Martijn, (Warning: This post contains somewhat of a long rant followed by a question.) I realize that NULL is the unknown value in SQL and that (most) functions therefore treat it as such. I have no problem with "RETURNS NULL ON NULL INPUT" except when a function returns NULL for no good reason. If I were the ruler of the world I would declare that: 1) All functions correctly treat NULL input as "unknown" and if this prevents them from returning a logical value they must return NULL. (SQL has this already) 2) If an error occurs within a function which prevents the function from returning a logical value (such as invalid inputs) the function must raise an exception and not return. IE: No value is returned at all -- not even NULL. PostgreSQL seems to already do this. 3) If a function can not calculate a return value due to some ambiguity it should return NULL. This is really just an extention of rule 1. 4) If a function can calculate a return value (no processing error, NULL input or other ambiguity exists) it MUST NOT return NULL. My problem is with SUBSTRING. When it fails to find a match for the regexp pattern within the source string it returns NULL. Why?! There is no ambiguity nor NULL inputs! The result of the function is known to logically by "No Results". So the only question is this: How do you logically return that from a function? a) An empty string is logically incorrect because an empty string is a value and returning it would imply that it was matched. b) NULL is logically incorrect because the result was known. NULL is supposed to be returned when the result is unknown. Does Postgres have a "EMPTY" or "NOTHING" return value? What does a SELECT return when there are no matching records? NULL? Hell no! Then what? Because whatever SELECT returns when it finds no matching records is what SUBSTRING should return when there are no matching substrings. Of course SELECT was never intended to return a SCALAR value either while SUBSTRING was. Maybe "EMPTY" is something that SCALAR values simply are no capable of specifying? I am suggesting that the behaviour of SUBSTRING returning NULL when no matches is found is either a bug in PostgreSQL or a flaw in the SQL specification. It is not logical. Another words I wanted by function to do this (in english): Replace the next occurance of <pattern> in <sourcestring> with <replacementstring>; if found. Repeat for all additional occurances; if any. At no point should any ambiguity ever be introduced into this function so long as none of the three inputs are NULL since no external data source is used. Therefore nothing should ever return NULL during the operation of the function. In my experience having to code "special cases" around the NULL value is one of the biggest annoyances in SQL. Special cases should never have to be coded for specific values that have no bearing on the business logic of the application. As a programmer I want to be reasonably assured that I can anticipate NULL popping up and that anywhere that it might pop up it will be handled gracefully. Obviously I'm somewhat new to SQL (less than 2 years) and especially to PostgreSQL (about 2 months). In addition to returning null on null input, what are the best programming practices for dealing with NULL within a procedure/query? Was coalesce the best way to handle this? Should I have just checked for NULL in my EXIT statement instead? -Robert
