IBM DB2 Question:

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Updating a column: You can update columns in the rows that
you retrieve. Updating a row after you use a cursor to
retrieve it is called a positioned update. If you intend to
perform any positioned updates on the identified table,
include the FOR UPDATE clause. The FOR UPDATE clause has
two forms:
• The first form is FOR UPDATE OF column-list. Use
this form when you know in advance which columns you need
to update.
• The second form is FOR UPDATE, with no column list.
Use this form when you might use the cursor to update any
of the columns of the table.
For example, you can use this cursor to update only the
SALARY column of the employee table:

EXEC SQL
DECLARE C1 CURSOR FOR
SELECT EMPNO, FIRSTNME, MIDINIT, LASTNAME, SALARY
FROM DSN8810.EMP X
WHERE EXISTS
(SELECT *
FROM DSN8810.PROJ Y
WHERE X.EMPNO=Y.RESPEMP
AND Y.PROJNO=:GOODPROJ)
FOR UPDATE OF SALARY;

If you might use the cursor to update any column of the
employee table, define the cursor like this:

EXEC SQL
DECLARE C1 CURSOR FOR
SELECT EMPNO, FIRSTNME, MIDINIT, LASTNAME, SALARY
FROM DSN8810.EMP X
WHERE EXISTS
(SELECT *
FROM DSN8810.PROJ Y
WHERE X.EMPNO=Y.RESPEMP
AND Y.PROJNO=:GOODPROJ)
FOR UPDATE;

DB2 must do more processing when you use the FOR UPDATE
clause without a column list than when you use the FOR
UPDATE clause with a column list.