[dba-SQLServer] Update using 'Select (TOP or COUNT) SQL String':

I'm trying to stuff a flag when changes exist in metadata and have been
unsuccessful using a SELECT TOP 1 fieldname, as well as SELECT
COUNT(fieldname).  Currently trying the count approach as that would give me
the magnitudes of changes, and the SQL string looks like:

 

UPDATE           PLAN_STAGE.dbo.S2B_MD_DT_CHANGES

SET      MD_CHANGES = (SELECT COUNT(CHILD)

                                    FROM
PLAN_STAGE.dbo.DATA_SOURCE_ACCOUNT_TMP

                                    WHERE CHILD IS NOT NULL)

FROM   PLAN_STAGE.dbo.S2B_MD_DT_CHANGES,
PLAN_STAGE.dbo.DATA_SOURCE_ACCOUNT_TMP

 

I have also tried substituting the following WHERE EXISTS in place of the
last FROM line in above SQL string:

 

WHERE EXISTS            (SELECT COUNT(CHILD)

                                    FROM
PLAN_STAGE.dbo.DATA_SOURCE_ACCOUNT_TMP

                                    WHERE CHILD IS NOT NULL)

 

The select portion by itself works, but the update returns '(0 row(s)
affected)'.  What am I missing here?

Thanks in Advance,

Ron Moore