Introduction to Cursors in DB2

DB2 is a database management system that supports the use of cursors. Cursors provide a way for programmers to manipulate rows in a result set one at a time. This can be useful in situations where it is not practical to retrieve the entire result set at once, or where the programmer needs to perform some processing on each row before moving to the next one.

What is a Cursor?

A cursor is a database object that allows a programmer to retrieve rows from a result set one at a time. The programmer can move forward or backward through the result set, or skip to a specific row. Cursors are used to process result sets in a procedural manner, rather than a set-based manner.

Types of Cursors

DB2 supports the following types of cursors:

• Static Cursor: A static cursor retrieves a snapshot of the result set. The cursor does not change as other users modify the underlying data. This type of cursor is useful when the result set is small and changes to the data are infrequent.

• Dynamic Cursor: A dynamic cursor retrieves the current rows in the result set. The cursor changes as other users modify the underlying data. This type of cursor is useful when the result set is large and changes to the data are frequent.

• Keyset-Driven Cursor: A keyset-driven cursor retrieves a subset of the result set based on a key range. The cursor changes as other users modify the underlying data. This type of cursor is useful when the result set is large and changes to the data are frequent, but the subset of data that the programmer is interested in is relatively small.

Using Cursors

To use a cursor in DB2, the programmer must first declare the cursor and define its properties, such as the type of cursor, the SQL statement that will be used to retrieve the result set, and any parameters that are required for the statement. Once the cursor is declared, the programmer can open it, which retrieves the first row of the result set. The programmer can then perform whatever processing is required on the row, and then move to the next row using a fetch statement. When all rows have been processed, the programmer can close the cursor.

Here is an example of how to use a cursor in DB2:

DECLARE c1 CURSOR FOR
   SELECT col1, col2, col3 
   FROM mytable 
   WHERE col1 > 100;

OPEN c1;

FETCH FROM c1 INTO :var1, :var2, :var3;

WHILE SQLCODE = 0 DO
   -- process the row here
   -- ...

   FETCH FROM c1 INTO :var1, :var2, :var3;
END WHILE;

CLOSE c1;

In this example, a cursor named c1 is declared, which retrieves the columns col1, col2, and col3 from the table mytable where col1 is greater than 100. The cursor is then opened, and the first row is fetched into the variables var1, var2, and var3. The programmer can then process the row, and fetch the next row using a loop. When all rows have been processed, the cursor is closed.

Conclusion

Cursors provide an additional level of granularity for processing result sets in DB2. They allow programmers to manipulate rows one at a time, and provide more control over the processing of large result sets. By using a cursor, a programmer can perform processing on each row before moving to the next one, or skip over rows that are not needed.