除了此答案之外，如果您需要根据tableA.value动态更改tableB.value，则可以执行以下操作:

In addition to this answer if you need to change tableB.value according to tableA.value dynamically you can do for example:

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'