INF: Aggregates and SET Clause in UPDATE Statement

  - Microsoft SQL Server version 4.2 for OS/2

SUMMARY

In an UPDATE statement, an aggregate function cannot appear directly in the SET list. For example, an attempt to execute a query such as,

   UPDATE <table name>
   SET <column name> = count(*)
   FROM <table names>
   WHERE <condition List>

   An aggregate may not appear in the set list of an UPDATE statement.
   (Msg 157, Level 15, State 1).

MORE INFORMATION

When an aggregate appears in the SET list of an UPDATE statement as above, there exists an ambiguity as to whether the associated WHERE clause qualifies the rows to be updated or it qualifies the rows on which to apply the aggregate function. The correct way to do this is:

   UPDATE <table name>
   SET <column name> = (select count(*) from <table name>
                        where <aggregate condition>)
   FROM <table names>
   WHERE <update condition>.