A、B为2个不同的表，将A表的a字段赋值给B表的b字段：
update B set b = (select a from A where A.id = B.id) where B.b is null
update B set b = (select a from A where A.id = B.id) where B.b is null
注：where B.b is null为将B表的b字段为空的值赋值为A表的a字段
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