MySQL NULL or NOT NULL That is The Question?

user_211191 0
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What is the difference between NULL and NOT NULL? And when should they be used?

sql-null mysql database
8 Answers

Also note that NULL is not equal to anything else, even not to NULL itself.

For example:

mysql> select if(NULL=NULL, "null=null", "null!=null");
+------------------------------------------+
| if(NULL=NULL, "null=null", "null!=null") |
+------------------------------------------+
| null!=null                               |
+------------------------------------------+
1 row in set (0.00 sec)

This definition of NULL is very useful when you need a unique key on a column that is partially filled. In such case you can just leave all the empty values as NULL, and it will not cause any violation of the uniqueness key, since NULL != NULL.

Here is an example of how you can see if something is NULL:

mysql> select if(null is null, "null is null", "null is not null");
+------------------------------------------------------+
| if(null is null, "null is null", "null is not null") |
+------------------------------------------------------+
| null is null                                         |
+------------------------------------------------------+
1 row in set (0.01 sec)
user_11687360

If you're not sure, use NOT NULL.

    Despite the common belief, NOT NULL doesn't require you to fill all fields; it just means whatever you omit will have the default value. So no, it doesn't mean pain. Also, NULL is less efficient in terms of indexing, and causes many edge case situations when processing what you receive from a query.

    So, while of course NULL values have a theoretical meaning (and in rare cases you can benefit from this), most of the time NOT NULL is the way to go. NOT NULL makes your fields work like any variable: they always have a value, and you decide if that value means something or not. And yes, if you need all the possible values and one extra value that tells you there's simply nothing there, you can still use NULL.

So why do they love NULL so much?

    Because it's descriptive. It has a semantic meaning, like "Nnah, wait, this is not just an empty string, this is a lot more exotic - it's the lack of information!" They will explain how it's different to say "time is 00:00" and "i don't knot what time it is". And this is valid; it just takes some extra effort to handle this. Because the system will allocate extra space for the information "is there a value at all" and it will constantly struggle checking it out. So for the tiny piece of semantic beauty, you sacrifice time and storage. Not much, but still. (Instead, you could have said "99:99" which is clearly an invalid time and you can assign a constant to it. No, don't even start, it's just an example.)

    The whole phenomenon reminds me of the good old isset debate where people are somehow obsessed with the beauty of looking at a nonexistent array index and getting an error message. This is completely pointless. Practical defaults are a blessing, they simplify the work in a "you know what I mean" style and you can write more readable, more concise, more expressive code that will make sense even after you spend 4 years with other projects.

Inevitable NULLs

    If you have JOINs, you will encounter NULLs sooner or later, when you want to join another row but it's not there. This is the only valid case where you can't really live without NULLs; you must know it's not just an empty record you got, it's no record at all.

    Otherwise? NOT NULL is convenient, efficient, and gives you fewer surprises. In return, it will be called ignorant and outrageous by some people with semantical-compulsive disorder. (Which is not a thing but should be.)

TL;DR

Prefer NOT NULL when possible.
It's a weird thing - for the machine, too.

user_18926070
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