SQL While Loop

Galaxy Glossary

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

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.

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;

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.

SQL While Loop

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

Description

Why SQL While Loop is important

Example Usage

Common Mistakes

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