Select for all entries

Question

Hi,
      I am new in abap reports. Now i want to know why we should use select for all entries in query. We can do retrieve directly by accessing the table in database dictionary.
      Experts please give me the reasons I want to know the concepts behind it.It will be better if you kindly explain this with help of code.
     With regards,
       Abir.

Answer 1

HI
GOOD
SELECT
Basic form
SELECT result [target] FROM source [where] [GROUP BY fields] [ORDER BY order].
Effect
Retrieves an extract and/or a set of data from a database table or view (see Relational database ). SELECT belongs to the OPEN SQL command set.
Each SELECT command consists of a series of clauses specifying different tasks:
The SELECT result clause specifies
whether the result of the selection is a table or a single record,
which columns the result is meant to have and
whether the result is allowed to include identical lines.
The INTO target clause specifies the target area into which the selected data is to be read. If the target area is an internal table, the INTO clause specifies
whether the selected data is to overwrite the contents of the internal table or
whether the selected data is to be appended to the contents and
whether the selected data is to be placed in the internal table all at once or in several packets.
The INTO clause can also follow the FROM clause.
You can omit the INTO clause. The system then makes the data available in the table work area (see TABLES ) dbtab . If the SELECT clause includes a "*", the command is processed like the identical SELECT * INTO dbtab FROM dbtab statement. If the SELECT clause contains a list a1 ... an , the command is executed like SELECT a1 ... an INTO CORRESPONDING FIELDS OF dbtab FROM dbtab .
If the result of the selection is meant to be a table, the data is usually (for further information, see INTO -Klausel ) read line by line within a processing loop introduced by SELECT and concluded by ENDSELECT . For each line read, the processing passes through the loop once. If the result of the selection is meant to be a single record, the closing ENDSELECT is omitted.
The FROM source clause the source (database table or view ) from which the data is to be selected. It also determines
the type of client handling,
the behavior for buffered tables and
the maximum number of lines to be read.
The WHERE where clause specifies the conditions which the result of the selection must satisfy. It thus determines the lines of the result table. Normally - i.e. unless a client field is specified in the WHERE clause - only data of the current client is selected. If you want to select across other clients, the FROM clause must include the addition ... CLIENT SPECIFIED .
The GROUP-BY fields clause combines groups of lines together into single lines. A group is a set of lines which contain the same value for every database field in the GROUP BY clause.
The ORDER-BY order clause stipulates how the lines of the result table are to be ordered.
Each time the SELECT statement is executed, the system field SY-DBCNT contains the number of lines read so far. After ENDSELECT , SY-DBCNT contains the total number of lines read.
The return code value is set as follows:
SY-SUBRC = 0 At least one line was read.
SY_SUBRC = 4 No lines were read.
SY-SUBRC = 8 The search key was not fully qualified.
(nur bei SELECT SINGLE ). The returned single record is any line of the solution set.
Example
Output the passenger list for the Lufthansa flight 0400 on 28.02.1995:
TABLES SBOOK.
SELECT * FROM SBOOK
  WHERE
    CARRID   = 'LH '      AND
    CONNID   = '0400'     AND
    FLDATE   = '19950228'
  ORDER BY PRIMARY KEY.
  WRITE: / SBOOK-BOOKID, SBOOK-CUSTOMID,   SBOOK-CUSTTYPE,
           SBOOK-SMOKER, SBOOK-LUGGWEIGHT, SBOOK-WUNIT,
           SBOOK-INVOICE.
ENDSELECT.
Note
Performance
In client/server environments, storing database tables in local buffers (see SAP buffering ) can save considerable amounts of time because the time required to make an access via the network is much more than that needed to access a locally buffered table.
Notes
A SELECT command on a table for which SAP buffering is defined in the ABAP/4 Dictionary is normally satisfied from the SAP buffer by bypassing the database. This does not apply with
- SELECT SINGLE FOR UPDATE
- SELECT DISTINCT in the SELECT clause ,
- BYPASSING BUFFER in the FROM clause ,
- ORDER BY f1 ... fn in the ORDER-BY clause ,
- aggregate functions in the SELECT clause ,
- when using IS [NOT] NULL WHERE condition ,
or if the generic key part is not qualified in the WHERE-Bedingung for a generically buffered table.
Authorization checks are not supported by the SELECT statement, so you must program these yourself.
In dialog systems, the database system locking mechanism cannot always guarantee to synchronize the simultaneous access of several users to the same dataset. In many cases, it is therefore advisable to use the SAP locking mechanism .
Changes to data in a database are only finalized after a database commit (see LUW ). Prior to this, any database update can be reversed by a database rollback (see Programming transactions ). At the lowest isolation level (see the section on the "uncommitted read" under Locking mechanism ), this can result in the dataset selected by the SELECT command not really being written to the database. While a program is selecting data, a second program can add, change or delete lines at the same time. Then, the changes made by the second program are reversed by rolling back the database system. The selection of the first program thus reflects only a very temporary state of the database. If such "phantom data" is not acceptable for a program, you must either use the SAP locking mechanism or at least set the isolation level of the database system to "committed read" (see Locking mechanism ).
In a SELECT-ENDSELECT loop, the CONTINUE statement terminates the current loop pass prematurely and starts the next.
If one of the statements in a SELECT ... ENDSELECT loop results in a database commit, the cursor belonging to the SELECT ... ENDSELECT loop is lost and the processing terminates with a runtime error. Since each screen change automatically generates a database commit, statements such as CALL SCREEN , CALL DIALOG , CALL TRANSACTION or MESSAGE are not allowed within a SELECT ... ENDSELECT loop.
Related OPEN CURSOR , FETCH und CLOSE CURSOR
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http://www.geocities.com/SiliconValley/Campus/6345/select.htm
THANKS
MRUTYUN