Apply運算符可以實現兩個查詢結果的全組合結果，又稱為交叉集合。例如兩個數據組合（A,B）、（A,B），他們的交叉集合為（AA，AB，AA，AB）。
Apply分為Cross Apply和Outer Apply兩種使用方式。具體分析如下：
首先先建立兩個表StudentList和ScoreInfo。腳本語言如下：
create table StudentList(
id int Identity(1,1) not null,
Name nvarchar(20) not null,
Sex bit not null,
Birthday date not null,
Class nvarchar(2) not null,
Grade nvarchar(2) not null,
regdate date not null,
Primary key (id));
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);
其中ScoreInfo中的StudentID為StudentList中id的外鍵
插入數據，腳本如下
insert into StudentList(Name, Sex, Birthday, Class, Grade, regdate) values('張三', 1, '1988-05-28', 1, 8, '2010-05-05');
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('王麗', 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');
兩個表結構建立完畢，數據也成功插入進去了。為了便于講解在StudentList表中再插入一條記錄
insert into StudentList(Name, Sex, Birthday, Class, Grade, regdate)
values('李銘', 1, '1989-05-04', 2, 7, '2010-05-05');
輸入以下語句
select * from StudentList a
cross apply
(select ClassID, Score from ScoreInfo where StudentID=a.id) b;
結果如下
再輸入以下語句
select * from StudentList a
outer apply
(select ClassID, Score from ScoreInfo where StudentID=a.id) b;
結果如下
可以看出Cross Apply和Outer Apply的區別
Cross Apply把語句兩邊的兩個Select查詢結果進行交叉配對，將所有結果展示出來。Cross Apply查詢確保在查詢兩個子集數據的交集時，只有有效信息的集合才被列出來。
OuterApply查詢是把兩個子集的所有組合列了出來，不管數據是否有交叉，全部顯示要配對的數據。
