SQL While Loop

How can I repeat a block of SQL code multiple times based on a condition?

SQL doesn't have a direct `WHILE` loop like some programming languages. Instead, you can achieve repeated execution using `WHILE` loops within stored procedures or by combining `WHILE` loops with `cursors`. This allows you to iterate through data and perform actions based on conditions.

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Description

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Example H3

SQL, unlike procedural languages like Python or Java, doesn't have built-in `WHILE` loops for directly iterating through data. While you can't use a `WHILE` loop directly within a standard SQL query, you can achieve similar functionality using stored procedures and cursors. Stored procedures allow you to encapsulate a series of SQL statements, including conditional logic. A cursor acts as a pointer to a result set, enabling you to fetch rows one by one and perform operations on them. This approach is often used when you need to process data row by row, update multiple rows based on a condition, or perform complex calculations on a dataset.

Why SQL While Loop is important

Stored procedures and cursors, while not direct `WHILE` loops, are crucial for automating complex data manipulation tasks. They improve code organization, reusability, and security by encapsulating logic within the database.

SQL While Loop Example Usage


-- Declare a variable to store the result of a query
DECLARE @customerID INT;

-- Select the customer ID for customer with name 'John Doe'
SELECT @customerID = customerID
FROM Customers
WHERE customerName = 'John Doe';

-- Check if the variable was successfully assigned
IF @customerID IS NOT NULL
BEGIN
    -- Retrieve order details for the customer
    SELECT orderID, orderDate
    FROM Orders
    WHERE customerID = @customerID;
END
ELSE
BEGIN
    PRINT 'Customer not found.';
END;

SQL While Loop Syntax

Common Mistakes

Trying to use a `WHILE` loop directly within a standard SQL query.
Incorrectly using `FETCH` statements within the cursor loop.
Forgetting to `CLOSE` the cursor after use.
Failing to handle the `NOT FOUND` condition in the cursor loop.

Frequently Asked Questions (FAQs)

Why doesn't standard SQL include a built-in WHILE loop?

SQL is a declarative language designed to describe what data you want, not how to retrieve it. Set-based operations are therefore preferred over row-by-row iteration. Embedding an imperative WHILE loop in the core language would conflict with this philosophy and hurt query-planner optimizations that make SQL engines fast.

How can I iterate over rows when a single SELECT isn't enough?

You can wrap your logic inside a stored procedure and use a cursor to fetch rows one at a time. The cursor acts as a pointer to the result set, allowing you to apply conditional updates, perform complex calculations, or call additional SQL statements for each row—effectively mimicking a WHILE loop.

What advantages does a tool like Galaxy offer when working with stored procedures and cursors?

Galaxy’s context-aware AI copilot can auto-generate stored-procedure templates, suggest cursor management best practices, and even refactor set-based alternatives when possible. Combined with instant feedback, version history, and team sharing, Galaxy helps engineers implement or replace cursor-based logic faster and with fewer errors.