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NULL in MySQL

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)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
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)


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