SQL - Cursor Functions

SQL - Cursors

In SQL, a cursor is a database object that allows you to traverse the rows of a result set one by one. It acts as a pointer to a single row, enabling row-by-row processing, which is contrary to the set-based nature of SQL.

Warning: Avoid Cursors When Possible

In modern SQL development, cursors are generally considered an anti-pattern and should be avoided. SQL is designed for powerful, efficient set-based operations (operating on all rows at once). Cursors force a procedural, row-by-row approach (RBAR - Row By Agonizing Row), which is typically slow, inefficient, and harder to read and maintain.

Use cursors only as a last resort for tasks that are impossible to perform with set-based logic, such as complex administrative tasks, executing dynamic SQL row by row, or interfacing with legacy systems that require it.

Modern Alternatives to Cursors

Most problems that seem to require a cursor can be solved more efficiently with modern SQL features like:

• Window Functions: For calculations that depend on other rows, like running totals, rankings, and moving averages.
• Common Table Expressions (CTEs): For breaking down complex queries into logical, readable steps.
• Temporary Tables: For storing intermediate results in a complex multi-step process.

Example: The Modern Way vs. The Cursor Way

Let’s say we want to calculate a running total of sales. Here is the modern, efficient approach using a window function:

-- Modern, Set-Based Approach (Efficient)
SELECT
    SaleDate,
    Amount,
    SUM(Amount) OVER (ORDER BY SaleDate) AS RunningTotal
FROM Sales;

The cursor-based approach (shown for demonstration purposes only) is much more verbose, complex, and significantly slower on large datasets.

Cursor Functions (SQL Server / T-SQL)

The functions described below are specific to Microsoft SQL Server’s Transact-SQL (T-SQL) dialect. They return information about the state of a cursor.

All cursor functions are nondeterministic, meaning they can return different results each time they are called, even with the same input values, because they depend on the cursor’s current state.

Function	Description
@@CURSOR_ROWS	Returns the number of rows in the last-opened cursor. Returns 0 if no cursor is open or the last cursor was closed or deallocated.
@@FETCH_STATUS	Returns the status of the last FETCH statement issued against any cursor currently open. Returns 0 for a successful fetch, -1 for a fetch beyond the end of the result set, and -2 for a missing row.
CURSOR_STATUS	A scalar function that indicates the status of a specific cursor by name or a cursor variable. Helps determine if a cursor is open, closed, or does not exist.

A Note on Learning

While it’s useful to know what cursors are, especially if you encounter them in legacy code, your learning efforts should be focused on mastering set-based operations and modern SQL features. This will lead to more efficient, scalable, and maintainable solutions.