Question

table structure

table 1

userid   username 
 1        joe
 2        matt

table 2

 userid      username 
  1            

Need to update username in table2 if user id is match table 1
like select userid from table 1 if exist in table 2 then update username in table 2 ( username fetched from table 1 )

Answer 1

Try it by joining the tables using INNER JOIN

UPDATE table2 a 
       INNER JOIN table1 b
          ON a.userid = b.userid
SET    a.username = b.username

SQLFiddle Demo