Quickest way to check for pre-existing record before insert [mysql_errno()]

Question

My question will use emails as an example, but this could apply to anything.

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Normally before registering a new user (including inserting his/her email) I check

Answer 1

Use

INSERT IGNORE INTO Users VALUES(...);

with a unique key on email field, then check row count with mysql_affected_rows();

This will result in a single query to the DB and rule out the race condition of the time window between SELECT and INSERT