How to lookup a date value in the recordset in another table.

I want to lookup a date value in the recordset in another table.

I have a report that is showing the result of a query. One field in this query is a datetime field. Now I want to check whether the datepart of the datetime field exists in another table. If so I want to place the result of that on the report.

How can I do that?
I can not find in Crystal reports a function like exists or so?

I already tried it in de SQL query by using a left outer join to that specific table.
But I can not convert the null values to a value like 'Not found'.