Cross apply不是很懂

if object_id('T_a','U') is not null
drop table T_a
GO
CREATE TABLE T_a(    id int unique not null,
    name varchar(50),
)
GO
if object_id('T_b',N'U') is not null
drop table T_b
GO
create table T_b
(
    id int unique not null,
    name varchar(10),
    a_ids varchar(100) null --要在这一列中存放t_a表的ID序列,这样做连第一范式都没有满足，但是有时候考虑性能或设计我们可能会像这么用
)
GO
--初始化数据
INSERT INTO T_a VALUES(1,'A-1')
INSERT INTO T_a VALUES(2,'A-2')
INSERT INTO T_a VALUES(3,'A-3')
INSERT INTO T_a VALUES(4,'A-4')
INSERT INTO T_a VALUES(5,'A-5')
INSERT INTO T_b VALUES(1,'B-1','1,2,4')
INSERT INTO T_b VALUES(2,'B-1','2,3,5')
INSERT INTO T_b VALUES(3,'B-1','6,7,8')
GO
--创建一个表值函数，用来拆分用逗号分割的数字串，返回只有一列数字的表
if object_id('splitIDs','TF') is not null
drop function splitIDs;
GO
create function splitIDs(
    @Ids nvarchar(1000)
)
returns @t_id TABLE (id bigint)
as
begin
    declare @i int,@j int,@l int,@v bigint;
    set @i = 0;
    set @j = 0;
    set @l = len(@Ids);
    while(@j < @l)
    begin
       set @j = charindex(',',@Ids,@i+1);
       if(@j = 0) set @j = @l+1;
       set @v = cast(SUBSTRING(@Ids,@i+1,@j-@i-1) as bigint);
       INSERT INTO @t_id VALUES(@v)
       set @i = @j;
    end
    return;
end
GO
--测试splitIDs的执行效果
select * from splitIDs('1,2,4,3')
select * from splitIDs('100')
select * from splitIDs(NULL)
GO


select * from T_a
select * from T_b
--使用cross apply获得t_b表中指定行对应的所有t_a表中的记录
select *
--,
--    aid = t_a.id
--    ,aname = t_a.name
--    ,bid = t_b.id
from t_b
outer apply splitIDs(a_ids) tbl_Ids
INNER JOIN t_a ON tbl_Ids .id = t_a.id
where t_b.id = 1