In this blog post, we'll delve into the workings of generated columns and showcase a practical example where they can be harnessed to prevent users from signing up with subtly different, yet essentially identical, email addresses.

Understanding Generated Columns

Generated columns in MySQL are virtual columns whose values are determined by an expression rather than being directly stored in the database. These columns can be computed based on other columns in the same table, constants, or functions.

To create a generated column, you specify a generation expression when defining the column. This expression can involve mathematical operations, string manipulations, or even date functions. The column's value is then computed automatically, saving the need for manual intervention.

Example Scenario: Unifying Email Addresses

Consider a scenario where you want to ensure that users cannot sign up with email addresses that are technically the same but appear different due to the use of the '+' symbol and additional strings. For instance, user@gmail.com, user+123@gmail.com, and user+test@gmail.com should all be treated as the same email address.

Creating the Users Table

Let's create a simplified 'users' table to illustrate this:

CREATE TABLE users_signup (
    user_id INT AUTO_INCREMENT PRIMARY KEY,
    email VARCHAR(255) NOT NULL,
    normalized_email VARCHAR(255) GENERATED ALWAYS AS (
        CONCAT_WS(
            '@',
            SUBSTRING_INDEX(SUBSTRING_INDEX(email, '@', 1), '+', 1),
            SUBSTRING_INDEX(email, '@', -1)
        )
    ) STORED,
    UNIQUE KEY idx_normalized_email (normalized_email)
);

Lets understand this create table command

In MySQL, the STORED keyword in the context of a generated column specifies that the values for that column will be stored physically on disk. When you define a generated column, you have the option to store the computed values on disk or to make them virtual (computed on-the-fly when queried).

Here's a breakdown:

• VIRTUAL: This is the default behavior. The value of a virtual column is not physically stored on disk; instead, it is computed when queried. Virtual columns are useful when you want to derive a value based on other columns without consuming additional storage.

• STORED: This option indicates that the computed values for the generated column will be stored on disk. Storing the values can be beneficial if you frequently query or index the generated column, as it avoids the need to recompute the value each time it is accessed.

Here we are generating a vitual column, storing that on disk, and the value is determined by this function

 CONCAT_WS(
            '@',
             SUBSTRING_INDEX(SUBSTRING_INDEX(email, '@', 1), '+', 1),
              SUBSTRING_INDEX(email, '@', -1)
             )

We are getting a substring before '@' then taking substring of that result before '+'

and concating it with '@' and substring after '@' in the original input.

So now when you try to insert values like this, you will get an error

INSERT INTO users_signup (email) VALUES ('user+123@gmail.com');

-- This should fail because 'user+test@gmail.com' normalizes to 'user@gmail.com'
INSERT INTO users_signup (email) VALUES ('user+test@gmail.com');

-- This should fail because 'uSer@gmail.com' normalizes to 'user@gmail.com' (case-insensitive)
INSERT INTO users_signup (email) VALUES ('uSer@gmail.com');

And only one entry is present in database.

mysql> select * from users_signup;
+---------+--------------------+------------------+
| user_id | email              | normalized_email |
+---------+--------------------+------------------+
|       1 | user+123@gmail.com | user@gmail.com   |
+---------+--------------------+------------------+
1 row in set (0.00 sec)

This way you can stop "smart" people trying to hack the free tier of your application.

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