CONSTRAINT sj_fk2 FOREIGN KEY(jectid) REFERENCES ject(id);
//删除外键约束 //ALTER TABLE sj DROP FOREIGN KEY sj_fk1; //ALTER TABLE sj DROP FOREIGN KEY sj_fk2;
3、添加一些演示数据 //实体表1 INSERT INTO stud VALUES('S001','Jack'); INSERT INTO stud VALUES('S002','Rose'); INSERT INTO stud VALUES('S003','Tom');
//实体表2 INSERT INTO ject VALUES('J001','Java'); INSERT INTO ject VALUES('J002','Oracle'); INSERT INTO ject VALUES('J003','XML'); INSERT INTO ject VALUES('J004','JSP'); INSERT INTO ject VALUES('J005','Game');
//关系表 INSERT INTO sj VALUES('S001','J001'); INSERT INTO sj VALUES('S001','J003'); INSERT INTO sj VALUES('S001','J004'); INSERT INTO sj VALUES('S002','J002'); INSERT INTO sj VALUES('S002','J003'); INSERT INTO sj VALUES('S002','J004');
//查询哪些人选了哪些课 //SQL组织的1992标准，可用，但效率不高 SELECT stud.name, ject.NAME FROM stud,ject,sj WHERE stud.id=sj.studid AND ject.id=sj.jectid;
//SQL组织的1996标准，效率高，推荐使用---关联 SELECT stud.name, ject.NAME FROM stud INNER JOIN sj ON stud.id=sj.studid INNER JOIN ject ON ject.id=sj.jectid;
//查询哪些人没选课 SELECT stud.name FROM stud LEFT JOIN sj ON stud.id=sj.studid LEFT JOIN ject ON ject.id=sj.jectid where ject.id is NULL;
//查询哪些课没人选 SELECT ject.name FROM stud RIGHT JOIN sj ON stud.id=sj.studid RIGHT JOIN ject ON ject.id=sj.jectid where stud.id is NULL;
