已经有类似此请求的帖子-请查看：
Copy data from one column to other column (which is in a different table)
MySQL: Copy table to another table with an extra column
Join results from tables with same name from different databases
我相信还有其他的，所以你可以找到一个与你正在使用的数据库有关。

Something like:

update alpha.comments
    set comments = (select b.comments from beta.comments b where b.id = comments.id);

In SQL Server, you can can express this as:

update a
    set comments = b.comments
    from alpha.comments a join
         beta.comments b
         on a.id = b.id;

This handles duplicates in an unfortunate way. It arbitrarily chooses one for the assignment, so no error is generated.

可以这样更新

UPDATE Alpha SET Comments=(
SELECT Comments
FROM Beta
WHERE Beta.ID = Alpha.ID
);