by Markus Winand.

NULL in the Oracle Database


SQL's NULL frequently causes confusion. Although the basic idea of NULLto represent missing data—is rather simple, there are some peculiarities. You have to use IS NULL instead of = NULL, for example. Moreover the Oracle database has additional NULL oddities, on the one hand because it does not always handle NULL as required by the standard and on the other hand because it has a very “special” handling of NULL in indexes.

The SQL standard does not define NULL as a value but rather as a placeholder for a missing or unknown value. Consequently, no value can be NULL. Instead the Oracle database treats an empty string as NULL:

   SELECT     '0 IS NULL???' AS "what is NULL?" FROM dual
    WHERE      0 IS NULL
UNION ALL
   SELECT    '0 is not null' FROM dual
    WHERE     0 IS NOT NULL
UNION ALL
   SELECT ''''' IS NULL???'  FROM dual
    WHERE    '' IS NULL
UNION ALL
   SELECT ''''' is not null' FROM dual 
    WHERE    '' IS NOT NULL
what is NULL?
--------------
0 is not null
'' IS NULL???

To add to the confusion, there is even a case when the Oracle database treats NULL as empty string:

SELECT dummy
     , dummy || ''
     , dummy || NULL
  FROM dual
D D D
- - -
X X X

Concatenating the DUMMY column (always containing 'X') with NULL should return NULL.

The concept of NULL is used in many programming languages. No matter where you look, an empty string is never NULL…except in the Oracle database. It is, in fact, impossible to store an empty string in a VARCHAR2 field. If you try, the Oracle database just stores NULL.

This peculiarity is not only strange; it is also dangerous. Additionally the Oracle database's NULL oddity does not stop here—it continues with indexing.

If you like my way of explaining things, you'll love my book.

Contents

  1. NULL in Indexes — Every index is a partial index

  2. NOT NULL Constraints — affect index usage

  3. Emulating Partial Indexes — using function-based indexing

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About the Author

Photo of Markus Winand

Markus Winand teaches efficient SQL—inhouse and online. He minimizes the development time using modern SQL and optimizes the runtime with smart indexing—for that he also published the book SQL Performance Explained.

“Use The Index, Luke!” by Markus Winand is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Unported License.
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