1、左外连接将左表中要丢弃的记录保留在结果集中，右表中对应的部分用null填充。
mysql> select a.*,b.* from course as a left join rollcall as b on a.course_id=b.course_id; +-----------+-------------+------+---------+-----------+-----------+------+-----------+------------+--------------+--------------+---------+ | course_id | course_name | time | teacher | classroom | tmand | db | course_id | student_id | student_name | teacher_name | mand | +-----------+-------------+------+---------+-----------+-----------+------+-----------+------------+--------------+--------------+---------+ | 1 | 1234 | 123 | 123 | 123 | 123 | 2 | 1 | 1501010096 | 于宗云 | 123 | 123 | | 2 | 123 | 123 | 123 | 123 | 123 | NULL | NULL | NULL | NULL | NULL | NULL | +-----------+-------------+------+---------+-----------+-----------+------+-----------+------------+--------------+--------------+---------+
2、右外连接，将右表中要丢弃的记录保留在结果集中，左表中对应的部分用null填充。
mysql> select a.*,b.* from course as a right join rollcall as b on a.course_id=b.course_id; +-----------+-------------+------+---------+-----------+-----------+----+-----------+------------+--------------+--------------+---------+ | course_id | course_name | time | teacher | classroom | tmand | db | course_id | student_id | student_name | teacher_name | mand | +-----------+-------------+------+---------+-----------+-----------+----+-----------+------------+--------------+--------------+---------+ | 1 | 1234 | 123 | 123 | 123 | 123 | 2 | 1 | 1501010096 | 于宗云 | 123 | 123 | +-----------+-------------+------+---------+-----------+-----------+----+-----------+------------+--------------+--------------+---------+ 1 row in set (0.00 sec)
3、全外连接：将左表和右表中要丢弃的记录保留在结果集中。
右表和左表中相应的部分用null填充。
# 由于本机没有安装oracle所以没有验证第一句sql的正确性 Select * from table1 full join table2 on table1.C=table2.C
