NULL is not a value - it's a condition. So if you try and write something "= NULL" you'll get an empty set rather than the results you're looking for. Try "IS NULL" instead.

example

Joining 2 tables connecting records that match



mysql> select * from demo_people join demo_property on demo_people.pid = demo_property.pid;

+-----+-----------+--------------+------+------+----------------------+

| pid | name      | phone        | pid  | spid | selling              |

+-----+-----------+--------------+------+------+----------------------+

|   1 | Mr Brown  | 01225 708225 |    1 |    1 | Old House Farm       |

|   3 | Mr Pullen | 01380 724040 |    3 |    2 | The Willows          |

|   3 | Mr Pullen | 01380 724040 |    3 |    3 | Tall Trees           |

|   3 | Mr Pullen | 01380 724040 |    3 |    4 | The Melksham Florist |

|   3 | Mr Pullen | 01380 724040 |    3 |    6 | The Beetle Drive     |

+-----+-----------+--------------+------+------+----------------------+

5 rows in set (0.05 sec)



Joining two tables connecting records that match PLUS an extra for each "orphan" on left



mysql> select * from demo_people left join demo_property on demo_people.pid = demo_property.pid;

+-----+------------+--------------+------+------+----------------------+

| pid | name       | phone        | pid  | spid | selling              |

+-----+------------+--------------+------+------+----------------------+

|   1 | Mr Brown   | 01225 708225 |    1 |    1 | Old House Farm       |

|   2 | Miss Smith | 01225 899360 | NULL | NULL | NULL                 |

|   3 | Mr Pullen  | 01380 724040 |    3 |    2 | The Willows          |

|   3 | Mr Pullen  | 01380 724040 |    3 |    3 | Tall Trees           |

|   3 | Mr Pullen  | 01380 724040 |    3 |    4 | The Melksham Florist |

|   3 | Mr Pullen  | 01380 724040 |    3 |    6 | The Beetle Drive     |

+-----+------------+--------------+------+------+----------------------+

6 rows in set (0.01 sec)



Selecting only orphans from the join - doing it the CORRECT way



mysql> select * from demo_people left join demo_property on demo_people.pid = demo_property.pid where spid is NULL;

+-----+------------+--------------+------+------+---------+

| pid | name       | phone        | pid  | spid | selling |

+-----+------------+--------------+------+------+---------+

|   2 | Miss Smith | 01225 899360 | NULL | NULL | NULL    |

+-----+------------+--------------+------+------+---------+

1 row in set (0.00 sec)



Trying to select the orphans, but failing because NULL is not a value 



mysql> select * from demo_people left join demo_property on demo_people.pid = demo_property.pid where spid = NULL;

Empty set (0.00 sec)



mysql> 

See More SQL commands for further information sources
(written 2005-04-06, updated 2006-06-05)

 S152 - SQL Primer as Used in MySQL
  [4007] Which database should I use? MySQL v SQLite - (2013-02-16)
  [3061] Databases - why data is split into separate tables, and how to join them - (2010-11-20)
  [3060] INSERT, DELETE, REPLACE and UPDATE - changing the content of SQL tables - (2010-11-19)
  [2240] How do I query a database (MySQL)? - (2009-06-15)
  [591] Key facts - SQL and MySQL - (2006-02-04)
  [515] MySQL - an FAQ - (2005-12-03)
  [502] SELECT in MySQL - choosing the rows you want - (2005-11-22)
  [494] MySQL - a score of things to remember - (2005-11-12)
  [158] MySQL - LEFT JOIN and RIGHT JOIN, INNER JOIN and OUTER JOIN - (2004-12-20)

Back to Free parking for short errands in Melksham

Previous and next or Horse's mouth home

Forward to Different course every day

Our most popular resources
Course Picture
More to programming than just programming
Different course every day
NULL in MySQL
Free parking for short errands in Melksham
Information request forms, cleaning up spam
Searching security holes
A beautiful place to live and learn
Business practise, 2005 style