表TAB_A：

表TAB_B：

最终需要的结果如下：

1、笛卡尔积(不是本例)

select * from TAB_A join TAB_B;

2、两表不关联连接

不关联连接需要找一个公共字段，所以我们取 rownum 进行关联，mysql没有full join，所以我们 使用 left join union right join 查询：

select A.* , B.* from
( select TAB_A.*, @rownum:=@rownum+1 as rn from TAB_A ,(select @rownum :=0 ) t) A 
LEFT JOIN 
( select TAB_B.*, @rownum:=@rownum+1 as rn from TAB_B ,(select @rownum :=0 ) t) B
on A.rn = B.rn
UNION
select A.* , B.* from
( select TAB_A.*, @rownum:=@rownum+1 as rn from TAB_A ,(select @rownum :=0 ) t) A 
RIGHT JOIN 
( select TAB_B.*, @rownum:=@rownum+1 as rn from TAB_B ,(select @rownum :=0 ) t) B
on A.rn = B.rn

结果如下：


rownum怎么跟预想的不太一样？？？

后来测试把四个 (select @rownum :=0 ) t 修改为不同的参数名：
( select TAB_B., @rownum:=@rownum+1 as rn from TAB_B ,(select @rownum :=0 ) t) B
修改为：
( select TAB_B., @rownum1:=@rownum1+1 as rn from TAB_B ,(select @rownum1 :=0 ) t) B

修改后的sql如下：

select A.* , B.* from
( select TAB_A.*, @rownum:=@rownum+1 as rn from TAB_A ,(select @rownum :=0 ) t) A 
LEFT JOIN 
( select TAB_B.*, @rownum1:=@rownum1+1 as rn from TAB_B ,(select @rownum1 :=0 ) t) B
on A.rn = B.rn
UNION
select A.* , B.* from
( select TAB_A.*, @rownum2:=@rownum2+1 as rn from TAB_A ,(select @rownum2 :=0 ) t) A 
RIGHT JOIN 
( select TAB_B.*, @rownum3:=@rownum3+1 as rn from TAB_B ,(select @rownum3 :=0 ) t) B
on A.rn = B.rn

结果：

完结撒花