Apply运算符可以实现两个查询结果的全组合结果，又称为交叉集合。例如两个数据组合（A,B）、（A,B），他们的交叉集合为（AA，AB，AA，AB）。
Apply分为Cross Apply和Outer Apply两种使用方式。具体分析如下：
首先先建立两个表StudentList和ScoreInfo。脚本语言如下：
create table ScoreInfo(
id int Identity(1,1) not null primary key,
StudentID int not null,
ClassID int not null,
Score int not null,
TestDate date not null,
regdate date not null);
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其中ScoreInfo中的StudentID为StudentList中id的外键
插入数据，脚本如下
insert into StudentList(Name, Sex, Birthday, Class, Grade, regdate) values('李四', 1, '1985-09-13', 4, 4, '2010-05-05');
insert into StudentList(Name, Sex, Birthday, Class, Grade, regdate) values('王丽', 0, '1987-11-05', 1, 7, '2010-05-05');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(1, 1, 98, '2010-04-15', '2010-05-01');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(1, 2, 92, '2010-04-15', '2010-05-01');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(1, 3, 86, '2010-04-15', '2010-05-01');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(2, 1, 95, '2010-04-15', '2010-05-01');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(2, 2, 94, '2010-04-15', '2010-05-01');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(2, 3, 91, '2010-04-15', '2010-05-01');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(3, 1, 90, '2010-04-15', '2010-05-01');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(3, 2, 88, '2010-04-15', '2010-05-01');
insert into ScoreInfo(StudentID, ClassID, Score, TestDate, regdate) values(3, 3, 90, '2010-04-15', '2010-05-01');
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两个表结构建立完毕，数据也成功插入进去了。为了便于讲解在StudentList表中再插入一条记录
再输入以下语句
select * from StudentList a
outer apply
(select ClassID, Score from ScoreInfo where StudentID=a.id) b;
结果如下
可以看出Cross Apply和Outer Apply的区别
Cross Apply把语句两边的两个Select查询结果进行交叉配对，将所有结果展示出来。Cross Apply查询确保在查询两个子集数据的交集时，只有有效信息的集合才被列出来。
OuterApply查询是把两个子集的所有组合列了出来，不管数据是否有交叉，全部显示要配对的数据。
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