1. The phrase 'NULL string' has a well-known and precise meaning in C ('""').
   
   
2. Whatever meaning this phrase has in SQL is derived from it's meaning in C. It does not have the well-known and precise meaning in the former as in the latter and the question 'How do I test for a SQL NULL string?' is better rephrased 'How do I test if a char/varchar field declared 'NULL' contains a C-style NULL string ('""')?'.
   
   
3. Employing the eminent philosopher of language Donald Davidson, I introduce "the principle of charity", according to which a listener wishing to interpret a speaker accurately "must maximize the self-consistency attribut(ed) to him, on pain of not understanding him" (From "Truth and Meaning").

While the term 'NULL' is obviously part of SQL, and it is not impossible to define the phrase 'NULL string' within the semantics of SQL, it does not follow that it is equally likely that the questioner was in fact importing those semantics rather than the semantics of C.

Similarly, although the romance languages share cognates derived from Latin, take libre for example, when I speak Spanish I am not also speaking French by virtue of the shared term. In such a situation the listener must determine from the context which language I am speaking. If I make the utterance "Creo en habla libre, y cerveza gratis", it is wholly incorrect to believe that I am speaking French, but poorly, because an interpretation of the utterance that attributes self-consistency to me is available. To conclude that I am speaking French on the basis of the appearance of "libre" is to attribute to me a great deal of nonsense surrounding that one term, and this is not sound interpretation.

Therefore, because there is a well-known and precise meaning available to the phrase 'NULL string', and because I believe that the principle of charity will yield in general better interpretation, to conclude that the speaker refers to it's meaning in C is a reasonable conclusion, indeed, more reasonable than any other, including SQL.

Now according to merlyn's clarifications, his point is that the listener must be sensitive to different meanings possibly at work in a question. This is fine, however I would suggest that an orthogonal pressure here ought to be the principle of charity. It is better to risk thinking too highly of your interlocutor than to assume immediately that his utterances are largely nonsense. If the point here is not to give a real answer to the question 'How do I test for a NULL string', for which there is a sensible answer in Perl, but to give an exhortation to be careful about importing meanings carelessly across language boundaries, then I suggest the question be rephrased in such a way that the principle of charity not demand a particular interpretation. The rather more innucuous "How do I test for a string that is null?" would make the point better.

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