We have seen SQL SELECT command along with WHERE clause to fetch data from MySQL table, but when we try to give a condition, which compare field or column value to NULL, it does not work properly.
To handle such situation MySQL provides three operators
- IS NULL: operator returns true if column value is NULL.
- IS NOT NULL: operator returns true if column value is not NULL.
- <=>: operator compares values, which (unlike the = operator) is true even for two NULL values.
Conditions involving NULL are special. You cannot use = NULL or != NULL to look for NULL values in columns. Such comparisons always fail because it's impossible to tell whether or not they are true. Even NULL = NULL fails.
To look for columns that are or are not NULL, use IS NULL or IS NOT NULL.
Using NULL values at Command Prompt:
Suppose a table tcount_tbl in TUTORIALS database and it contains two columnstutorial_author and tutorial_count, where a NULL tutorial_count indicates that the value is unknown:
Example:
Try out the following examples:
root@host# mysql -u root -p password; Enter password:******* mysql> use TUTORIALS; Database changed mysql> create table tcount_tbl -> ( -> tutorial_author varchar(40) NOT NULL, -> tutorial_count INT -> ); Query OK, 0 rows affected (0.05 sec) mysql> INSERT INTO tcount_tbl -> (tutorial_author, tutorial_count) values ('mahran', 20); mysql> INSERT INTO tcount_tbl -> (tutorial_author, tutorial_count) values ('mahnaz', NULL); mysql> INSERT INTO tcount_tbl -> (tutorial_author, tutorial_count) values ('Jen', NULL); mysql> INSERT INTO tcount_tbl -> (tutorial_author, tutorial_count) values ('Gill', 20); mysql> SELECT * from tcount_tbl; +-----------------+----------------+ | tutorial_author | tutorial_count | +-----------------+----------------+ | mahran | 20 | | mahnaz | NULL | | Jen | NULL | | Gill | 20 | +-----------------+----------------+ 4 rows in set (0.00 sec) mysql>
You can see that = and != do not work with NULL values as follows:
mysql> SELECT * FROM tcount_tbl WHERE tutorial_count = NULL; Empty set (0.00 sec) mysql> SELECT * FROM tcount_tbl WHERE tutorial_count != NULL; Empty set (0.01 sec)
To find records where the tutorial_count column is or is not NULL, the queries should be written like this:
mysql> SELECT * FROM tcount_tbl -> WHERE tutorial_count IS NULL; +-----------------+----------------+ | tutorial_author | tutorial_count | +-----------------+----------------+ | mahnaz | NULL | | Jen | NULL | +-----------------+----------------+ 2 rows in set (0.00 sec) mysql> SELECT * from tcount_tbl -> WHERE tutorial_count IS NOT NULL; +-----------------+----------------+ | tutorial_author | tutorial_count | +-----------------+----------------+ | mahran | 20 | | Gill | 20 | +-----------------+----------------+ 2 rows in set (0.00 sec)
Handling NULL Values in PHP Script:
You can use if...else condition to prepare a query based on NULL value.
Example:
Following example takes tutorial_count from outside and then compares it with the value available in the table.
<?php $dbhost = 'localhost:3036'; $dbuser = 'root'; $dbpass = 'rootpassword'; $conn = mysql_connect($dbhost, $dbuser, $dbpass); if(! $conn ) { die('Could not connect: ' . mysql_error()); } if( isset($tutorial_count )) { $sql = 'SELECT tutorial_author, tutorial_count FROM tcount_tbl WHERE tutorial_count = $tutorial_count'; } else { $sql = 'SELECT tutorial_author, tutorial_count FROM tcount_tbl WHERE tutorial_count IS $tutorial_count'; } mysql_select_db('TUTORIALS'); $retval = mysql_query( $sql, $conn ); if(! $retval ) { die('Could not get data: ' . mysql_error()); } while($row = mysql_fetch_array($retval, MYSQL_ASSOC)) { echo "Author:{$row['tutorial_author']} <br> ". "Count: {$row['tutorial_count']} <br> ". "--------------------------------<br>"; } echo "Fetched data successfully\n"; mysql_close($conn); ?>
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