Kendra Little takes a peek at a view:
Views let you do dumb things by accident in SQL Server. Then they make you have to think way too hard to fix them.
Most of the time when people create views, they start by refining a
SELECTquery, then turn it into a view. People also often create multiple views that pull different slices of data and UNION the results together.Combined, these two things easily lead to undeclared datatypes in views with problematic implicit conversions.
Read on for an example of this problem in action. Kendra’s example involved a view and a separate table, but you can also see this kind of thing pop up in a view that itself contains set operators like UNION.