In SQL, when working with cursors, the FETCH statement is used to retrieve rows from a result set one at a time or in specified sequences. The FETCH statement can be combined with different keywords such as NEXT, PRIOR, FIRST, and LAST to control which row to fetch relative to the cursor's position. Below is a brief overview of each of these keywords and how they can be used with a cursor.
Setting Up a Cursor
Before using FETCH, you need to declare and open a cursor, and then you can use the FETCH statement to retrieve rows. Here's a basic structure for working with cursors:
DECLARE cursor_name CURSOR FOR
SELECT column1, column2 FROM table_name;
OPEN cursor_name;
-- Now you can use FETCH statements
FETCH Statements
1. FETCH NEXT
- Retrieves the next row in the result set from the cursor's current position. If the cursor is positioned on the last row, it will not return any rows (and the cursor will be considered closed).
FETCH NEXT FROM cursor_name;
2. FETCH PRIOR
- Retrieves the previous row from the cursor's current position. If the cursor is positioned on the first row, it will not return any rows.
FETCH PRIOR FROM cursor_name;
3. FETCH FIRST
- Retrieves the first row of the result set regardless of the current cursor position. If you need to retrieve the very first row at any time, this is the command to use.
FETCH FIRST FROM cursor_name;
4. FETCH LAST
- Retrieves the last row of the result set. Similar to FETCH FIRST, it allows you to jump to the end of the result set.
FETCH LAST FROM cursor_name;
Example SQL Cursor Usage
Here’s an example demonstrating how to use these FETCH statements in a cursor context:
-- Declare the cursor
DECLARE my_cursor CURSOR FOR
SELECT column1 FROM my_table ORDER BY column1;
-- Open the cursor
OPEN my_cursor;
-- Fetch the first row
FETCH FIRST FROM my_cursor;
-- Process the first row here
-- Fetch the next row
FETCH NEXT FROM my_cursor;
-- Process the next row here
-- Fetch the last row
FETCH LAST FROM my_cursor;
-- Process the last row here
-- Fetch the previous row from the last position
FETCH PRIOR FROM my_cursor;
-- Process the previous row here
-- Clean up
CLOSE my_cursor;
DEALLOCATE my_cursor;
Important Notes
- Always remember to close and deallocate your cursor after your operations to free up resources.
- The behavior of FETCH PRIOR might be different if the cursor is declared as forward-only.
- Make sure to handle the cases where fetching a row might be out of bounds (for example, trying to fetch prior when the cursor is on the first row).
Conclusion
Using FETCH NEXT, FETCH PRIOR, FETCH FIRST, and FETCH LAST, you can navigate through the rows returned by a cursor effectively, according to your needs.