SELECT * FROM a LEFT JOIN b ON a.aID = b.bID;  a为主，a的数据全显示，连不上b的对应字段为空

 

SELECT * FROM a RIGHT JOIN b ON a.aID = b.bID; b为主，b的数据全显示，连不上a的对应字段为空

 

SELECT * FROM a INNER JOIN a ON a.aID = b.bID; 相当于where a.aID=b.bID