Cursors in SQL Server

Question

Can you please explain about Cursors and when it will be useful in real time applications or scenarios

Jaish Mathews · Answer

What is a SQL Server Cursor? A SQL Server Cursor is a database object used to retrieve manipulate and navigate through a result set one row at a time . Unlike a set-based operation (the usual SQL operations) which processes all rows at once a cursor allows row-by-row processing of a query result. Cursors are particularly useful when you need to perform operations on each row individually which cannot be achieved easily using set-based SQL queries. Types of Cursors in SQL Server Static Cursor : Creates a snapshot of the data and does not reflect changes made to the underlying data after the cursor is opened. Useful for reporting or scenarios where consistent data is required. Dynamic Cursor : Reflects all changes made to the underlying data (like inserts updates or deletes). Useful when you need real-time updates. Forward-Only Cursor : Allows movement in one direction (forward) through the result set. More efficient and lightweight than other cursor types. Use this if you only need sequential row processing. Keyset-Driven Cursor : Reflects changes to the data but does not show new rows added to the underlying data. Useful when you need updates but don't care about new rows. Syntax of a SQL Server Cursor DECLARE cursor_name CURSOR FOR SELECT column1 column2 FROM table_name WHERE condition; -- Open the cursor OPEN cursor_name; -- Fetch the first row FETCH NEXT FROM cursor_name INTO @variable1 @variable2; -- Loop through rows WHILE @@FETCH_STATUS = 0 BEGIN -- Perform operations on the current row PRINT @variable1; PRINT @variable2; -- Fetch the next row FETCH NEXT FROM cursor_name INTO @variable1 @variable2; END; -- Close and deallocate the cursor CLOSE cursor_name; DEALLOCATE cursor_name; When to Use Cursors in Real-Time Scenarios Cursors are often considered inefficient compared to set-based operations but can be useful in specific real-time scenarios: 1. Row-by-Row Processing Example: Processing payroll for employees where specific computations depend on the previous row's results or custom logic. Why Use a Cursor? The row-by-row logic cannot be expressed in a single SQL query. 2. Batch Processing Example: Splitting data into smaller chunks for processing or export to external systems. Why Use a Cursor? Allows controlled iteration over large datasets. 3. Dynamic SQL Execution Example: Executing a series of stored procedures dynamically based on the data in a table. Why Use a Cursor? Enables sequential execution of dynamically generated commands. 4. Debugging and Logging Example: Logging changes or debugging issues in a result set row by row. Why Use a Cursor? Easy to log or capture data from each row for review. 5. Processing Data with Dependencies Example: Updating inventory quantities based on sales transactions in a sequential order. Why Use a Cursor? Ensures that updates occur in the correct sequence. Disadvantages of Cursors Performance Overhead : Cursors are slower than set-based operations due to row-by-row processing. Resource-Intensive :Cursors consume memory and server resources especially with large result sets. Complexity : Writing and maintaining cursor-based logic can be more complex than set-based SQL. Best Practices Avoid Cursors if Possible : Try to rewrite logic using set-based operations joins or window functions. Use Temporary Tables or Table Variables : Store intermediate results in a temporary table and process them with set-based queries. Use Forward-Only Cursors for Performance : If row-by-row processing is mandatory prefer forward-only cursors for better performance. Close and Deallocate Cursors : Always close and deallocate cursors to free up resources.