一. 初始化SQL语句
/*join 建表语句*/
drop database if exists test;
create database test;
use test;
/* 左表t1*/
drop table if exists t1;
create table t1 (id int not null,name varchar(20));
insert into t1 values (1,'t1a');
insert into t1 values (2,'t1b');
insert into t1 values (3,'t1c');
insert into t1 values (4,'t1d');
insert into t1 values (5,'t1f');
/* 右表 t2*/
drop table if exists t2;
create table t2 (id int not null,name varchar(20));
insert into t2 values (2,'t2b');
insert into t2 values (3,'t2c');
insert into t2 values (4,'t2d');
insert into t2 values (5,'t2f');
insert into t2 values (6,'t2a');
二. 笛卡尔积
两表关联，把左表的列和右表的列通过笛卡尔积的形式表达出来
select * from t1 join t2;
三. 左连接
两表关联，左表全部保留，右表关联不上用null表示
select * from t1 left join t2 on t1.id=t2.id;
四. 右连接
右表全部保留，左表关联不上的用null表示
select * from t1 right join t2 on t1.id=t2.id;
五. 内连接
两表关联，保留两表中交集的记录
