獲取最終的值

select a.id, if(a.content is null, b.content, a.content) as content from a left join b on a.link = b.id;

將這個當成臨時表，更新 a 即可 (這里寫if，是假設(shè)a表有content字段)

update a as c,
(
  select a.id, if(a.content is null, b.content, a.content) as content 
  from a 
  left join b on a.link = b.id
) as d 
set c.content = d.content 
where d.id=c.id ;

SELECT [Id],[link],ISNULL((SELECT [content] FROM [Table_B] WHERE Id=[Tabe_A].[link]),'')  AS [content] FROM [Table_A]