R/3 ABAP開發學習筆記(轉)

2、使用聚合函式
不推薦
Maxnu = 0.
Select * from zflight where airln = ‘LF’ and cntry = ‘IN’.
Check zflight-fligh > maxnu.
Maxnu = zflight-fligh.
Endselect.
推薦
Select max( fligh ) from zflight into maxnu where airln = ‘LF’ and cntry = ‘IN’.

3、使用檢視代替基本表查詢
不推薦
Select * from zcntry where cntry like ‘IN%’.
Select single * from zflight where cntry = zcntry-cntry and airln = ‘LF’.
Endselect.
推薦
Select * from zcnfl where cntry like ‘IN%’ and airln = ‘LF’.
Endselect.

4、使用INTO table 代替select endselect
不推薦
Refresh: int_fligh.
Select * from zflight into int_fligh.
Append int_fligh. Clear int_fligh.
Endselect.
推薦
Refresh: int_fligh.
Select * from zflight into table int_fligh.

5、使用批次修改內表代替逐行修改
不推薦
Loop at int_fligh.
If int_fligh-flag is initial.
Int_fligh-flag = ‘X’.
Endif.
Modify int_fligh.
Endloop.
推薦
Int_fligh-flag = ‘X’.
Modify int_fligh transporting flag where flag is initial.

6、使用二分法查詢，提高查詢內表資料速度
不推薦
Read table int_fligh with key airln = ‘LF’.
推薦
Read table int_fligh with key airln = ‘LF’ binary search.

7、兩個內表新增使用批次增加代替逐行
不推薦
Loop at int_fligh1.
Append int_fligh1 to int_fligh2.
Endloop.
推薦
Append lines of int_fligh1 to int_fligh2.

8、使用table buffering
Use of buffered tables is recommended to improve the performance considerably. The buffer is bypassed while using the following statementsSelect distinct
Select … for update
Order by, group by, having clause
Joins
Use the Bypass buffer addition to the select clause in order to explicitly bypass the buffer while selecting the data.

9、 使用FOR ALL Entries
不推薦
Loop at int_cntry. Select single * from zfligh into int_fligh where cntry = int_cntry-cntry. Append int_fligh. Endloop.
推薦
Select * from zfligh appending table int_fligh
For all entries in int_cntry
Where cntry = int_cntry-cntry.

10、正確地使用where語句，使查詢能使用索引When a base table has multiple indices, the where clause should be in the order of the index, either a primary or a secondary index
To choose an index, the optimizer checks the field names specified in the where clause and then uses an index that has the same order of the fields. One more tip is that if a table begins with MANDT, while an index does not, there is a high possibility that the optimizer might not use that index.

11、正確地使用MOVE語句
Instead of using the move-corresponding clause it is advisable to use the move statement instead. Attempt should be made to move entire internal table headers in a single shot, rather than moving the fields one by one.

12、正確地使用inner joinLet us take an example of 2 tables, zairln and zflight. The table zairln has the field airln, which is the airline code and the field lnnam, which is the name of the airline. The table zflight has the field airln, the airline code and other fields which hold the details of the flights that an airline operates.
Since these 2 tables a re logically joined by the airln field, it is advisable to use the inner join.
Select a~airln a~lnnam b~fligh b~cntry into table int_airdet
From zairln as a inner join zflight as b on a~airln = b~airln.
In order to restrict the data as per the selection criteria, a where clause can be added to the above inner join.

13、使用sort by 代替order by

14、避免使用SELECT DISTINCT語句
使用的 ABAP SORT + DELETE ADJACENT DUPLICATES 代替.

定義內表與工作區最方便的方法

*定義 名為 ITAB 的內表, 內表結構 參照表 TABLE 。

DATA: ITAB TYPE TABLE OF TABLE.

*定義 名為 WA 的工作區, 其 行結構與 內表 ITAB 相同 。

DATA: WA LIKE LINE OF ITAB.

----------------------------------------------------------------

1.使用occurs 0，定義的不再是物件，而是internal table
2.使用with header line字尾，定義為internal table的同時也定義了一個同名物件，因此可以用以下語句：
LOOP AT STH.
WRITE: / STH.
ENDLOOP.
3.TYPE後面接結構，LIKE後面接物件
4.OBLIGATORY為必輸欄位
5.DATA SEPARATER . = DATA SEPARATER TYPE C.
6.關於內表的結構描述，它的當前記錄資料是放在header line中的,Occurs 是分配資料緩衝區，大小不重要，系統會自動分配。但定義內表不用occurs就需要用with header line,occurs語句記得是為了向下相容。
7.occurs 指明的數量是有一點學問的.
1.當你知道可能每次用Select命中或交換的紀錄數xxx時,可指明 occurs xxx.
2.如用occurs 0 宣告時, buffers 由系統自動分配.
8.SELECT在into時記得一般都要加上table，不然是into一個工作區，即wa，而工作區要寫入內表，則需要再append，所以直接定放內表即可，內表和工作區的區別就在於工作區就相當於表頭，是有一行，data定義begin of itab時不加occurs就是工作區，加了就是內表，occurs *，後面表示系統初始分配給此內表多少行，每次滿時再多分配多少行，我們平常為了節約記憶體，一般直接用0，with header line是為了定義含表頭的內表，平常occurs就直接帶表頭，而with header line一般是在itab1 like itab occurs 0 with header line時用，這是參照一個內表定義另一內表，如果要帶表頭，一定要加with header line。
你這樣問不是辦法，最好不懂時直接接F1，查到SAP的幫助即可. check是檢查後面的邏緝是否滿足，不滿足則在上例是跳出form，不的執行下面的語句。

說實在，初略的看了一下上面的程式，寫得太爛了，竟然將usr01或usr03透明表中的欄位按條件取到一個表工作區，竟然不加single，象這種不加single的select按理說應該是調不過的，必須在後面再對應一個endselect,而這種select加endselect用每次去讀一次透明表，訪問資料庫的次數太多了，換個好一點程式自己研究吧。

9.透明表可以連線查詢，簇表只能透過簡單的select查詢，如bseg.
內表相當於陣列，可以透過loop,read進行查詢。
內表中的記錄都是放在工作區中進行編輯和察看的。結構同內表。程式中宣告內表時，表名為 ITAB_XXXX，字尾儘可能為關聯DBTab或內表用途