To efficiently perform an SQL query where one column equals another column, you can use a self-join. A self-join allows you to combine rows from the same table based on a related column. In this case, you'll be matching the values of two columns within the same table. Here's how you can write the SQL query using a self-join:

Let's assume you have a table called users with columns id and referrer_id, where referrer_id represents the ID of another user who referred the current user.

sql
SELECT u1.id AS user_id, u1.name AS user_name, u2.id AS referrer_id, u2.name AS referrer_name FROM users u1 LEFT JOIN users u2 ON u1.referrer_id = u2.id;

In this query:

  • users u1 represents the main user table, where you want to find the referrer's information.
  • users u2 is a second instance of the same users table, representing the referrers' data.
  • The LEFT JOIN connects the main user table (u1) to the referrer table (u2) using the referrer_id column in u1 and the id column in u2.
  • The SELECT statement retrieves the id and name of the main user (u1) and the id and name of the referrer (u2) if there is a match. If there is no match (no referrer), the columns from u2 will be NULL.

This query efficiently retrieves the user information along with the corresponding referrer information (if available) in a single SQL query. Using a self-join in this scenario can be more performant than running separate queries or using subqueries.

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