- LEFT JOIN
FROM a LEFT JOIN b ON a.id = b.id
-以a表的记录为基础,如果B表中没有符合a.id = b.id的,则在a表记录后用NULL填充。
-WHERE中的判断不会影响到b表,否则系统报错。
-LEFT OUTER JOIN = LEFT JOIN
- RIGHT JOIN
与LEFT JOIN相反
- INNER JOIN
a INNER JOIN b on a.id = b.id <==> FROM a, b WHERE a.id = b.id