HP 3000 Manuals

ALLBASE/SQL FORTRAN Application Programming Guide

Introducing the Cursor 

You use a cursor to manage a query result that may contain more than one
row when you want to make all the qualifying rows available to the
program user.  Cursors are used in sequential table processing as shown
later in this chapter.

Like the cursor on a terminal screen, an ALLBASE/SQL cursor is a position
indicator.  It does not, however, point to a column.  Rather, it points
to one row in an active set.  An active set is a query result obtained
when a SELECT command associated with a cursor (defined in a DECLARE
CURSOR command) is executed (using the OPEN command).

Each cursor used in a program must be declared before it is used.  You
use the DECLARE CURSOR command to declare a cursor.  The DECLARE CURSOR
command names the cursor and associates it with a particular SELECT
command:

      EXEC SQL DECLARE Cursor1
     1           CURSOR FOR
     2           SELECT PartName,
     3                  SalesPrice
     4             FROM PurchDB.Parts
     5            WHERE PartNumber BETWEEN :LowValue AND :HighValue
     6         ORDER BY PartName

All cursor names within one program must be unique.  You use a cursor
name when you perform data manipulation operations using the cursor.

The SELECT command in the cursor declaration does not specify any output 
host variables.  The SELECT command can, however, contain input host 
variables, as in the WHERE clause of the cursor declaration above.

Rows in the active set are returned to output host variables when the
FETCH command is executed:

     EXEC SQL OPEN Cursor1
     .
     .     The OPEN command examines any input host
     .       variables and determines the active set.
          .
     EXEC SQL FETCH Cursor1 INTO OutputHostVariables 

          .       The FETCH command delivers one row of 
          .       the active set into output host variables.
          .
          .

If a serial scan will be used to retrieve the active set, ALLBASE/SQL
locks the table(s) when the OPEN command is executed.  If an index scan 
will be used, locks are placed when rows are fetched.  Any locks obtained
are held until the transaction terminates or the CLOSE command is
executed.

Both the OPEN and the FETCH commands position the cursor:

   *   The OPEN command positions the cursor before the first row of the
       active set.

   *   The FETCH command advances the cursor to the next row of the
       active set and delivers that row to the output host variables.

The row at which the cursor points at any one time is called the current 
row.  When a row is a current row, you can delete it as follows:

      EXEC SQL DELETE FROM PurchDB.Parts
     1                WHERE CURRENT OF Cursor1

When you delete the current row, the cursor remains between the row
deleted and the next row in the active set until you execute the FETCH
command again:

      EXEC SQL FETCH  Cursor1
     1           INTO :PartName :PartNameInd,
     2                :SalesPrice :SalesPriceInd

When a row is a current row you can update it if the cursor declaration
contains a FOR UPDATE OF clause naming the column(s) you want to change.
The following cursor, for example, can be used to update the SalesPrice 
column of the current row by using the WHERE CURRENT OF option in the
UPDATE command:

      EXEC SQL DECLARE Cursor2
     1           CURSOR FOR
     2           SELECT PartName, SalesPrice
     3             FROM PurchDB.Parts
     4            WHERE PartNumber BETWEEN :LowValue AND :HighValue
     5       FOR UPDATE OF SalesPrice
      .
      .     Because the DECLARE CURSOR command is not 
      .     executed at runtime, no status checking code 
      .     needs to appear here.
      .
      EXEC SQL OPEN Cursor2
      .
      .     The OPEN command examines any input host 
      .     variables and determines the active set. 
      .     Then the program fetches one row at a time.
      .
      EXEC SQL FETCH  Cursor2
     1           INTO :PartName :PartNameInd,
     2                :SalesPrice :SalesPriceInd
      .
      .     If the program user wants to change the SalesPrice 
      .     of the row displayed (the current row), the UPDATE 
      .     command is executed.  The new SalesPrice entered by 
      .     the user is stored in an input host variable named 
      .     NewSalesPrice.
            .
      EXEC SQL UPDATE PurchDB.Parts
     1             SET SalesPrice = :NewSalesPrice
     2           WHERE CURRENT OF Cursor2

      .     After the UPDATE command is executed, the updated 
      .     row remains the current row until the FETCH command 
      .     is executed again.
      .

The restrictions that govern deletions and updates using a view also 
govern deletions and updates using a cursor.  You cannot delete or update
a row using a cursor if the cursor declaration contains any of the
following:

   *   Join operation

   *   Aggregate function

   *   DISTINCT

   *   GROUP BY

   *   ORDER BY

   *   UNION

After the last row in the active set has been fetched, the cursor is
positioned after the last row fetched and the value in SQLCode is equal
to 100.  Therefore to retrieve all rows in the active set, you execute
the FETCH command until SQLCode is not 0:

      DO WHILE (SQLCode .EQ. 0)
       EXEC SQL FETCH  Cursor3
     1           INTO :PartNumber,
     2                :PartName :PartNameInd,
     3                :SalesPrice :SalesPriceInd

       IF (SQLCode .EQ. 0) THEN
         CALL DisplayRow
       ELSEIF (SQLCA.SQLCode .EQ.100) THEN
         CALL WriteOut ('Row Not Found or No More Rows!')
        ELSE
         CALL SQLStatusCheck
       ENDIF
      END DO

When you are finished operating on an active set, you use the CLOSE
command:

     EXEC SQL CLOSE Cursor3

When you close a cursor, the active set becomes undefined and you cannot
use the cursor again unless you issue an OPEN command to reopen it.  The
COMMIT WORK and ROLLBACK WORK commands also close any open cursors,
automatically.  Figure 6-2 summarizes the effect of the cursor-related
commands on the position of the cursor and on the active set.


          Figure 6-2.  Effect of SQL Commands on Cursor and Active Sets 

MPE/iX 5.0 Documentation