Alternative Approaches to Conditional Data in MariaDB: Beyond Virtual Columns

• MariaDB allows defining virtual columns within a table.
• These columns don't physically store data.
• Instead, their values are calculated based on an expression whenever you query the table.
• This can improve readability and simplify queries.

Case Statements in Virtual Columns:

• You can use a case statement within the expression to define the virtual column's value based on certain conditions.
• For example, you could create a column categorizing product prices ("Low", "Medium", "High") based on another price column.

Challenges:

• There are a couple of reasons why creating a virtual column with a case statement might fail in MariaDB:

  1. MariaDB Version: This functionality is only available in MariaDB versions 10.2.27, 10.3.18, and 10.4.8 onwards. If you're using an older version, it won't work.
  2. SQL Mode: In MariaDB 10.5 and above, there might be compatibility issues if the case statement's outcome can change depending on the server's SQL mode settings. In such cases, you'll get an error.

Solutions:

• Check your MariaDB version. If it's older than 10.2.27, you can't create virtual columns with case statements.
• If you're on MariaDB 10.5 or later and facing errors, review your SQL mode settings and ensure they won't affect the case statement's outcome.

This code creates a virtual column named category in the products table. The category is assigned "Low", "Medium", or "High" based on the value in the price column:

CREATE TABLE products (
  id INT AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(255) NOT NULL,
  price DECIMAL(10,2) NOT NULL,
  category VARCHAR(10) AS (
    CASE WHEN price < 10 THEN 'Low'
         WHEN price < 20 THEN 'Medium'
         ELSE 'High'
    END
  ) VIRTUAL
);

Explanation:

• We define a products table with id, name, and price columns.
• We create a virtual column named category.
• The category is defined using the AS keyword followed by a CASE statement.
• The CASE statement checks the value in the price column.
  • If price is less than 10, category becomes "Low".
  • If price is less than 20 but not less than 10, category becomes "Medium".
  • Otherwise, category becomes "High".
• Finally, we specify VIRTUAL to indicate it's a virtual column.

Possible Error Scenario:

This code might fail in MariaDB versions below 10.2.27:

CREATE TABLE customers (
  id INT AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(255) NOT NULL,
  country VARCHAR(50) NOT NULL,
  is_vip BOOLEAN AS (
    CASE WHEN country IN ('US', 'CA') THEN TRUE
         ELSE FALSE
    END
  ) VIRTUAL
);

• This code attempts to create a virtual column is_vip based on the customer's country.
• However, if you're using an older MariaDB version, you'll encounter an error because virtual columns with expressions were introduced in later versions.

1. Stored Procedures:

• Create a stored procedure that takes the necessary columns as input and calculates the desired value based on a case statement.
• In your queries, call this stored procedure to get the calculated value for each row.

Example:

CREATE PROCEDURE get_category(price DECIMAL(10,2))
BEGIN
  DECLARE category VARCHAR(10);
  SET category = (CASE WHEN price < 10 THEN 'Low'
                       WHEN price < 20 THEN 'Medium'
                       ELSE 'High'
                  END);
  SELECT category;
END;

Then, in your queries:

SELECT id, name, price, get_category(price) AS category
FROM products;

1. Views:

• Create a view that joins the original table with a subquery containing the case statement logic.
• This view will act as a virtual table reflecting the data with the calculated column.

CREATE VIEW product_categories AS
SELECT p.id, p.name, p.price,
       (CASE WHEN p.price < 10 THEN 'Low'
            WHEN p.price < 20 THEN 'Medium'
            ELSE 'High'
       END) AS category
FROM products AS p;

Then, you can query the view as if it were a regular table:

SELECT * FROM product_categories;

1. Triggers:

• Less recommended for this scenario, but you could create a trigger on the original table that updates a separate column with the calculated value based on a case statement whenever a new row is inserted or updated.

Choosing the Right Method:

• Stored procedures offer flexibility but require an additional call in your queries.
• Views provide a virtual table representation but might have performance implications for complex logic.
• Triggers can introduce overhead and complexity for simple calculations.