What is a Cursor in SQL?

In this blog, we’ll cover the concept of cursors in SQL, including both purpose and functionality. This will include explaining the types of cursors, how to use them, syntax, and practical examples to get a great grasp. We will, in addition, explore the merits, demerits, and how cursors contrast with loops in SQL. 

Table of content

• What is Cursor in SQL?
• Types of Cursors in SQL
• Life Cycle of Cursor
• How to Use a Cursor in SQL?
• Difference between Cursor vs Loop in SQL
• SQL Cursor Syntax
• SQL Cursor Example
• Advantages of Cursor in SQL
• Disadvantages of Cursor in SQL
• Conclusion

What is Cursor in SQL?

A cursor in SQL is a database object that allows you to retrieve and manipulate data one row at a time. Cursors are typically used when performing operations on each result set row, such as updating or deleting. Cursors are similar to pointers in programming, allowing you to move through a result set and access the data in each row.

Types of Cursors in SQL

When working with cursors in SQL, there are two main types: Implicit Cursors and Explicit Cursors. These cursors serve similar purposes, but there are some significant differences.

Implicit Cursors

Implicit cursors are created by the SQL engine whenever you execute a SELECT statement. The cursor is automatically opened, and you can fetch rows from the result set one at a time. 

Implicit cursors are helpful when retrieving a small amount of data and performing simple operations on each row.

Explicit Cursors

Explicit cursors are created by the programmer to retrieve and manipulate data from a result set. They provide greater control over the cursor, such as the ability to open and close it, fetch rows, and also, perform operations on the data.

In addition, Explicit cursors are specifically useful while dealing with large amounts of data and also performing complex operations on each row. Thus, they enable the programmer to process the data one row at a time. This can improve performance and reduce memory usage.

Lifecycle of Cursor

SQL Cursor Lifecycle contains five crucial stages: declare, open, fetch, update, and close. Each stage is significant in managing the cursor and ensuring it is operated efficiently and resources are managed properly. Now, let’s break down each stage below:

1. Declare Cursor

Declare the cursor. Declaring the cursor is a declaration of the cursor name and the SQL SELECT statement that defines the result set to be processed. The declaration of the cursor is declared using the following syntax:

DECLARE cursor_name CURSOR FOR select_statement;  

2. Open Cursor

After declaring a cursor, opening the cursor will come next. When opening a cursor, it first initializes it then holds the fetched data by the SELECT statement within the cursor. Here is its syntax:

OPEN cursor_name;  

3. Fetch Cursor

Fetching refers to the fetching of rows from the cursor one by one or in blocks. This is applicable for all the operations, for instance, inserting, updating, or deleting every row. The syntax for fetching rows are:

FETCH NEXT FROM cursor_name INTO variable_list; 

4. Close Cursor

Once you have used the cursor it should be closed. Closing a cursor de-activates the cursor but releases no memory in relation to that cursor. This is achieved as follows:

CLOSE cursor_name; 

5. Deallocate Cursor

This includes the deallocation of the cursor. This de-allocates the cursor definition and frees up all associated system resources. The syntax is as follows:

DEALLOCATE cursor_name;  

How to Use a Cursor in SQL?

Cursors are a valuable asset for developers working with databases. They enable the efficient processing of vast amounts of data and are widely utilized in numerous database applications.

To use a cursor in SQL, you first need to declare the cursor and specify the SELECT statement that will be used to retrieve the data. 

Here is an example of how to declare a cursor:

DECLARE cursor_name CURSOR FOR
SELECT column1, column2, column3
FROM table_name
WHERE condition;

Once you have declared the cursor, you can open it and fetch rows from the result set using the FETCH statement. Here is an example of how to open and fetch rows from a cursor:

OPEN cursor_name;
FETCH NEXT FROM cursor_name INTO variable1, variable2, variable3;

You can then perform data operations using the variables you fetched. Once you have finished processing the row, you can re-fetch the next row using the FETCH statement.

Here is an example of how to fetch the next row:

FETCH NEXT FROM cursor_name INTO variable1, variable2, variable3;

You can continue fetching rows and performing operations on them until you have processed all the rows in the result set. Once you are finished, you can close the cursor using the CLOSE statement.

Difference between Cursor vs Loop in SQL 

Below is a comparison between Cursor and Loop in SQL presented in tabular form:

SQL Cursor Syntax

The SQL feature known as cursors makes it possible to traverse and manipulate data one row at a time. But to use a cursor, you need to know the correct syntax when declaring and working with it.

Here is the basic syntax for creating a cursor in SQL:

DECLARE cursor_name CURSOR FOR
SELECT column1, column2, column3
FROM table_name
WHERE condition;
OPEN cursor_name;
FETCH NEXT FROM cursor_name INTO variable1, variable2, variable3;
WHILE @@FETCH_STATUS = 0
BEGIN

-- Do some processing here

FETCH NEXT FROM cursor_name INTO variable1, variable2, variable3;
END
CLOSE cursor_name;
DEALLOCATE cursor_name;

SQL Cursor Example

Let’s take an example of how to use an explicit cursor in SQL. In this example, we will create a cursor that retrieves all the employees from the “employees” table and calculates their average salary.

DECLARE employee_cursor CURSOR FOR
SELECT employee_id, first_name, last_name, salary
FROM employees;
DECLARE @employee_id INT;
DECLARE @first_name VARCHAR(50);
DECLARE @last_name VARCHAR(50);
DECLARE @salary DECIMAL(18, 2);
DECLARE @total_salary DECIMAL(18, 2) = 0;
DECLARE @num_employees INT = 0;
DECLARE @avg_salary DECIMAL(18, 2) = 0;
OPEN employee_cursor;
FETCH NEXT FROM employee_cursor INTO @employee_id, @first_name, @last_name, @salary;
WHILE @@FETCH_STATUS = 0
BEGIN
SET @total_salary = @total_salary + @salary;
SET @num_employees = @num_employees + 1;
FETCH NEXT FROM employee_cursor INTO @employee_id, @first_name, @last_name, @salary;
END
CLOSE employee_cursor;
DEALLOCATE employee_cursor;
SET @avg_salary = @total_salary / @num_employees;
SELECT 'Average Salary' AS Metric, @avg_salary AS Value;

In this given example, a cursor called “employee_cursor”, retrieves all the employees from the “employees” table. It then reflects declare variables to hold the employee data and initialize variables to calculate the average salary.

We open the cursor and fetch the first row into the variables. 

Then enter a loop that calculates the total salary and the number of employees. We fetch the next row until there are no more rows to fetch.

Once we have processed all the rows, we close and deallocate the cursor. We then calculate the average salary by dividing the total salary by the number of employees and output the result.

Advantages of Cursor in SQL

Below are the advantages of using a cursor in SQL:

• Row-Level Processing: Cursors enable row-level processing, allowing developers to access and manipulate data one row at a time. This level of control is beneficial for complex tasks that require specific operations on individual rows.
• Data Navigation: Cursors provide explicit navigation commands like FETCH NEXT or FETCH PRIOR, enabling easy traversal through the result set. This feature allows developers to efficiently process data in a specified order.
• Transaction Control: Cursors can be used within transactions, allowing developers to manage data integrity and consistency during complex operations, ensuring that changes are made in a controlled manner.
• Complex Query Handling: Cursors are useful when dealing with complex queries that involve multiple tables and require procedural logic, as they provide a way to process and manage the data step-by-step, simplifying the code implementation.

Disadvantages of Cursor in SQL

Cursors in SQL have certain disadvantages that developers should consider when using them:

• Performance Overhead: Cursors can incur significant performance overhead, especially with large result sets, as individual fetch operations can be time-consuming.
• Resource Consumption: Cursors consume server resources while they are open, which can impact overall system performance, particularly in high-concurrency environments.
• Complexity: Implementing and managing cursors can lead to complex and less readable code, making the codebase harder to maintain and debug.
• Locking and Blocking: Cursors can cause locking and blocking issues, affecting other processes that need access to the same data, potentially leading to contention and reduced system responsiveness.

Conclusion

In conclusion, cursors in SQL can be a helpful tool for handling complex queries requiring individual row processing or executing a stored procedure or trigger for each row. However, they can also have some drawbacks, such as decreased efficiency and potential locking issues in a multi-user environment.

Understanding when and how to use cursors properly is essential to avoid these potential problems. Generally, it is recommended to use cursors sparingly and only when necessary. It is best to use set-based operations whenever possible to improve performance when working with large datasets.