How to rewrite IS DISTINCT FROM and IS NOT DISTINCT FROM?

Question

How do you rewrite expressions containing the standard IS DISTINCT FROM and IS NOT DISTINCT FROM operators in SQL implementations such as Microsoft SQL Server 2008R2 that do not support them?

Answer 1

The IS DISTINCT FROM predicate was introduced as feature T151 of SQL:1999, and its readable negation, IS NOT DISTINCT FROM, was added as feature T152 of SQL:2003. The purpose of these predicates is to guarantee that the result of comparing two values is either True or False, never Unknown.

These predicates work with any comparable type (including rows, arrays and multisets) making it rather complicated to emulate them exactly. However, SQL Server doesn't support most of these types, so we can get pretty far by checking for null arguments/operands:

• a IS DISTINCT FROM b can be rewritten as:

  ((a <> b OR a IS NULL OR b IS NULL) AND NOT (a IS NULL AND b IS NULL))
  

• a IS NOT DISTINCT FROM b can be rewritten as:

  (NOT (a <> b OR a IS NULL OR b IS NULL) OR (a IS NULL AND b IS NULL))
  

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Your own answer is incorrect as it fails to consider that FALSE OR NULL evaluates to Unknown. For example, NULL IS DISTINCT FROM NULL should evaluate to False. Similarly, 1 IS NOT DISTINCT FROM NULL should evaluate to False. In both cases, your expressions yield Unknown.