Update table based on another table

问题:

I'm trying to update a column in a table based on another column in another table.

UPDATE eval e
   SET rank = (SELECT p.desc
                 FROM Position p
                WHERE p.id = e.faculty 
                  AND p.date >= '2011-05-20'
              )

p.id and e.faculty correspond. I want to update rank with p.desc if the id's are the same. (e.faculty and p.id)

Any help will be great! :)

回答1:

Try this for SQL Server:

UPDATE dbo.eval 
SET rank = p.desc
FROM dbo.Position p
WHERE p.id = eval.faculty and p.date >= '2011-05-20'

or if you need an alias on the base table (for whatever reason), you need to do this:

UPDATE dbo.eval 
SET rank = p.desc
FROM dbo.eval e
INNER JOIN dbo.Position p ON p.id = e.faculty 
WHERE p.date >= '2011-05-20'

回答2:

You need a restriction in the form of a WHERE clause; if you use EXISTS you can based it on you scalar subquery e.g.

UPDATE eval
   SET rank = (
               SELECT p.desc
                 FROM Position p
                WHERE p.id = eval.faculty 
                      AND p.date >= '2011-05-20'
              )
 WHERE EXISTS (
               SELECT *
                 FROM Position p
                WHERE p.id = eval.faculty 
                      AND p.date >= '2011-05-20'
              );

Note the above targets the UPDATE on the base table eval rather than the correlation name e. This makes a lot more sense when you think of an SQL UPDATE in terms of relational assignment i.e. you don't want to assign to e because it (unlike the base table) will go out of scope!