CURSOR concept and usage in SQL

Introduction

A cursor lets you go through rows in a table one by one. It helps when you want to do something with each row separately.

When you need to process each row of a query result step by step.

When you want to update or check rows one at a time.

When a task depends on the data from the previous row.

When you cannot do the work with a single SQL statement.

When you want to print or log each row's data during processing.

Syntax

SQL

DECLARE cursor_name CURSOR FOR select_statement;
OPEN cursor_name;
FETCH NEXT FROM cursor_name INTO variable_list;
-- Repeat FETCH as needed
CLOSE cursor_name;
DEALLOCATE cursor_name;

You first declare the cursor with a SELECT query.

Then you open it to start reading rows.

FETCH gets the next row into variables.

Close and deallocate when done to free resources.

Examples

Declare a cursor named emp_cursor to select employee IDs and names.

SQL

DECLARE emp_cursor CURSOR FOR
SELECT employee_id, name FROM employees;

Open the cursor and fetch the first row into variables.

SQL

OPEN emp_cursor;
FETCH NEXT FROM emp_cursor INTO @emp_id, @emp_name;

Loop through all rows, printing each employee's name.

SQL

WHILE @@FETCH_STATUS = 0
BEGIN
  PRINT 'Employee: ' + @emp_name;
  FETCH NEXT FROM emp_cursor INTO @emp_id, @emp_name;
END

Close and remove the cursor when finished.

SQL

CLOSE emp_cursor;
DEALLOCATE emp_cursor;

Sample Program

This program goes through each employee row, printing their ID and name one by one.

SQL

DECLARE @emp_id INT, @emp_name NVARCHAR(50);
DECLARE emp_cursor CURSOR FOR
SELECT employee_id, name FROM employees;

OPEN emp_cursor;
FETCH NEXT FROM emp_cursor INTO @emp_id, @emp_name;

WHILE @@FETCH_STATUS = 0
BEGIN
  PRINT 'Employee ID: ' + CAST(@emp_id AS NVARCHAR(10)) + ', Name: ' + @emp_name;
  FETCH NEXT FROM emp_cursor INTO @emp_id, @emp_name;
END

CLOSE emp_cursor;
DEALLOCATE emp_cursor;

Important Notes

Cursors can be slow for large data because they process rows one at a time.

Always close and deallocate cursors to avoid using extra memory.

Try to use set-based SQL operations first before using cursors.

Summary

A cursor lets you handle query rows one by one.

You declare, open, fetch rows, then close and deallocate.

Use cursors when row-by-row processing is needed.

Practice

1. What is the main purpose of using a CURSOR in SQL?

easy

A. To process query results row by row

B. To speed up bulk data insertion

C. To create new tables dynamically

D. To automatically backup the database

Solution

1. Step 1: Understand what a cursor does

   A cursor allows you to handle each row returned by a query individually, instead of all at once.

2. Step 2: Compare options to cursor purpose

   Options B, C, and D describe tasks unrelated to row-by-row processing, which is the cursor's main use.

3. Final Answer:

   To process query results row by row -> Option A

4. Quick Check:

   Cursor = row-by-row processing [OK]

Hint: Remember: Cursor = handle rows one at a time [OK]

Common Mistakes:

• Thinking cursor speeds up bulk inserts
• Confusing cursor with table creation
• Assuming cursor automates backups

2. Which of the following is the correct order of cursor operations in SQL?

easy

A. DECLARE, FETCH, OPEN, CLOSE, DEALLOCATE

B. OPEN, DECLARE, FETCH, CLOSE, DEALLOCATE

C. DECLARE, OPEN, FETCH, CLOSE, DEALLOCATE

D. FETCH, DECLARE, OPEN, CLOSE, DEALLOCATE

Solution

1. Step 1: Recall the cursor lifecycle

   You first DECLARE the cursor, then OPEN it to start fetching rows, FETCH rows one by one, CLOSE it when done, and finally DEALLOCATE to free resources.

2. Step 2: Match the correct sequence

   Only DECLARE, OPEN, FETCH, CLOSE, DEALLOCATE follows this exact order. Other options mix the steps incorrectly.

3. Final Answer:

   DECLARE, OPEN, FETCH, CLOSE, DEALLOCATE -> Option C

4. Quick Check:

   Cursor lifecycle order = C [OK]

Hint: Remember: Declare first, then open, fetch, close, deallocate [OK]

Common Mistakes:

• Opening cursor before declaring
• Fetching before opening
• Not deallocating after closing

3. Given the following SQL cursor code snippet, what will be the output?

DECLARE @Name VARCHAR(50);
DECLARE name_cursor CURSOR FOR SELECT Name FROM Employees WHERE Department = 'Sales';
OPEN name_cursor;
FETCH NEXT FROM name_cursor INTO @Name;
WHILE @@FETCH_STATUS = 0
BEGIN
  PRINT @Name;
  FETCH NEXT FROM name_cursor INTO @Name;
END
CLOSE name_cursor;
DEALLOCATE name_cursor;

Assuming the Employees table has Sales department names: 'Alice', 'Bob', 'Carol'.

medium

A. Only 'Alice' is printed

B. Syntax error due to missing FETCH

C. No output because cursor is not opened

D. Alice Bob Carol (each printed on a new line)

Solution

1. Step 1: Understand cursor fetch loop

   The cursor fetches each row where Department = 'Sales'. It prints the @Name variable for each fetched row.

2. Step 2: Identify the printed names

   Since the table has 'Alice', 'Bob', and 'Carol' in Sales, all three names will be printed one by one.

3. Final Answer:

   Alice Bob Carol (each printed on a new line) -> Option D

4. Quick Check:

   Cursor prints all fetched rows [OK]

Hint: Cursor loops print all fetched rows until none left [OK]

Common Mistakes:

• Assuming only first row prints
• Forgetting to open cursor
• Confusing fetch status check

4. Identify the error in the following cursor code:

DECLARE cur CURSOR FOR SELECT Id FROM Products;
OPEN cur;
FETCH NEXT FROM cur INTO @ProductId;
WHILE @@FETCH_STATUS = 0
BEGIN
  PRINT @ProductId;
END
CLOSE cur;
DEALLOCATE cur;

medium

A. Missing FETCH NEXT inside the WHILE loop

B. Cursor not declared properly

C. Cursor not opened before fetching

D. No error, code runs fine

Solution

1. Step 1: Check fetch inside loop

   The code fetches once before the loop but does not fetch again inside the loop, causing an infinite loop.

2. Step 2: Identify missing statement

   To move to next row, FETCH NEXT must be inside the WHILE loop after PRINT.

3. Final Answer:

   Missing FETCH NEXT inside the WHILE loop -> Option A

4. Quick Check:

   Fetch inside loop needed to advance cursor [OK]

Hint: Always fetch next row inside the loop to avoid infinite loop [OK]

Common Mistakes:

• Forgetting FETCH NEXT inside loop
• Assuming one fetch is enough
• Not opening cursor before fetch

5. You want to update the salary of employees one by one using a cursor. Which approach correctly uses a cursor for this task?

hard

A. Open cursor, but never fetch rows, then update salary

B. Declare cursor for SELECT, open it, fetch each employee, update salary inside loop, close and deallocate

C. Declare cursor, but update salary before opening cursor

D. Use a single UPDATE statement without cursor for all employees

Solution

1. Step 1: Understand row-by-row update need

   When updating salaries individually, you must fetch each employee row and update inside the loop.

2. Step 2: Identify correct cursor usage

   Declare cursor for SELECT, open it, fetch each employee, update salary inside loop, close and deallocate correctly declares, opens, fetches, updates inside loop, then closes and deallocates cursor.

3. Final Answer:

   Declare cursor for SELECT, open it, fetch each employee, update salary inside loop, close and deallocate -> Option B

4. Quick Check:

   Cursor updates row-by-row inside loop [OK]

Hint: Update inside fetch loop after opening cursor [OK]

Common Mistakes:

• Trying to update before opening cursor
• Not fetching rows before update
• Using cursor but updating all at once