根据同一表的不同列中存在的主键更新列

Question

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id	topic_id	id
1	21	null
2	null	1

Answer 1

UPDATE table t1
JOIN table t2 ON t1.id = t2.id2 -- join 2 tables copies according to the relation
SET t2.topic_id = t1.topic_id     -- copy the value into the column
WHERE t2.topic_id IS NULL;        -- update only rows without the value in the column