The database already provides such a field. To use your previous example,

Custom Field: 

SELECT *
  FROM T
 WHERE HasAmount
   AND ( Amount = 10 OR NOT (Amount = 10) )

[download]

Provided Field: 

SELECT *
  FROM T
 WHERE NOT IsNULL(Amount)
   AND ( Amount = 10 OR NOT (Amount = 10) )

[download]

In reply to Re^6: DBI: passing undef as an argument by ikegami
in thread DBI: passing undef as an argument by fws

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