MySQL update table based on another tables value

Question

I have a two tables,

Here is my first table,

ID      SUBST_ID        CREATED_ID
1       031938          TEST123
2       930111          COOL123
3             


        

Answer 1

UPDATE TABLE2
       JOIN TABLE1
       ON TABLE1.SUBST_ID = TABLE2.SERIAL_ID
SET    TABLE2.BRANCH_ID = TABLE1.CREATED_ID 
WHERE TABLE2.BRANCH_ID IS NULL or TABLE2.BRANCH_ID='';