Hello, (please read all before replying)
> >> >> > Basics of database design: store what you know. > >> >> > > >> >> > Given that NULLs basically means the absence of a value > >> >> > (eg: unknown), you shouldn't be storing NULLs. > >> >> > > >> >> Nonsense!! > >> > > >> > That's a bold statement ... > >> > > >> >> That's simply wrong. A null means "unknown or not applicable" and is a > >> > > >> > Ah, so now things become clear, a NULL actually can mean > >> > two things? So much for clarity then... > >> > > >> Come on; that's not fair. "Unknown" and "not applicable" are more like > >> different senses of the same thing, not two opposite things. > > > > Read the literature on how to design databases. What you do > > is storing "true propositions". That is, each attribute defines a > > certain "true proposition". For example: > > > > Employee "Martijn" has Employee# 14. > > > > You should be able to derive these sentences from every row. > > Putting a NULL or N/A in there fails to meet this requirement. > > > Perhaps my theory is a bit rusty but I have never heard of this "true > propositions" business before. Do you have a citation where I can read about > this? I would certainly recommend this one: http://www.amazon.com/exec/obidos/ASIN/0321197844/databasede095-20/103-0880452-0854264?creative=327641&camp=14573&adid=0VQ1KDVJ56MV7389RPEQ&link_code=as1 > Frankly, I'm skeptical about your interpretation of this idea. While you > clearly don't want false information in a database, it isn't false to say > that student such-and-such's exam mark or such-and-such an employee's > termination date is unknown or not applicable. It _would_ be false to store > a grade of 0 for a student who didn't take the exam because it implies that > he got every question wrong when in fact he didn't write the test at all; Agreed. > that would be an example of 0 having two contradicatory meanings. Given a > phobia about nulls, it is a reasonable design to put students who don't take > the test in a separate table but it's not the ONLY way to handle the > situation: a null to indicate a student who didn't take the test is > perfectly reasonable too. Disagreed. Let me explain further: In this case, a "null" means (according to you and in your design) that the student did not take the test. It could also mean that we lots it's grade for now, and that we want to fill it in later, as in: "unknown", which is what NULL also means (your own words). In another case, null would be the employee isn't terminated yet, but could be in the future. That's the problems with nulls -> there's no actual defined meaning. You can never say: "This table holds test results, a row will mean: Student with StudentID 123 made test #12 and scored a 4" Cause you can have rows that can mean this: Student with StudentID 123 made test #12 and scored a yet unknown score or Student with StudentID 123 made test #12 and did not make the test or Student with StudentID 123 and test #12 doesn't apply to him In short: NULL is ambigious. > >> >> perfectly valid value to use in many, many situations. Two classic > >> > examples: > >> > > >> > Let me first state that there's a difference between storing NULLs > >> > and handling NULLs in your result. > >> > > >> Yes, of course there is: so what? You said NULLs should never be > > _stored_; > >> that's what I'm responding to. Using NULLs is not very _difficult_ > >> anyway, > >> just a bit tedious because it is another case to handle. > > > > There's no point in storing what you don't know. > > > But you _do_ know something: that the student didn't take the exam. That is > a piece of true knowledge all by itself. But that's not what the rows in the table would mean - see above. In the relational model, the idea is to store true propositions - that is, each row means the same thing, but with different attributes. See above. > >> > So, let's debunk these two classic examples... > >> > > >> Please note that I did not say that you MUST use NULLs, just that they > > were > >> perfectly valid to use in a design. > > > > Depends on who you're asking ... > > > > A few years ago, I was working at a company that had developed > > an application with an Oracle database with about 400 tables, nothing > > too large. > > > > Plenty of tables had NULLs, not because of missing business data, > > but rather used internally in the system. > > > I'm not sure what you mean by that phrase "used internally in the system". Missing business data, eg: no middle name. Some people would put in a NULL in there as well, while NULL can also mean: I do not KNOW this persons middle name. Ambiguity, see above. > > The number of problems we had with them, I cannot count on my > > two hands, nor can I count them on the hands of all other developers > > (5) for that project. > > > How about enumerating a few of them here? I've dealt with relational > databases in several capacities for over 20 years and I've never found nulls > to be a big problem. Not big, but a problem... :-) Do you often go around looking for (little) problems while you can easily avoid them? > > Just last week, I spoke to a guy who used to be my collegue there > > and we discussed the design a bit (he still does database design and > > development for a large company, on Sybase with over 4000 > > deployed systems) and we both agreed that using NULLs in most > > cases as part of the initial design was causing us more problems > > then helping us. > > > Again, how about listing some of these problems? Well, for one, we had a table that stored all kinds of different actions that should take place. Each action could be part of a list of actions. Someone decided that actions with a NULL for the "belongs to list of actions #" would be an action "by itself". Business wise, one could create an order for a product and that product had to be created by a list of certain actions. The order creator would then use any pre-defined list of actions, or create such a list himself by selecting the different actions. Asking for stand alone actions would then (obviously simplified): select * from bewerking where list is null This is a slow query. Period. One that could have easily been avoided by not storing NULLs, but using a separate table. Why a table? Because "standalone actions" do not equal "actions that belong to a list". > >> Okay, that might be acceptable, if it doesn't cause you to lose track of > > the > >> student altogether. But if this table was the only one that even recorded > >> the _existence_ of the student, you'd have a problem; > > > > Indeed, then I would have a problem, cause you cannot derive from > > the "test results" table that a student exists. This is a rather silly > > statement. > > If the "test results" table should ALSO store the existence of a student, > > you're design is wrong :-) > > > >>if someone tried to > >> verify that the student had attended this school, you wouldn't know that > >> they had. > > > > In your own words: nonsense. > > > > Each table should store what is was designed to store. If I would > > have to know if a student was enlisted in a certain course or would > > be attending this school, I would not be using the "test results" table. > > > Look, I agree that this scenario is not particularly likely to happen but it > COULD happen. It's not me who made up the example :-) > Suppose you run a small company that runs certification exams: > your only contact with the students is that they show up to take an exam and > you record the mark and send it to whoever awards the certification. In that > case, you likely wouldn't have an elaborate set of tables containing a > variety of student information the way you would if you were a university. > In that case, it's quite reasonable to believe that the only table in the > system is the one that records the test marks. That table may include the > student name, address, test name, score, etc. etc. That table may only be > populated as the student comes in the door: the 'greeter' welcomes the > student to the test center, asks his name and other details, points him to > the exam room, and records the test result when the student hands in the > exam. In that scenario, if student Joe Blow does not show up for the test > because his car broke down, he may never be entered in the system at all and > you have therefore lost sight of the fact that he ever existed. Again, this > is not a very likely scenario but it is not an impossible one and that's the > key point: if it COULD happen, you will have a problem. See above. There's no point in taking exams from random people. You are trying to come up with all sorts of unlikely scenarios to justify a solution that's, IMO, faulty. > >>Or in a more probable case, if that was the only test for that > >> course and the student missed it and then had no row in the table, you > > might > >> not have any way of knowing that they took the course! > > > > See above. > > > Dismissing the less probable case does not dismiss this more probable > scenario. Wrong problem description for task at hand. See above (my previous reply, not this one). Attack each problem with the right solution, don't try to find problems in solutions that aren't meant to solve the problem you just found. > >>And if they later > >> wanted to write the exam, having recovered from their illness, your query > >> might have the effect of keeping them from taking the second exam: your > >> query would report that they had never taken the first exam so an > >> adminstrator might refuse to let him/her take the second exam because > >> they > >> (apparently) had never been scheduled to take the first one. > > > > Different problem, see above. > > > Again, you can't dismiss the second scenario by dismissing the first one; > the scenarios are different and the second one is more probable than the > first. See above. The "Different problem" remark still holds. > >> Now, you could have a second table to record people who were scheduled to > >> take tests but failed to take them to cover that situation but I think it > > > > Sounds like a decent design to me. > > > Not surprisingly since it is the one you suggested as the "correct" way to > handle the situation. But the design that uses nulls is NOT invalid and you > have not demonstrated that it is. What I DID demonstrate, however, is that it's _very_ easy to create a design without NULLs, that is much clearer to read, doesn't have to handle NULLs, avoids NULLs in general and make it easy to understand what problems are solved. Considering maintainability, ability to refactor and without having to think up missing data, I prefer my design. Do note I don't claim to have the key to every problem, but I would like to know people that design WITH NULLs doesn't have to be the standard although SQL provides them. > >> would be easier to record all students in one table and then simply store > > a > >> null for any test that they fail to take and a zero for every case where > >> a > >> student got every answer wrong. When you compute the class average, the > >> avg() function would ensure that the students who got every question > >> wrong > >> would pull down the class average but that students who failed to write > > the > >> test at all would NOT skew the average because the avg() function ignores > >> nulls. > > > > Ah, an excellent example of why NULLs are tedious: they are ignored > > by some functions, but not by others. > > > _ALL_ of the column functions ignore nulls: it is NOT inconsistent. In fact, > it is the only logical thing that these functions can do, which you'd > realize if you gave it a bit of open-minded thought. Don't accuse me of not being open minded. I did develop systems with NULLs before. I became a database developer back then from practicing. However, I'm beginning to understand the theory more and more and dislike NULLs more and more because of it. > Suppose you had a table with a primary key of employee number and a column > containing bonuses. Some of the rows contain nulls in the bonus column > because the employee didn't earn a bonus yet. Now, you need to answer these > questions: what is the largest bonus amount? What is the smallest bonus > amount? What is the sum of the bonuses? What is the average bonus? Since a > null, by definition, is unknown, it isn't bigger or smaller or equal to any > other value, including another null so it can't qualify as the biggest value > or the smallest value. A number plus (or minus, or times, or divided by) a > null is undefined so it makes no sense to add it to the sum of the bonuses. > A null is NOT a zero so treating it as a zero in an average would unduly > skew the result. So, in each case, the function does the logical thing and > IGNORES the null. > > > Without having the record there in the first place, there would have > > to be no rule of AVG ignoring NULLs. Problem solved. > > > That is the other way of the solving the problem. But it is NOT the ONLY > way. > > >> > Why store a date column if you don't know? > >> > > >> > Why not use: > >> > > >> > EMPLOYEES > >> > EmployeeID int, > >> > StartingDate Date, > >> > ... > >> > > >> > TERMINATED_EMPLOYEES (albeit a bit agressive ;) ) > >> > EmployeeID > >> > TerminationDate > >> > > >> > Once more: perfectly valid design. > >> > > >> Yes, that is also a valid design but it means you have to have yet > >> another > >> table that you could have avoided simply by permitting a null in the > >> employee table. Hey, if you really want your tables to proliferate like > >> this, that's up to you. > > > > It seems you have a fear of creating new tables ;-) > > > No, I don't. It seems to me that you have an undue fear of nulls. > > > This is what database systems are designed to do... > > > Database systems are designed to accomodate large quantities of tables but > any designer with any real world experience will advise you not to create > tables simply for the sake of creating tables: that is an unnecessary > proliferation of tables. Look at normalization. As you probably know, each > phase of normalization tends to increase the number of tables in the > database. I know for a fact that the theorists have identified at least up > to 17NF (17th Normal Form) but how many levels of normalization do > businesses typically use in their normalizations? Answer: Three. > (Occasionally four). And I think you'll find that the main reason is that > the benefits of normalizing beyond 3NF are outweighed by the increasing > number of tables to manage. > > >> The key point is that NULLs _do_ work and are a legitimate design > > decision. > >> You don't _have_ to use them but they can save you some work and reduce > > the > >> number of tables you need. (They can also be a bit more work on the > >> programming side.) > >> > >> I took your remarks to mean that NULLs were always a bad idea and were a > >> symptom of bad design and I strongly disagree with that. > > > > So I've noticed. > > > I'm trying to be gracious here and accept that I may just have misunderstood > the emphasis in what you are saying. See above. > > The message I'm trying to get across is that: > > 1 - according to (proper) design literature, you should not use NULLs > > if you don't have to > > Perhaps you can cite some of this "proper" literature. I'm interested in > seeing their definition of what an appropriate time to use nulls is. > > > 2 - NULLs can cause you more problems than you can think of > > I'd like to hear what these problems are. I've never had any big problems > because of nulls. Ah well, SQL (these days) provides plenty of functions to avoid the problems. Coalesce being one of them... How often do people write: (probably not valid MySQL) select firstname || coalesce(middlename || ' ', ' ') || lastname from ... > > 3 - its sometimes much easier to avoid storing NULLs AND to be > > able to refactor your database because of it > > I agree that nulls can frequently be avoided and that avoiding them will > simplify _some_ situations. > > > 4 - the meaning of NULL can change, so why store it in the first place > > > Simple: because it happens to be true when it is stored. In the case of the > hypothetical employee, I store a null termination date when I hire him > because I don't know when he is going to leave. Why store something that you -don't- know. > If he laters gives his > notice, then I know when he is leaving and can store that date for his > termination date instead of a null; then his row of the table is true again, > based on the new facts. > > >> If you are simply saying that you don't like them and prefer to use > >> different designs to avoid them, then I don't have any problem with that. > > > > I have seen that when I avoid storing NULLs, my applications > > became more clear and easier to understand. > > > Beauty is in the eye of the beholder as we all know. I have no problem with > having nulls in my tables and consider that a better design than a separate > table for special cases most of the time. > > Again, if you are saying that you don't like nulls and prefer to avoid them > in your designs, I have no problem with that; that's just your personal > preference. It's the same as if we sat down to eat a meal and you asked for > chocolate ice cream for dessert and I had vanilla; neither choice is > "wrong", they are just personal preference. Well, to that we agree and I would order sugared whipped cream with it ;-) > But if you are stating categorically that nulls are always "bad" or "wrong", > I disagree strongly. That's like saying only chocolate ice cream is > acceptable and that all other flavours are evil. That's just wrong. We are not going to agree, that much is clear. I think we raised some very interesting points though (which is exactly why I started this in the first place). I would avoid NULLs if I could. It makes - very often - more sense to avoid them than to include them. You say that it's not "bad" to have NULLs every now and then, a statement to which I can agree up to a certain height. I would also like to state, that I've seen design where NULLs is grossly overused for all sorts of situations. Your simple examples can be among those. IF you're using NULLs, you'd better be aware of what you're doing. Just don't use NULL for the sole purpose of avoiding a table or relation. I'd still say that in general, the "true proposition" remark holds and is very very valid. With that as the basis, design your database. I think I've also described why using NULLs in a "true proposition" makes them invalid. I would advice against that. This topic will pop up every now and then. If there's one thing that I would like, is that people stop using NULLs way too often, I think we can agree on that. I for one had many big and small problems with NULLs stored in the database. That made me appreciate more how easy it is to avoid them and how to handle situations differently. Martijn Tonies Database Workbench - development tool for MySQL, and more! Upscene Productions http://www.upscene.com My thoughts: http://blog.upscene.com/martijn/ Database development questions? 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