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This is because you have used a parameter name "address" which has the same name as a column in the People table. So this line:

AND address = People.Address

is treated as:

AND People.address = People.Address

Therefore, only the "author" parameter is actually being used in the SELECT.

You should prefix your parameter names, either like this:

p_author

Or in the SQL like this:

AND CheckAuthor.address = People.Address