笔记:实现内连接

以操作数据库tb_train_cpp2和tb_train_cpp22为例:

DESC tb_train_cpp2;

DESC tb_train_cpp22;

数据表tb_train_cpp2中的字段country与tb_train_cpp22中的字段cname对应[可实现内连接]

SELECT tb_train_cpp2.userName,tb_train_cpp2.age,tb_train_cpp2.position,tb_train_cpp22.people_num

FROM tb_train_cpp2

JOIN tb_train_cpp22 ON tb_train_cpp2.country=tb_train_cpp22.cname;

以操作数据表tb_train_user和tb_train_province为例:

DESC tb_train_user;

DESC tb_train_province;

SELECT u.userid,u.username,u.score,p.proname

FROM tb_train_user AS u

JOIN tb_train_province AS p ON u.proid=p.id;

创建数据表tb_train_cpp31和tb_train_cpp32和tb_train_cpp33:

CREATE TABLE IF NOT EXISTS tb_train_cpp3(

userId TINYINT UNSIGNED KEY AUTO_INCREMENT,

userName VARCHAR(10) NOT NULL UNIQUE,

userCard CHAR(5) NOT NULL,

proName VARCHAR(10) DEFAULT '四川-成都'

)ENGINE=INNODB CHARSET=UTF8;

INSERT tb_train_cpp3(userName,userCard,proName) VALUES

('吕布','10058','河南-信阳'),

('张颌','17744','四川-绵阳'),

('曹操','11684','辽宁-鞍山'),

('赵子龙','10892','江苏-南京'),

('貂蝉','10906','湖北-武汉'),

('甘宁','12480',DEFAULT),

('张辽','18745','安徽-合肥'),

('夏侯惇','19003','河南-开封'),

('左慈','19869','广东-广州'),

('赵飞燕','16891','湖南-岳阳'),

('张飞','12565','陕西-西安'),

('袁绍','12007','新疆-乌鲁木齐');

CREATE TABLE IF NOT EXISTS tb_train_cpp33(

proName VARCHAR(10) NOT NULL,

proId TINYINT UNSIGNED

);

INSERT tb_train_cpp33 VALUES

('河南-信阳',12),

('四川-绵阳',25),

('辽宁-鞍山',38),

('江苏-南京',44),

('湖北-武汉',50),

('四川-成都',28),

('安徽-合肥',69),

('河南-开封',17),

('广东-广州',72),

('湖南-岳阳',86),

('陕西-西安',99),

('新疆-乌鲁木齐',105);

CREATE TABLE tb_train_cpp33(

userName VARCHAR(10) NOT NULL UNIQUE,

age INT UNSIGNED DEFAULT 18,

sex ENUM('男','女','保密'),

ablity FLOAT(4,2)

);

INSERT tb_train_cpp33 VALUES

('吕布',36,'男',78.24),

('张颌',32,'女',75.87),

('曹操',45,'男',94.66),

('赵子龙',DEFAULT,'保密',90.18),

('貂蝉',24,'女',82.51),

('甘宁',28,'男',85.90),

('张辽',40,'男',89.02),

('夏侯惇',37,'保密',76.34),

('左慈',56,'女',91.48),

('赵飞燕',20,'女',88.63),

('张飞',48,'男',64.99),

('袁绍',50,'保密',76.89);

SELECT t1.userName,t1.userCard,t2.proId

FROM tb_train_cpp31 AS t1

JOIN tb_train_cpp32 AS t2 ON t1.proName=t2.proName;

SELECT t1.userName,t1.userCard,t3.age,t3.sex,t3.ablity,t1.proName

FROM tb_train_cpp31 AS t1

JOIN tb_train_cpp33 AS t3 ON t1.userName=t3.userName;