I think I found an issue in VistaDB 5.1 Beta (5.0.9.1345).

When I do a query involving multiple dates in the Where clause zero results are being returned instead of the expected result.

-- result = 2087
SELECT Count(t1.StudentId)
FROM [tblTest] t1
INNER JOIN [tblEnrollment] t2 ON t1.[StudentID] = t2.[StudentID]

-- result = 2011
SELECT Count(t1.[StudentId])
FROM [tblTest] t1
INNER JOIN [tblEnrollment] t2 ON t1.[StudentID] = t2.[StudentID]
WHERE t1.[TimeTested] >= t2.[DateStart]

-- result = 2087
SELECT Count(t1.[StudentId])
FROM [tblTest] t1
INNER JOIN [tblEnrollment] t2 ON t1.[StudentID] = t2.[StudentID]
WHERE t1.[TimeTested] <= t2.[DateEnd]

-- result = 2011
SELECT Count(t1.[StudentId])
FROM [tblTest] t1
INNER JOIN [tblEnrollment] t2 ON t1.[StudentID] = t2.[StudentID]
WHERE t1.[TimeTested] >= t2.[DateStart] AND t1.[TimeTested] <= t2.[DateEnd]

-- result = 2011
SELECT Count(t1.[StudentId])
FROM [tblTest] t1
INNER JOIN [tblEnrollment] t2 ON t1.[StudentID] = t2.[StudentID]
WHERE t1.[TimeTested] BETWEEN t2.[DateStart] AND t2.[DateEnd]

-- result = 0, expected 2011
SELECT Count(t1.[StudentId])
FROM [tblTest] t1
INNER JOIN [tblEnrollment] t2 ON t1.[StudentID] = t2.[StudentID]
WHERE t1.[TimeTested] >= t2.[DateStart] OR (t1.[TimeTested] BETWEEN t2.[DateStart] AND t2.[DateEnd])

-- result = 0, expected 2011
SELECT Count(t1.[StudentId])
FROM [tblTest] t1
INNER JOIN [tblEnrollment] t2 ON t1.[StudentID] = t2.[StudentID]
WHERE t1.[TimeTested] >= t2.[DateStart] OR  (t1.[TimeTested] >= t2.[DateStart] AND t1.[TimeTested] <= t2.[DateEnd])

The same queries run under VistaDB 5.0.5.1338 return the correct results.

The attached file has the queries above plus example database.