abap loop at first last new end

loop 循环里面用可用at进行分组汇总，
at first 用于第一行；
at last 用于最后一行；
at new field 当field字段前面或者他本身的值与上一条记录的值不同时，才执行at endat里面的代码；
at end field 与at new相似，这里只是与下一条记录的值进行比较。

实例代码：
DATA: t1(4) TYPE c, t2 TYPE i.

FIELD-GROUPS: header.

INSERT t2 t1 INTO header.

t1 ='AABB'. t2 = 1. EXTRACT header.
t1 ='BBCC'. t2 = 2. EXTRACT header.
t1 ='AAAA'. t2 = 2. EXTRACT header.
t1 ='AABB'. t2 = 1. EXTRACT header.
t1 ='BBBB'. t2 = 2. EXTRACT header.
t1 ='BBCC'. t2 = 2. EXTRACT header.
t1 ='AAAA'. t2 = 1. EXTRACT header.
t1 ='BBBB'. t2 = 1. EXTRACT header.
t1 ='AAAA'. t2 = 3. EXTRACT header.
t1 ='AABB'. t2 = 1. EXTRACT header.

SORT BY t1 t2.

LOOP.

AT FIRST.
WRITE 'Start of LOOP'.
ULINE.
ENDAT.

AT NEW t1.
WRITE / ' New T1:'.
ENDAT.

AT NEW t2.
WRITE / ' New T2:'.
ENDAT.

WRITE: /14 t1, t2.

AT END OF t2.
WRITE / 'End of T2'.
ENDAT.

AT END OF t1.
WRITE / 'End of T1'.
ENDAT.

AT LAST.
ULINE.
ENDAT.

ENDLOOP.