Does this not work?

CREATE TABLE MyTable

(

_key_out VARCHAR(10)

, myval int

);

INSERT INTO mytable SELECT 'A', 1

INSERT INTO mytable SELECT 'A', 2

INSERT INTO mytable SELECT 'B', 1

INSERT INTO mytable SELECT 'C', 3

CREATE TABLE MyTable2

(

_key_out VARCHAR(10)

, myval int

);

INSERT INTO mytable2 SELECT 'A', 1

INSERT INTO mytable2 SELECT 'B', 2

INSERT INTO mytable2 SELECT 'B', 3

INSERT INTO mytable2 SELECT 'C', 3

EXEC sp_MSforeachtable 'select tablename=''?'', count(distinct _key_out) from ?'

Gives 3.

If I do this:

EXEC sp_MSforeachtable 'select tablename=''?'', count( _key_out) from ?'

I get 4.