How do you use the CURSOR data type to declare and use cursors in stored procedures and scripts?
PaulAnd
Posted by PaulAnd
Last Updated: July 06, 2024
The CURSOR data type in SQL is used to declare cursors, which allow you to iterate through a result set one row at a time. Cursors are particularly useful when you need to perform operations on a row-by-row basis in stored procedures or scripts. Below is a step-by-step guide on how to declare and use cursors within a stored procedure in SQL Server:
Declaring and Using Cursors
1. Declare the Cursor: You first declare a cursor using the DECLARE statement. You define the SQL query that generates the result set you want to iterate over. 2. Opening the Cursor: After the cursor is declared, you must open it using the OPEN statement. 3. Fetching Data: You then fetch the rows from the cursor using the FETCH NEXT statement. This retrieves the current row and moves to the next one. 4. Processing the Data: You can process the data fetched from the cursor by using the values in your business logic. 5. Closing and Deallocating the Cursor: Once you are done with the cursor, you should CLOSE and DEALLOCATE it to free up resources.
Example Stored Procedure
Here's an example of using a cursor within a stored procedure in SQL Server: 
CREATE PROCEDURE ProcessEmployeeSalaries
AS
BEGIN
    DECLARE @EmployeeID INT,
            @EmployeeName NVARCHAR(100),
            @Salary DECIMAL(10, 2);

    -- Step 1: Declare the cursor
    DECLARE EmployeeCursor CURSOR FOR
        SELECT EmployeeID, EmployeeName, Salary
        FROM Employees
        WHERE Salary > 50000;  -- Example filter

    -- Step 2: Open the cursor
    OPEN EmployeeCursor;

    -- Step 3: Fetch the first row
    FETCH NEXT FROM EmployeeCursor INTO @EmployeeID, @EmployeeName, @Salary;

    -- Step 4: Loop through the rows
    WHILE @@FETCH_STATUS = 0
    BEGIN
        -- Process data (example: print employee information)
        PRINT 'Employee ID: ' + CAST(@EmployeeID AS NVARCHAR(10)) +
              ', Name: ' + @EmployeeName +
              ', Salary: ' + CAST(@Salary AS NVARCHAR(10));

        -- Fetch the next row
        FETCH NEXT FROM EmployeeCursor INTO @EmployeeID, @EmployeeName, @Salary;
    END

    -- Step 5: Close and deallocate the cursor
    CLOSE EmployeeCursor;
    DEALLOCATE EmployeeCursor;
END
Explanation of the Code
1. Cursor Declaration: The cursor EmployeeCursor is declared to select EmployeeID, EmployeeName, and Salary from the Employees table where salaries exceed 50,000. 2. Cursor Opening: The OPEN EmployeeCursor; statement opens the cursor and prepares it for fetching. 3. Fetching Rows: Initial fetch is done to get the first row. It continues to fetch subsequent rows within a WHILE loop as long as the @@FETCH_STATUS is 0 (indicating that a row was fetched successfully). 4. Data Processing: In this example, the employee details are printed out. You can replace this logic with any processing you need to perform. 5. Resource Management: Finally, remember to close the cursor when done, and deallocate it to free the associated resources.
Best Practices
- Avoid Cursors When Possible: Cursors can lead to performance issues because they process rows one at a time. Whenever possible, try to use set-based operations. - Use FAST_FORWARD Option: If you only need read-only access and do not need to scroll backwards, using FAST_FORWARD cursors can improve performance. - Minimal Usage: Only use cursors where necessary and in circumstances that cannot be handled by set-based operations.
Conclusion
Using cursors allows more granular control over data processing when you need to work with result sets on a row-by-row basis. However, they should be used judiciously to avoid performance pitfalls associated with iterative processing.
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