<p style="white-space:normal;">I currently have three tables: </p><p style="white-space:normal;">users table//Data is inserted during registration <pre class="brush:php;toolbar:false;">usersId //PRIMARY KEY userfName userlName userUid email userPwd dateTime</pre> <p><code>parent</code> table</p> <pre class="brush:php;toolbar:false;">id //PRIMARY KEY ikB mLname mEmail mPhone fname fLname fEmail fPhone addressL1 addressL2 city stateAbbr zip created_at user_id //FOREIGN KEY</pre> <p>I used the email column from the users table and mEmail or fEmail to insert the usersId into the parents table. </p> <p><code>Children</code> Table</p> <pre class="brush:php;toolbar:false;">child_id //PRIMARY KEY child1Name dobChild1 ageChild1 child2Name dobChild2 ageChild2 child3Name dobChild3 ageChild3 child4Name dobChild4 ageChild4 child5Name dobChild5 ageChild5 child6Name dobChild6 ageChild6 child7Name dobChild7 ageChild7 child8Name dobChild8 ageChild8 child9Name dobChild9 ageChild9 child10Name dobChild10 ageChild10 parent_id //FOREIGN KEY</pre> <p>There are no common values ​​between the children's table and the other two tables. My expectation is to do a validation between the users table and the parent table, and if the users table's usersId and the parent table's user_id are equal, insert that value into the children table's parent_id. Is this possible? I considered using a cross join, but I'm concerned that performance will suffer as the number of entries in the database grows. </p> <pre class="brush:php;toolbar:false;">SELECT usersId FROM users; SELECT user_id FROM parent; UPDATE children (parent_Id) INNER JOIN parent ON children.parent_id = users.usersId UPDATE children SELECT usersId FROM users cross JOIN children.parent_id ON users.usersId = children.parent_id UPDATE children INNER JOIN users (INNER JOIN parent ON users.userId = parent.userId) ON users.usersId = parent.user_id SET children.parent_id = parent.user_Id;</pre> <p>I have tried every method with no success. I also tried changing the order of the connections but that didn't work either. Thank you very much for any help you can provide. </p>

UPDATE Children c
SET c.parent_id = (
  SELECT p.user_id
  FROM users u
  JOIN parent p ON u.email = p.mEmail OR u.email = p.fEmail
  WHERE c.parent_id IS NULL -- Only update rows with NULL parent_id to avoid overwriting existing values
  LIMIT 1 -- Assuming you want to update one parent_id per child record; you may modify this as needed
);