oracle的SQL语句中的，+ 左外连接

SELECT s.name AS 姓名,g.grade AS 分数,c.cid AS 课程名
FROM tbl_student s LEFT OUTER JOIN tbl_grade g ON s.StudentId=g.sid LEFT OUTER JOIN tbl_class c ON g.cid=c.ClassId
它跟下面等价
SELECT s.name AS 姓名,g.grade AS 分数,c.classname AS 课程名
FROM tbl_student s,tbl_grade g,tbl_class c
WHERE (s.StudentId=g.sid(+)) AND (g.cid=c.ClassId(+))