And why not just use constraints to prevent entering invalid data such as nulls or blanks?

This is a rant, too be honest.  Other than syntactical differences, what is different between the following two queries:

select
    [a].[co_num]
    , .[cust_po]
    , [a].[co_line]
    , [a].[co_release]

Also, please run the following and let us know what it returns:

select
    [a].[co_num]
    , .[cust_po]
    , [a].[co_line]
    , [a].[co_release]
    , [a].[item]
    , [a].[u_m]
    , [a].[qty-ordered]
    ,...