Thread: Concept of NULL
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Old December 29th, 2010, 10:17 AM
rseni rseni is offline
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Default Concept of NULL

Hy Guys,

Beginning with mysql. I am not able to grasp the concept of NULL. Check flickr screen-shots. In it when I specifically declared 'name' field to be NOT NULL. When i run the 'desc test' table command, the table description shows default value for name field to be NULL.Why is that so?

From what I have read about NULL, it connotes a missing or information that is not applicable. So when I declare a field to be NOT NULL it implies (as per my understanding) that user must enter a value for the name field else the DB engine should generate an error i.e. record will not be entered in DB. However when i run 'insert into test value();' the DB engine enters the record in table. Check flickr screen-shot(second-one).

FLICKR LINKS *declare_not_null* http://www.flickr.com/photos/55097319@N03/5302758813/

*empty_values* Check the second screenshot on flickr

Q.2 what would be sql statement to drop a primary key from a table's field. If I use 'ALTER TABLE test drop key id;' it gives the following:

ERROR: Incorrect table definition; there can be only one auto column and it must be defined as a key.

Thanks for your help.