Go Premium for a chance to win a PS4. Enter to Win

x
?
Solved

MySQL - I don't understand

Posted on 2014-12-11
2
Medium Priority
?
215 Views
Last Modified: 2014-12-11
See attached. This is a small sample of a table (named "customer") that I have.

Notice the column called cutype in most cases the value is NULL (in italics), I don't know what that means.

I run this query: DELETE from customer where cutype <> 'i'

It deletes the 79 rows that have blank (not NULL) in the column.

It leaves all the rest.

Actually, at this moment, there are NO rows in the table where cutype = 'i'.

Why does it leave the NULL ones? That is not a value of 'i'.

Thanks
cutype.jpg
0
Comment
Question by:Richard Korts
2 Comments
 
LVL 24

Accepted Solution

by:
mankowitz earned 2000 total points
ID: 40495213
Null isn't really an empty value so much as it is the absense of any value. The way to check for null is by using IS NULL. For example, to delete rows where cutype is null, you'd have to do this:

DELETE FROM customer WHERE cutype IS NULL;

if you want to get rid of anything that is either null or NOT i, do this:

DELETE FROM customer WHERE cutype IS NULL OR cutype <> 'i';
0
 
LVL 58

Expert Comment

by:Gary
ID: 40495214
NULL values are a bit special
http://dev.mysql.com/doc/refman/5.1/en/working-with-null.html

If you want to delete rows with NULL values you have to declare them

    DELETE from customer where cutype <> 'i' OR cutype IS NULL
0

Featured Post

Free Tool: Subnet Calculator

The subnet calculator helps you design networks by taking an IP address and network mask and returning information such as network, broadcast address, and host range.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Does the idea of dealing with bits scare or confuse you? Does it seem like a waste of time in an age where we all have terabytes of storage? If so, you're missing out on one of the core tools in every professional programmer's toolbox. Learn how to …
This post looks at MongoDB and MySQL, and covers high-level MongoDB strengths, weaknesses, features, and uses from the perspective of an SQL user.
In this video, Percona Solution Engineer Rick Golba discuss how (and why) you implement high availability in a database environment. To discuss how Percona Consulting can help with your design and architecture needs for your database and infrastr…
In this video, Percona Solutions Engineer Barrett Chambers discusses some of the basic syntax differences between MySQL and MongoDB. To learn more check out our webinar on MongoDB administration for MySQL DBA: https://www.percona.com/resources/we…

916 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question