左外连接

--连接操作只有匹配的结果保留
--外连接对非匹配元组填充null
--左外连接将保留左关系的所有元组

create table course
(
   id number,
   name varchar2(30)
);

insert into course values(1, '程序设计');
insert into course values(2, '数据结构');
insert into course values(3, '编译原理');

create table student
(
   id number,
   name varchar2(30),
   score number(4,2),
   course_id number
);

insert into student values(9801, '张三', 60.10, 1);
insert into student values(9802, '李四', 96.02, 1);
insert into student values(9803, '王五', 70.16, 2);
insert into student values(9803, '王五', 86.00, 1);

--左外连接
select c.name course_name, s.name, s.score from course c
left join student s on (c.id = s.course_id);

--注意'编译原理'右面，有两个空值哦!!
COURSE_NAME                    NAME                            SCORE
------------------------------ ------------------------------ ------
程序设计                       张三                            60.10
程序设计                       李四                            96.02
数据结构                       王五                            70.16
程序设计                       王五                            86.00
编译原理                       null                            null