如果不为空，则使用其他表中的数据更新表?
基本上我想要做的是将一列的值从一个表复制到另一个表的另一列.
Basically what I want to do is copy the value of a column from one table to another column in another table.
我使用的查询是:
UPDATE t1
SET product_code =
(SELECT `value` FROM t2 WHERE t2.variant_id = t1.variant_id AND key_id = 10);
哪个工作正常，但列不匹配，
Which is working fine, but there is a mismatch in columns,
所以我需要添加一个子句，如果子查询不返回 null，它只会对该行进行更新.
so I need to add in a clause which will only do the update on that row, if the subquery does not return null.
我该怎么做?
推荐答案
你应该只是在这样的连接中进行更新
You should just being doing the update across a join like this
UPDATE
t1 INNER JOIN t2 ON t1.variant_id = t2.variant_id
SET t1.product_code = t2.value
WHERE t2.key_id = 10
AND t2.value IS NOT NULL
在这种情况下无需担心空值，因为内连接只会选择两个表中都存在 variant_id 的行.
There is no need to worry about nulls in that case as the inner join will only select rows where the variant_id exists in both tables.
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