mysql update table from another table

UPDATE  t1
LEFT JOIN
        t2
ON      t2.id = t1.id
SET     t1.col1 = newvalue
WHERE   t2.id IS NULL

UPDATE tableB
INNER JOIN tableA ON tableB.name = tableA.name
SET tableB.value = IF(tableA.value > 0, tableA.value, tableB.value)
WHERE tableA.name = 'Joe'