Database Designs: MySQL NULL Fields
Does your table designs support NULL values?
There is a debate about creating tables which permit NULL values. I personally make every effort to make every field in a table NOT NULL. The argument for this: I want to be able to depend on my SQL statements. In this post I will illustrate how an insignificant field that allows NULL values can create an issue. This is a simple illustration. More complex queries can break down as easily where NULL values are allowed.
It’s in my opinion, that NULL fields should be a topic for discussion, when you begin talking about your designs and the integrity behind those designs.
Example: NULL fields are allowed.
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CREATE TABLE SECURITY_USERS (
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SECURITY_USER_ID Integer NOT NULL,
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USER_NAME Varchar(30) NOT NULL,
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FIRST_NAME Varchar(20),
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LAST_NAME Varchar(20),
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MIDDLE_INITIAL Varchar(1),
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CONSTRAINT PK_SECURITY_USERS PRIMARY KEY (SECURITY_USER_ID),
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CONSTRAINT UNQ_SECURITY_USERS_USER_NAME UNIQUE (USER_NAME)
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) TYPE=InnoDB DEFAULT CHARSET=utf8;
There are several things wrong with this table, but for the moment I would like to simply use it as an example of a table that supports NULL values. There are plenty of other fields in this table, but I’ve removed them for the sake of this post. They all permit NULL values.
In my opinion, the problem with this style design is that “I can’t always depend on my queries to produce good results“. Most of the time the system has entered in FIRST_NAME and LAST_NAME into the table and sometimes but not always, they entered a MIDDLE_INITIAL.
So, if we have two users in the system and the data looks like this:
Example: Sample data.
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/*
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SECURITY_USER_ID | USER_NAME | LAST_NAME | FIRST_NAME | MIDDLE_INITIAL
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1 admin Master Web (NULL)
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2 jdoe Doe John B
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*/
…and one of them has a middle initial, while the other does not. Then this creates the potential for unpredictable results in advanced queries.
Example: SQL Statement.
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SELECT concat(LAST_NAME, ‘, ‘, FIRST_NAME, ‘ ‘, MIDDLE_INITIAL) AS FULL_NAME
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FROM security_users WHERE security_user_id = 1 OR security_user_id = 2
All I want to do is concatenate the users name in SQL, so it displays in the format of my choice without my having to do it in code. Sometimes, it’s nice to bring back data in the format you want. Besides, if I execute this query in more than one place in my application I don’t want to have to continue formatting it in code. I just like to have the query do it.
The problem is, when NULL fields are allowed, you cannot depend on good results. Especially if I built my query and was testing it with people who had middle names. The first time the query came across someone who had a NULL middle name in the application, the application would receive unexpected results.
Example: Query Results
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/*
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FULL_NAME
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(NULL)
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Doe, John B
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*/
Despite the fact there is data in FIRST_NAME (= Web) and LAST_NAME (= Master), the returned result is still NULL. The MySQL concat() function failed to return the proper results, because MIDDLE_INITIAL contains a NULL value.
Using simple straight forward SQL statements failed, due to a NULL field in MIDDLE_INITIAL. This is a visual example; something we can see in a result window. However, if I had a more complicated query and was using a NULL field in a JOIN, then the problem isn’t as interesting. The problem can even become frustrating.
Example: NOT NULL table redesign (following the data structure)
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CREATE TABLE security_users (
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id bigint(20) NOT NULL AUTO_INCREMENT,
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user_name varchar(30) NOT NULL,
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user_first_name varchar(20) NOT NULL,
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user_last_name varchar(20) NOT NULL,
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user_middle_initial varchar(1) NOT NULL DEFAULT ”,
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UNIQUE KEY user_name (user_name),
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PRIMARY KEY (id)
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) TYPE=InnoDB DEFAULT CHARSET=UTF8 AUTO_INCREMENT=1000;
In this table, MIDDLE_INITIAL has a value, even if the value is empty. An empty value is not the same as NULL and the results would return as I might expect them. The query in this post would execute properly and return the right results.
