🧵 SQL CURSORS – Complete Guide with Syntax & Examples

🧲 Introduction – Why Use SQL Cursors?

SQL is designed to handle set-based operations efficiently. However, there are times when you need to process records one row at a time—like generating custom reports, performing procedural logic, or iterating through complex datasets. That’s where SQL Cursors come in.

A cursor in SQL acts like a pointer that traverses a result set row-by-row. While not ideal for performance, they provide essential control in scenarios where row-level processing is unavoidable.

🎯 In this guide, you’ll learn:

• What cursors are and how they work
• Cursor types and lifecycle
• Syntax and examples across databases
• Best practices and performance tips

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🔑 1. What Are SQL Cursors?

A cursor is a temporary work area created in the system memory where the result set is stored. It enables row-by-row processing rather than the typical set-based processing.

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🔁 2. Cursor Workflow Steps

Step	Description
Declare	Define the cursor with a SELECT statement
Open	Execute the SELECT and populate the cursor
Fetch	Retrieve each row into variables
Process	Execute logic for each row
Close	Release the cursor
Deallocate	Free the memory associated with the cursor

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🛠️ 3. SQL Server Cursor Example

DECLARE @Name VARCHAR(50);

DECLARE employee_cursor CURSOR FOR
SELECT name FROM employees;

OPEN employee_cursor;

FETCH NEXT FROM employee_cursor INTO @Name;

WHILE @@FETCH_STATUS = 0
BEGIN
    PRINT 'Employee Name: ' + @Name;
    FETCH NEXT FROM employee_cursor INTO @Name;
END;

CLOSE employee_cursor;
DEALLOCATE employee_cursor;

✅ Explanation:

• DECLARE CURSOR – Sets up the result set.
• OPEN – Loads the rows.
• FETCH NEXT – Iterates through rows.
• @@FETCH_STATUS – Checks for end of result set.

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🔎 4. MySQL Cursor Example (Stored Procedure)

DELIMITER //

CREATE PROCEDURE list_names()
BEGIN
  DECLARE done INT DEFAULT FALSE;
  DECLARE emp_name VARCHAR(100);
  DECLARE cur CURSOR FOR SELECT name FROM employees;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

  OPEN cur;

  read_loop: LOOP
    FETCH cur INTO emp_name;
    IF done THEN
      LEAVE read_loop;
    END IF;
    SELECT emp_name;
  END LOOP;

  CLOSE cur;
END //

DELIMITER ;

✅ Explanation:

• MySQL cursors must be used inside stored procedures.
• HANDLER FOR NOT FOUND is used to exit the loop.

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🧪 5. Cursor Types in SQL Server

Cursor Type	Description
STATIC	Copies data into temp table; changes not visible
DYNAMIC	Reflects all changes in underlying data
FORWARD_ONLY	Allows only forward movement
KEYSET	Key values static, data dynamic

💡 Use FORWARD_ONLY for best performance when possible.

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📘 Cursor Use Cases

Scenario	Example Purpose
Row-level data manipulation	Generating custom invoices
Complex validation logic	Checking constraints across tables
Auditing or logging	Row-based tracking of changes
Conditional processing	Running logic only on filtered records

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💡 Best Practices & ⚠️ Gotchas

✅ Best Practices	⚠️ Avoid This
Use cursors only when necessary	Using cursors for simple data logic
Prefer FORWARD_ONLY for efficiency	Updating/deleting from open cursors
Always CLOSE and DEALLOCATE properly	Leaving cursors open (memory leaks)
Minimize logic inside loops	Nested cursors (unless justified)

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📊 Cursor vs WHILE Loop

Feature	CURSOR	WHILE Loop + SELECT
Performance	Slower (row-by-row)	Better for set-based logic
Control	Higher (row-level granularity)	Less control, but often sufficient
Usage Scope	Complex procedures, audits	Simple loops or bulk actions

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📌 Summary – Recap & Next Steps

SQL Cursors provide row-by-row control over data, useful in specific scenarios like report generation, data validation, or when looping is required. Use them wisely due to their performance cost.

🔍 Key Takeaways:

• SQL Cursors are ideal for procedural row-level operations.
• Must follow a clear DECLARE → OPEN → FETCH → CLOSE → DEALLOCATE pattern.
• Always evaluate if a set-based alternative is more efficient.

⚙️ Real-World Relevance:
Cursors are used in data migration scripts, legacy systems, and batch-processing jobs in banking, ERP, and data warehousing systems.

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❓ FAQ – SQL Cursors

❓ When should I use a SQL Cursor?

✅ Use cursors when you need row-by-row logic like custom calculations, auditing, or sequential data transformation.

❓ Are SQL Cursors bad for performance?

✅ Yes, they are slower than set-based operations. Use them only when necessary and keep logic minimal.

❓ Can I update data using a cursor?

✅ Yes, but only with updatable cursors. Be cautious as this can lead to locks or performance issues.

❓ What’s the alternative to cursors?

✅ Use window functions, ROW_NUMBER(), CTEs, or WHILE loops with set operations when possible.

❓ Are cursors supported in all databases?

✅ Yes, but the syntax varies. Example:

• SQL Server: supports full cursor control.
• MySQL: only in stored procedures.
• PostgreSQL: supports cursors with DECLARE and FETCH.

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