How to Concatenate a String with an Auto-Incrementing Primary Key in MySQL Insert?

Concatenating String with Auto-Incrementing Primary Key in MySQL Insert

When inserting data into a MySQL database, it can be useful to concatenate a string with the auto-incrementing primary key ID to create a unique identifier.

Consider the following user table:

CREATE TABLE users (
  user_id   INT NOT NULL AUTO_INCREMENT,
  username   VARCHAR(255) NOT NULL,
  PRIMARY KEY (user_id)
);

Suppose you want to set the username column to a string prefix (e.g., 'user') followed by the auto-incrementing user_id. However, the user_id value is not generated until after the insert operation.

An attempt to accomplish this with a single statement using a BEFORE INSERT trigger is not feasible because the user_id value is not yet available. Similarly, using an AFTER INSERT trigger to update the username field is not allowed.

Solution 1: Insert and Update

One approach is to perform the insert operation first, then immediately execute an UPDATE statement to concatenate the string and user_id. This can be done in PHP as follows:

// Insert user data
$stmt = $mysqli->prepare(...);
$stmt->bind_param(...);
$stmt->execute();

// Get the auto-incremented user ID
$user_id = $mysqli->insert_id;

// Update username
$update_stmt = $mysqli->prepare("UPDATE users SET username = 'user$user_id' WHERE user_id = ?");
$update_stmt->bind_param("i", $user_id);
$update_stmt->execute();

Solution 2: Custom INSERT Query

If the user_id is not an auto-incrementing value, you can insert the concatenated username in the original INSERT statement. This requires modifying the SQL query to include the concatenation:

INSERT INTO users (username, ...) VALUES ('user' || ?, ...);

In this query, replace the placeholder ? with the PHP variable containing the user_id that you specify during insert.

Generated Columns (MySQL 5.7 and Later)

MySQL 5.7 introduced generated columns, which allow you to create columns based on expressions. However, it's not possible to define a generated column that refers to an auto-incrementing column.

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