Sequential Update of Array Column in MariaDB

In this article, we will explore how to update a column with values from an array sequentially. This problem is particularly useful when you need to apply different settings or updates based on certain conditions.

We’ll start by discussing the general approach to updating arrays in MySQL and then dive into the specifics of sequential updates using window functions and conditional logic.

Background: Updating Arrays in MariaDB

MariaDB provides a built-in way to update arrays, known as the LIST type. However, when dealing with arrays containing multiple values, the traditional approach involves concatenating or repeating the array elements for each row. This can lead to performance issues and is often not the most efficient solution.

Overview of Window Functions

Window functions are a class of SQL functions that perform calculations across a set of rows that are related to the current row. In MariaDB, window functions have been supported since version 5.7.

One of the key features of window functions is their ability to apply complex logic based on the relative positions of rows within the result set. This makes them particularly useful for tasks like sequential updates and aggregations.

Approach: Using Window Functions for Sequential Updates

In our case, we want to update a column with values from an array in a sequential manner. To achieve this, we’ll use a combination of window functions and conditional logic to determine which value to apply to each row.

Step 1: Adding an Order Column

To enable the use of window functions, we need to add an order column that represents the order for our update sequentially. In our example, we’re using the ID column as the order column.

-- Create a new column 'color' and initialize it with NULL values
ALTER TABLE table_name ADD COLUMN color VARCHAR(10);

-- Add an order column (in this case, ID)
ALTER TABLE table_name ADD COLUMN order_id INT;

Step 2: Applying Window Function

Next, we’ll use the row_number() window function to assign a unique number to each row within the result set. This will allow us to apply different values from the array based on the relative position of each row.

-- Apply row numbering using the ROW_NUMBER() function
UPDATE table_name AS t
INNER JOIN (
  SELECT Id,
         ROW_NUMBER() OVER (ORDER BY Id) AS rn
  FROM table_name
) AS sub ON t.Id = sub.Id

Step 3: Conditional Logic for Updating

Now that we have the row numbers, we can apply conditional logic to determine which value from the array to update each row with.

In our example, we’re using a CASE statement with the MOD() function to determine the position of each row within the result set. We’ll then use this position to select the corresponding value from the array.

-- Update the 'color' column based on the row number (rn)
SET color = (
  CASE WHEN MOD(rn,3) = 1 THEN '#F00'
       WHEN MOD(rn,3) = 2 THEN '#0F0'
       ELSE '#00F' END
);

Putting It All Together

Here’s the complete SQL statement that updates the color column in a sequential manner:

UPDATE table_name AS t
INNER JOIN (
  SELECT Id,
         ROW_NUMBER() OVER (ORDER BY Id) AS rn
  FROM table_name
) AS sub ON t.Id = sub.Id
SET color = (
  CASE WHEN MOD(rn,3) = 1 THEN '#F00'
       WHEN MOD(rn,3) = 2 THEN '#0F0'
       ELSE '#00F' END
);

Example Use Case

Suppose we have a table orders with an order_id column and want to update the status column with values from an array in a sequential manner:

CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  customer_name VARCHAR(50),
  status VARCHAR(10)
);

INSERT INTO orders (order_id, customer_name, status) VALUES
(1, 'John Doe', NULL),
(2, 'Jane Smith', NULL),
(3, 'Bob Johnson', NULL);

To update the status column with values from the array ('pending', 'in_progress', 'shipped'), we can use the following SQL statement:

UPDATE orders AS t
INNER JOIN (
  SELECT order_id,
         ROW_NUMBER() OVER (ORDER BY order_id) AS rn
  FROM orders
) AS sub ON t.order_id = sub.order_id
SET status = (
  CASE WHEN MOD(rn,3) = 1 THEN 'pending'
       WHEN MOD(rn,3) = 2 THEN 'in_progress'
       ELSE 'shipped' END
);

This will update the status column in the following order: pending, in_progress, shipped.

Conclusion

In this article, we explored how to update a column with values from an array sequentially using MariaDB window functions and conditional logic. We discussed the benefits of using window functions for complex updates and provided a step-by-step guide on how to implement sequential updates in your own database queries.

By following these techniques, you’ll be able to efficiently apply different settings or updates based on certain conditions, making your database queries more flexible and powerful.

Last modified on 2023-05-12