( Problems with NULL

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 A.5.3 Problems with `NULL' Values
 The concept of the `NULL' value is a common source of confusion for
 newcomers to SQL, who often think that `NULL' is the same thing as an
 empty string `'''.  This is not the case. For example, the following
 statements are completely different:
      mysql> INSERT INTO my_table (phone) VALUES (NULL);
      mysql> INSERT INTO my_table (phone) VALUES ('');
 Both statements insert a value into the `phone' column, but the first
 inserts a `NULL' value and the second inserts an empty string.  The
 meaning of the first can be regarded as "phone number is not known" and
 the meaning of the second can be regarded as "the person is known to
 have no phone, and thus no phone number."
 To help with `NULL' handling, you can use the `IS NULL' and `IS NOT
 NULL' operators and the `IFNULL()' function.
 In SQL, the `NULL' value is never true in comparison to any other
 value, even `NULL'.  An expression that contains `NULL' always produces
 a `NULL' value unless otherwise indicated in the documentation for the
 operators and functions involved in the expression. All columns in the
 following example return `NULL':
      mysql> SELECT NULL, 1+NULL, CONCAT('Invisible',NULL);
 If you want to search for column values that are `NULL', you cannot use
 an `expr = NULL' test. The following statement returns no rows, because
 `expr = NULL' is never true for any expression:
      mysql> SELECT * FROM my_table WHERE phone = NULL;
 To look for `NULL' values, you must use the `IS NULL' test.  The
 following statements show how to find the `NULL' phone number and the
 empty phone number:
      mysql> SELECT * FROM my_table WHERE phone IS NULL;
      mysql> SELECT * FROM my_table WHERE phone = '';
 You can add an index on a column that can have `NULL' values if you are
 using MySQL 3.23.2 or newer and are using the `MyISAM', `InnoDB', or
 `BDB' storage engine.  As of MySQL 4.0.2, the `MEMORY' storage engine
 also supports `NULL' values in indexes.  Otherwise, you must declare an
 indexed column `NOT NULL' and you cannot insert `NULL' into the column.
 When reading data with `LOAD DATA INFILE', empty or missing columns are
 updated with `'''. If you want a `NULL' value in a column, you should
 use `\N' in the data file. The literal word "`NULL'" may also be used
 under some circumstances.   `LOAD DATA' LOAD DATA.
 When using `DISTINCT', `GROUP BY', or `ORDER BY', all `NULL' values are
 regarded as equal.
 When using `ORDER BY', `NULL' values are presented first, or last if
 you specify `DESC' to sort in descending order. Exception: In MySQL
 4.0.2 through 4.0.10, `NULL' values sort first regardless of sort order.
 Aggregate (summary) functions such as `COUNT()', `MIN()', and `SUM()'
 ignore `NULL' values. The exception to this is `COUNT(*)', which counts
 rows and not individual column values.  For example, the following
 statement produces two counts.  The first is a count of the number of
 rows in the table, and the second is a count of the number of
 non-`NULL' values in the `age' column:
      mysql> SELECT COUNT(*), COUNT(age) FROM person;
 For some column types, MySQL handles `NULL' values specially.  If you
 insert `NULL' into a `TIMESTAMP' column, the current date and time is
 inserted.  If you insert `NULL' into an integer column that has the
 `AUTO_INCREMENT' attribute, the next number in the sequence is inserted.
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