t1表內容如下：
t2表內容如下：
下面來簡述join和left join/right join的區別：
inner join
select * from t1 inner join t2 on t1.id = t2.id;
公共部分的數據才會被查詢出來；
left join
select * from t1 left join t2 on t1.id = t2.id;
查詢出來的結果和前表記錄數一樣多；
right join
select * from t1 right join t2 on t1.id = t2.id;
能轉化為
select * from t2 left join t1 on t1.id = t2.id;