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Find Count of Existence of Row not Null within data set

Last Modified: 2016-03-09
I have a table called "VisitDetails" which tracks pages viewed within a Visit.

Looks like this:

VisitID     PageID     CategoryID      ProductID  (all are int, VisitID FK to a table called Visits which hold data about the visitor)

A Visit consists of multiple page views - they may be Pages, Categories, or Products.

I am trying to figure out how many Visits have a ProductID in any rows.  For example data:

VisitID     PageID     CategoryID      ProductID
1               NULL          NULL                100
2                1000          NULL                NULL
1              1000            NULL                NULL
1               NULL           10000               NULL
2              NULL             NULL                101

My ideal results would show me that 100% (count 2 of 2 unique Visits had a ProductID).  This will tell me what proportion of Visits viewed any Product.

Struggling to write an effective query to do this.
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