表與表之間一般存在三種關係，即一對一，一對多，多對多關係。
下面分別就三種關係講解數據庫相關設計的思路和思考過程；
(1)一對一關係
例如，下面的一張表，保存了人的相關信息，有男有女，要求查處所有的夫妻。
sql代碼:
CREATE TABLE IF NOT EXISTS person(
id INT PRIMARY KEY AUTO_INCREMENT,
sname VARCHAR(10),
sex CHAR(1),
husband INT,
wife INT
);
INSERT INTO person VALUES(1,'小花','0',3,0);
INSERT INTO person VALUES(2,'小明','1',0,4);
INSERT INTO person VALUES(3,'張三','1',0,1);
INSERT INTO person VALUES(4,'小麗','0',2,0);
INSERT INTO person VALUES(5,'王五','1',0,0);- 1
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sql代碼對應的person表: 
從表中可以看出，小花和張三是夫妻，小明和小麗是夫妻。通常爲了查詢方便，需要兩個表，但實際項目中爲了省空間，通常只建一個表，要實現一對一查詢，可以建立兩個視圖(虛表)
* 有個小細節提一下，視圖只是個邏輯概念，並不獨立佔用物理內存，它依附於數據表，對視圖進行修改時相應的表數據也會蓋改動*
CREATE VIEW men AS (SELECT * FROM person WHERE sex='1');
CREATE VIEW women AS (SELECT * FROM person WHERE sex='0');- 1
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men視圖: 
women視圖:
執行查詢:
方式一(原始方法):SELECT men.sname AS husband,women.sname AS wife FROM men,women WHERE men.id = women.husband AND women.id = men.wife;
方式二(連接方法):SELECT men.sname AS husband,women.sname AS wife FROM men INNER JOIN women ON men.id = women.husband AND women.id = men.wife;
二、一對多關係
例如:一個人可以擁有多輛汽車，要求查詢某個人擁有的所有車輛。
分析：這種情況其實也可以採用 一張表，但因爲一個人可以擁有多輛汽車，如果採用一張表，會造成冗餘信息過多。好的設計方式是，人和車輛分別單獨建表，那麼如何將兩個表關聯呢？有個巧妙的方法，在車輛的表中加個外鍵字段(人的編號)即可。
* (思路小結：’建兩個表，一’方不動，’多’方添加一個外鍵字段)*
sql代碼:
//建立人員表
CREATE TABLE human(
id VARCHAR(12) PRIMARY KEY,
sname VARCHAR(12),
age INT,
sex CHAR(1)
);
INSERT INTO human VALUES('H001','小王',27,'1');
INSERT INTO human VALUES('H002','小明',24,'1');
INSERT INTO human VALUES('H003','張慧',28,'0');
INSERT INTO human VALUES('H004','李小燕',35,'0');
INSERT INTO human VALUES('H005','王大拿',29,'1');
INSERT INTO human VALUES('H006','周強',36,'1');
//建立車輛信息表
CREATE TABLE car(
id VARCHAR(12) PRIMARY KEY,
mark VARCHAR(24),
price NUMERIC(6,2),
hid VARCHAR(12),
CONSTRAINT fk_human FOREIGN KEY(hid) REFERENCES human(id)
);
INSERT INTO car VALUES('C001','BMW',65.99,'H001');
INSERT INTO car VALUES('C002','BenZ',75.99,'H002');
INSERT INTO car VALUES('C003','Skoda',23.99,'H001');
INSERT INTO car VALUES('C004','Peugeot',20.99,'H003');
INSERT INTO car VALUES('C005','Porsche',295.99,'H004');
INSERT INTO car VALUES('C006','Honda',24.99,'H005');
INSERT INTO car VALUES('C007','Toyota',27.99,'H006');
INSERT INTO car VALUES('C008','Kia',18.99,'H002');
INSERT INTO car VALUES('C009','Bentley',309.99,'H005');- 1
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sql代碼對應的人員表: 
sql代碼對應的車輛信息表: 
執行查詢:SELECT human.sname AS 車主,car.mark AS 車輛 FROM human,car WHERE human.id = car.hid;
`SELECT human.sname AS 車主,car.mark AS 車輛 FROM human INNER JOIN car WHERE human.id = car.hid;
三、多對多關係`
例如：學生選課，一個學生可以選修多門課程，每門課程可供多個學生選擇。
分析：這種方式可以按照類似一對多方式建表，但冗餘信息太多，好的方式是實體和關係分離並單獨建表，實體表爲學生表和課程表，關係表爲選修表，其中關係表採用聯合主鍵的方式(由學生表主鍵和課程表主鍵組成)建表。
sql代碼：
//建立學生表
CREATE TABLE student(
id VARCHAR(10) PRIMARY KEY,
sname VARCHAR(12),
age INT,
sex CHAR(1),
class VARCHAR(6)
);
INSERT INTO student VALUES('p0001','王軍',20,1,'c101');
INSERT INTO student VALUES('p0002','張宇',21,1,'c101');
INSERT INTO student VALUES('p0003','劉飛',22,1,'c102');
INSERT INTO student VALUES('p0004','趙燕',18,0,'c103');
INSERT INTO student VALUES('p0005','曾婷',19,0,'c103');
INSERT INTO student VALUES('p0006','周慧',21,0,'c104');
INSERT INTO student VALUES('p0007','小紅',23,0,'c104');
INSERT INTO student VALUES('p0008','楊曉',18,0,'c104');
INSERT INTO student VALUES('p0009','李傑',20,1,'c105');
INSERT INTO student VALUES('p0010','張良',22,1,'c105');
//建立課程表
CREATE TABLE course(
id VARCHAR(10) PRIMARY KEY,
sname VARCHAR(12),
credit NUMERIC(2,1),
teacher VARCHAR(12)
);
INSERT INTO course VALUES('C001','Java',3.5,'李老師');
INSERT INTO course VALUES('C002','高等數學',5.0,'趙老師');
INSERT INTO course VALUES('C003','JavaScript',3.5,'王老師');
INSERT INTO course VALUES('C004','離散數學',3.5,'卜老師');
INSERT INTO course VALUES('C005','數據庫',3.5,'廖老師');
INSERT INTO course VALUES('C006','操作系統',3.5,'張老師');
//建立選修表
CREATE TABLE sc(
sid VARCHAR(10),
cid VARCHAR(10)
);
ALTER TABLE sc ADD CONSTRAINT pk_sc PRIMARY KEY(sid,cid);
ALTER TABLE sc ADD CONSTRAINT fk_student FOREIGN KEY(sid) REFERENCES student(id);
ALTER TABLE sc ADD CONSTRAINT fk_course FOREIGN KEY(cid) REFERENCES course(id);
INSERT INTO sc VALUES('S0001','C001');
INSERT INTO sc VALUES('S0001','C002');
INSERT INTO sc VALUES('S0001','C003');
INSERT INTO sc VALUES('S0002','C001');
INSERT INTO sc VALUES('S0002','C004');
INSERT INTO sc VALUES('S0003','C002');
INSERT INTO sc VALUES('S0003','C005');
INSERT INTO sc VALUES('S0004','C003');
INSERT INTO sc VALUES('S0005','C001');
INSERT INTO sc VALUES('S0006','C004');
INSERT INTO sc VALUES('S0007','C002');
INSERT INTO sc VALUES('S0008','C003');
INSERT INTO sc VALUES('S0009','C001');
INSERT INTO sc VALUES('S0009','C005');- 1
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sql代碼對應的學生表： 
sql代碼對應的課程表： 
sql代碼對應的選課表： 
執行查詢：(沒有選課的學生信息和無學生選的課程信息)
普通方式：
SELECT * FROM student WHERE student.id NOT IN (SELECT sid FROM sc);
SELECT * FROM course WHERE course.id NOT IN (SELECT cid FROM sc);- 1
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關聯方式：
SELECT student.* FROM student LEFT JOIN sc ON student.id=sc.sid LEFT JOIN course ON course.id = sc.cid WHERE course.sname IS NULL;
SELECT course.* FROM course LEFT JOIN sc ON course.id=sc.cid LEFT JOIN student ON student.id = sc.sid WHERE student.sname IS NULL;- 1
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