MySQL: Trying to populate data into one column from another table

Question

Table A has several columns, including FirstName and LastName Table B has different columns, including FirstName , LastName and EmployeeID

I added EmployeeID into Table A. I now want to populate Table A's Employee ID 's from Table B's Employee ID 's, using the first and last name (we currently have no one working with the same name - the design of this table was weird like that)

I've attempted a few things, but I keep coming back to

INSERT INTO TableA (EMPLOYEE_ID) A
SELECT B.EMPLOYEE_ID FROM TableB B
WHERE A.First_name = B.First_name
AND A.Last_name = B.Last_name

But I keep getting a syntax error - MySQL server version for the right syntax to use near A. I don't know how to use this syntax when dealing with Insert statements, I think, or if this is the right way at all.

Answer 1

It sounds like you already have data in TableA that you want to update. If that is the case, you need to use an UPDATE statement. Here's an example:

UPDATE TableA A
SET EMPLOYEE_ID =
  (SELECT EMPLOYEE_ID
   FROM TableB B
   WHERE B.First_name = A.First_name
   AND B.Last_name = A.Last_name)
WHERE EXISTS
  (SELECT EMPLOYEE_ID
   FROM TableB B
   WHERE B.First_name = A.First_name
   AND B.Last_name = A.Last_name)

Answer 2

Question: Are first and last name guaranteed to be unique?

Also, this needs to be an update.

UPDATE TableA A
SET a.employee_id = (SELECT employee_id FROM TableB WHERE first_name = a.first_name AND last_name = a.last_name)
WHERE EXISTS (SELECT 1 FROM TableB WHERE first_name = a.first_name AND last_name = a.last_name)