SQL Cursor Example

How can I process data row by row using SQL?

Cursors in SQL are used to process data from a result set one row at a time. They provide a way to iterate through the rows and perform operations on each row individually. This is particularly useful for tasks requiring complex logic or updates based on the current row's data.

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Description

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Cursors are a powerful tool in SQL that allow you to traverse a result set one row at a time. Imagine you need to update a table based on conditions within the data itself. A cursor lets you do this. Unlike other methods that process the entire result set at once, cursors offer fine-grained control over each row. This is crucial for tasks like complex data validation, custom calculations, or intricate updates. For instance, you might need to update a customer's address only if their order total exceeds a certain threshold. A cursor allows you to check this condition for each order individually. Cursors are often used in conjunction with loops or other procedural logic to perform actions on each row. However, they can be less efficient than set-based operations for simple tasks, so consider alternative approaches when possible. A key aspect of cursors is that they manage the result set's position, allowing you to move forward or backward through the rows. This is essential for tasks requiring sequential processing or conditional updates.

Why SQL Cursor Example is important

Cursors are essential for tasks requiring row-by-row processing, enabling complex logic and updates based on individual row data. They are crucial for situations where set-based operations are insufficient, allowing for fine-grained control over data manipulation.

SQL Cursor Example Example Usage


-- Create a table named 'products'
CREATE TABLE products (
    product_id INT PRIMARY KEY,
    product_name VARCHAR(50),
    price DECIMAL(10, 2)
);

-- Insert some sample data into the 'products' table
INSERT INTO products (product_id, product_name, price)
VALUES
    (1, 'Laptop', 1200.00),
    (2, 'Mouse', 25.00),
    (3, 'Keyboard', 75.00);

-- Create a new table named 'products_backup' with the same structure as 'products'
CREATE TABLE products_backup (
    product_id INT PRIMARY KEY,
    product_name VARCHAR(50),
    price DECIMAL(10, 2)
);

-- Insert data from 'products' into 'products_backup' using INSERT...SELECT
INSERT INTO products_backup (product_id, product_name, price)
SELECT product_id, product_name, price
FROM products;

-- Verify the data in the 'products_backup' table
SELECT * FROM products_backup;

SQL Cursor Example Syntax

Common Mistakes

Forgetting to declare and open the cursor.
Forgetting to fetch data from the cursor within the loop.
Failing to close and deallocate the cursor after use.
Using cursors for tasks that can be efficiently handled with set-based operations.

Frequently Asked Questions (FAQs)

When is using a SQL cursor preferable to set-based operations?

Cursors shine when you must inspect or manipulate each row individually—such as performing complex validations, executing custom calculations, or conditionally updating data (e.g., updating a customer’s address only if their order total exceeds a threshold). While set-based SQL is faster for straightforward bulk updates, a cursor’s row-by-row control is invaluable for intricate, sequential logic that can’t be expressed cleanly in a single statement.

How does a cursor keep track of its current row in a result set?

A cursor maintains an internal pointer to its position within the result set. SQL commands like FETCH NEXT, FETCH PRIOR, or FETCH ABSOLUTE n move this pointer forward, backward, or to a specific row, enabling sequential processing. This positional awareness lets you loop through rows, evaluate conditions, and decide whether to continue, skip, or exit the cursor—a capability set-based queries don’t provide.

Can a modern SQL editor like Galaxy improve my workflow when working with cursors?

Yes. Galaxy offers a lightning-fast editor, context-aware AI copilot, and rich metadata visibility that streamline cursor development. The AI copilot can autocomplete cursor syntax, suggest loop logic, and even refactor cursor code into set-based alternatives when appropriate. Built-in collaboration features let teams review and endorse complex cursor scripts without pasting SQL into Slack or Notion, reducing errors and accelerating delivery.