Question

I’m performing a MERGE statement from a temp table t into the main table ts, matching on an ID field. It contains the following:

WHEN MATCHED THEN UPDATE
   SET ts.username = t.username, ts.password = t.password, ts.title = t.title, ts.firstname = t.firstname, ts.surname = t.surname, ts.email = t.email

If the username and password is blank in the temp table t, I don’t want to update the main table ts. How can I do this?

EDIT: Seems like a CASE statement or using COALESCE(NULLIF(t.username, ''), ts.username) will work. Which is best?

Answer 1

When dealing with blanks (i.e. empty string), then using a case statement

WHEN MATCHED THEN UPDATE
   SET case when t.username = '' then ts.username else t.username end

If you’re dealing with null, then use isnull

WHEN MATCHED THEN UPDATE
   SET ts.username = isnull(t.username, ts.username),

If it can be either blank or null, you can deal with both in a case statement

WHEN MATCHED THEN UPDATE
   SET case when t.username = '' or t.username is null then ts.username else t.username end