MySQL permits you to store a “zero”  value of ’0000-00-00’  as a “dummy date.”  This is in some cases more convenient than using NULL  values. If a date to be stored in a DATE  column cannot be converted to any reasonable value, MySQL stores ’0000-00-00’ . To disallow ’0000-00-00’ , enable the NO_ZERO_DATE  mode. 

To have MySQL check all dates and accept only legal dates (unless overridden by IGNORE ), set the sql_mode  system variable to "NO_ZERO_IN_DATE,NO_ZERO_DATE" . 

## B.3.4.3 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); 

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 = ’’; 

See Section 3.3.4.6, “Working with NULL Values” , for additional information and examples. 

You can add an index on a column that can have NULL  values if you are using the MyISAM , InnoDB , or MEMORY  storage engine. Otherwise, you must declare an indexed column NOT NULL , and you cannot insert NULL  into the column. 

When reading data with LOAD DATA , empty or missing columns are updated with ’’ . To load a NULL value into a column, use \N  in the data file. The literal word NULL  may also be used under some circumstances. See Section 13.2.7, “LOAD DATA Statement” . 

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. 

Aggregate (group) 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 

