[nycphp-talk] Need help understanding NULL
Kristina D. H. Anderson
ka at kacomputerconsulting.com
Sat Aug 29 16:47:09 EDT 2009
The test for and behavior of NULL varies somewhat from language to
language and database to database.
Let's take a basic example, you design a database table and you
describe your LastName field as NOT NULL.
That means that every time you do an insert into the table, the field
LastName MUST contain a value of some sort. The purpose of that is
basically to ensure that any query, unless it contains all of the
required values, will error out if you attempt to do the insert without
having a value in that field.
In most cases, not all cases, on INSERT, an empty string i.e. '' is not
considered a NULL value, but an empty string which will result in ...
well...an empty string being stored in your database, but the field
will not show as NULL. When you pull that out of the database, some
languages will treat that as a NULL value and some will treat it as an
Or, in your database, you can specify NULL as the default value for the
field, and therefore if you skip it in an insert, it will default to
NULL. The design of the database depends on the needs of the
As a test of how PHP treats NULL vs. empty strings with your database,
Create values of NULL, ' ' (a string containing a space), '' (an empty
string) and pull them out into variables and then you can begin to
determine how, in your application, they are being treated.
In most cases, you should see NULLs and empty strings look very
similar...but not always. And the devil is definitely in the details,
especially if your application is talking to other platforms that may
handle these values differently.
It can help you to understand NULL, if you contemplate the difference
between an empty string '' and NULL. NULL is not a string because it's
undetermined. But an empty string is a string that contains, well,
nothing, but it is not NULL.
> Matt Juszczak wrote:
> > something is NULL, then you don't know what it is. It could be
> > or it could be set. You just don't know.
> That's precise the part I can't seem to wrap my head around.
> > If something is empty, then you know it's empty. So if birthdate
> > to '', then you know they have no birthdate. But if it's set to
> > you don't know if they have one or not.
> Thanks for trying, Matt, but I still don't get the purpose of "knowing
> that you don't know" something, so to speak.
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