Q(uick)BASIC Function: SAVEPOINT

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SAVEPOINT

A function that marks the beginning of a subset of ISAM database operations in a transaction

Worth knowing

Useful and cross-version information about the programming environments of QBasic and QuickBasic.

Syntax

SAVEPOINT

Description/Parameter(s)

Usage Notes

The SAVEPOINT function marks the beginning of a subset of ISAM operations within a transaction that can be rescinded using a ROLLBACK statement. SAVEPOINT returns an integer that refers to the savepoint.
Transactions are a way to group a series of ISAM operations so that you can commit them as a whole, rescind them all, or rescind operations since a designated savepoint. Use BEGINTRANS to indicate the beginning of a transaction and COMMITTRANS to commit all operations since the beginning of a transaction.
Use ROLLBACK with a savepoint% argument to return the data affected by the transaction to its state at that savepoint.
Use ROLLBACK with no argument to return the data affected by the transaction to its state at the previous savepoint, or at the beginning of the transaction if there are no intermediate savepoints.
ROLLBACK ALL rescinds all ISAM operations in a transaction and returns the data to its initial state at the beginning of the transaction.
If there is no transaction pending when you use SAVEPOINT, BASIC generates the error message, "Illegal function call."

Example

This example uses the DELETE statement to remove records from an ISAM file. It creates a transaction with the BEGINTRANS and COMMITTRANS statements, and uses the SAVEPOINT function and ROLLBACK statement within the transaction to provide rollback of any or all of the deletions.
Note: To run this program, you must load the ISAM TSR program PROISAM.EXE. Also, this program assumes a file called BOOKS.MDB exists in the current directory. BOOKS.MDB is a sample ISAM file that SETUP copies to your disk.

DEFINT A-Z
TYPE Borrower
    Cardnum AS LONG             'Card number.
    TheName AS STRING * 36      'Name.
    Street AS STRING * 50       'Street.
    City AS STRING * 26         'City.
    State AS STRING * 2         'State.
    Zip AS LONG                 'Zip code.
END TYPE
 
DIM People AS Borrower          'Record structure variable.
CONST Database = "BOOKS.MDB"    'Name of the disk file.
CONST Tablename = "Cardholders" 'Name of the table.
CONST viewbottom = 17, viewtop = 3, msgtxt = " *** Deleted: Savepoint "
DIM SavePts(viewbottom - viewtop + 1)
 
    TableNum = FREEFILE
    OPEN Database FOR ISAM Borrower Tablename AS TableNum
 
    'Loop until user chooses to quit.
    DO
        VIEW PRINT
        CLS : COLOR 15, 0
        PRINT SPC(34); "Card Holders"
        PRINT "Card#"; SPC(2); "Name"; SPC(13); "Street";
        PRINT SPC(20); "City"; SPC(6); "State"; SPC(2); "Zip"
        LOCATE 24, 1: PRINT "Choose a key:    ";
        PRINT "D - Delete this record    N - Next record    Q - Quit";
        MOVEFIRST TableNum
        VIEW PRINT viewtop TO viewbottom: COLOR 7, 0: CLS
        vPos = viewtop
 
        'Loop through a screenful of records.
        DO
            'For each record, display and ask user what to do with it.
            RETRIEVE TableNum, People
            LOCATE vPos, 1
            PRINT USING ("#####"); People.Cardnum;
            PRINT "  "; LEFT$(People.TheName, 15); "  ";
            PRINT LEFT$(People.Street, 25); "  ";
            PRINT LEFT$(People.City, 10); "  "; People.State; "   ";
            PRINT USING ("#####"); People.Zip
 
            'Get keystroke from user.
            validkeys$ = "DNQ"
            DO
                keychoice$ = UCASE$(INKEY$)
            LOOP WHILE INSTR(validkeys$, keychoice$) = 0 OR keychoice$ = ""
 
            'Process keystroke.
            SELECT CASE keychoice$
            CASE "D"
                IF NOT inTransaction THEN
                    inTransaction = -1
                    BEGINTRANS
                END IF
                NumSavePts = NumSavePts + 1
                SavePts(NumSavePts) = SAVEPOINT
                DELETE TableNum
                LOCATE vPos, 7: PRINT msgtxt; NumSavePts; SPC(79 - POS(0));
            CASE "Q"
                EXIT DO
            CASE "N"
                MOVENEXT TableNum
            END SELECT
            vPos = vPos + 1
        LOOP UNTIL EOF(TableNum) OR vPos = viewbottom
 
        'If user didn't delete any records, simply quit.
        IF NumSavePts = 0 THEN EXIT DO
 
        'Allow user to commit deletions, or roll back some or all of them.
        VIEW PRINT: LOCATE 24, 1: PRINT "Choose a key:    ";
        PRINT "R - Rollback to a savepoint   A - Rollback all deletions"
        PRINT SPC(17); "Q - commit deletions and Quit";
        validkeys$ = "RAQ"
        DO
            keychoice$ = UCASE$(INKEY$)
        LOOP WHILE INSTR(validkeys$, keychoice$) = 0 OR keychoice$ = ""
        SELECT CASE keychoice$
        CASE "R"
            VIEW PRINT 24 TO 25: PRINT : PRINT
            DO
                PRINT "Roll back to which savepoint ( 1 -"; NumSavePts; ")";
                INPUT RollbackPt
            LOOP UNTIL RollbackPt > 0 AND RollbackPt <= NumSavePts
            ROLLBACK SavePts(RollbackPt)
            NumSavePts = RollbackPt - 1
        CASE "A"
            NumSavePts = 0
            ROLLBACK ALL
        CASE "Q"
            EXIT DO
        END SELECT
    LOOP
 
    IF inTransaction THEN COMMITTRANS
    CLOSE
    END