Answers for "how to update constrain on a table in mysql"

mysql update table from another table

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'

update table mysql

-- Things in brackets are optional
-- IGNORE modifier updates rows even if errors occur (ie: the rows that cause errors are simply not updated)
UPDATE [IGNORE] table_name 
SET 
    column_name1 = expr1,
    column_name2 = expr2,
    ...
[WHERE 
    condition]; -- WHERE tells us which rows to update based on said condition