What means the statement "Select ... for all Entries ..." - select

Question

I was trying to do select for all entries in select statement - abap. I'm not getting the clear idea what select for entries does. Does any one know ?
Kindly have a look at the statements below:
1.
select bukrs belnr xblnr budat
from bkpf
into table it_bkpf
where belnr in s_belnr
2.
select bukrs belnr buzei gsber zuonr wrbtr kunnr
from bseg
into table it_bseg
for all entries in it_bkpf
where belnr = it_bkpf-belnr.
Please let me know the difference in two statements.
Siva

Answer 1

Some obvious differences:
Different tables
Different target fields
The 2nd select had a syntax problem: You used form instead from (I corrected it with my edit)
Other differences:
The selection 1.) uses in in the where clause. So it uses a select-options (or a range-object).
for all entries in it_bkpf means, that the internal table it_bkpf contains a list of elements, you want to select. Or in other words: Select all entries in bseg, where a filed belnr is an element of hte internal table bseg.

Answer 2

You will get clear answer through ST05 transaction.
You could execute st05 transaction, choose trace SQL and activate
trace.
After that run your code.
Enter st05 again choose deactivate trace, then view trace result.
There you can see exact SQL code that is forwarded to database server. As BSEG is clustered table, you could not use intuitive header-item join to retrieve needed financial movements inforamation. It's just because there are several tables including BSEG are storing in single database table, so database server technically can not separate BSEG rows and find BSEG-specific fields to make proper join.
So you can do join-like construction at application server. First you are retrieving all header-related columns from header table ( BKPF). Next when SELECT ... FOR ALL ENTRIES IN ... is executed application server will take a little portions of header rows (typically 5) and construct SQL queries for retrieving packs of items, corresponding to that portions. Next all that portions will merged in single internal table. So there will be only items of desired document as it were if you could execute normal join.

Answer 3

Here's what happens the way I understand it. The two statements are probably executed after another:
The first statement selects a few entries from the bkpf table. These entries are stored in the internal table it_bkpf (say belnr 1, 2, 3).
Each of these entries is then used as part of the select #2. The "for all entries" matches the belnr in table bseg to those in the internal table it_bkpf from the first statement. The matching entries are then put into the internal table it_bseg.
With the example you've given this is pretty much the same if the where clause in SQL #2 was where belnr in s_belnr (instead of the whole for all entries). This would only make sense if you needed the contents of it_bkpf for some other purpose. Another typical situation is if you determine the contents of the internal table used in the for all entries clause with some program logic instead of reading it directly from the database.
One catch with "for all entries": Make sure the internal table in the for all entries is not empty - then the whole table in the from clause would be selected.