However, it goes without saying that this objective hasn’t yet been fully achieved, and so (as I’ve already said, more or less) the goal of using SQL relationally must sometimes be compromised in the interest of achieving satisfactory performance. So the referential integrity rule simply spells out the semantics of foreign keys; the name “referential integrity” derives from the fact that a foreign key value can be regarded as a reference to the tuple with that same value for the corresponding target key. That is, for beginners at least, it’s hard to see the relevance of the material, or it’s hard to understand the problems it’s meant to solve, or both. Date, including An Introduction to Database Systems, Eighth Edition, and The Database Relational Model: A Retrospective Review and Analysis : A Historical Account and Assessment of E. F. Codd's Contribution to the Field of Database Technology, and more on ThriftBooks.com. (By the way, it isn’t necessary, though it’s often a good idea, for attributes to have the same name as the corresponding type, and frequently they won’t. Instead, such definitions are supplied by some user. In passing, I observe that, more generally, there’s a logical difference between a thing of any kind and a picture of that thing. Indeed, distinct representations of the same value can appear at any number of distinct locations in time and space—meaning, loosely, that any number of different variables (see the definition immediately following) can have the same value, at the same time or different times. $34.09 #20. Listen to the Latest MapScape Podcast To be more specific, I’m sure you know something about SQL; but— and I apologize for the possibly offensive tone here—if your knowledge of the relational model derives only from your knowledge of SQL, then I’m afraid you won’t know the relational model as well as you should, and you’ll probably know “some things that ain’t so.” I can’t say it too strongly: SQL and the relational model aren’t the same thing. In this book, therefore, I usually will follow the primary key discipline—and in pictures like Fig. Partly for that reason, from this point forward I’ll use the term key, unqualified, to mean any candidate key, regardless of whether the candidate key in question has additionally been designated as primary. Date, is intended to give database practitioners an introduction to relational theory and its relationship with the SQL standard and syntax. For example, consider the tuple for supplier S1 in Fig. Perhaps most likely in practice, the model as such isn’t taught at all—the SQL language, or some specific dialect of that language, such as the Oracle dialect, is taught instead. What relvars does the suppliers-and-parts database contain? Date Get SQL and Relational Theory, 3rd Edition now with O’Reilly online learning. For the most part, my aim in this chapter has been just to tell you what I rather hope you knew already (and you might have felt the chapter was a little light on technical substance, therefore). For example, there’s no way to subvert the system by somehow “flopping over” from one attribute to another. If you haven’t done so already, go through the chapter again and identify all of the places where I used the term relation when I should by rights have used the term relvar instead. Or consider the concept join: Users have to know what a join is, they have to know how to invoke a join, they have to know what the result of a join looks like, and so on. For now, let me just point out that we’ve encountered quite a few important examples of such differences already. Instead, you can think of a view as being “materialized”—in effect, you can think of a base relation being constructed whose value is obtained by evaluating the specified relational expression—at the time the view in question is referenced. 1.3 and hence to pretend the heading is just a set of attribute names. Learning the theory behind T-SQL querying and programming is an important step in developing code. For example, projecting the suppliers relation of Fig. And while it might be acceptable to pretend otherwise in informal contexts—indeed, I often do so myself—I would argue that it’s acceptable only if all parties involved understand that those more user friendly terms are just an approximation to the truth and fail overall to capture the essence of what’s really going on. 1.13 Write CREATE TABLE statements for an SQL version of the suppliers-and-parts database. That tuple has four components, corresponding to the four attributes SNO, SNAME, STATUS, and CITY. Fig. Users have to understand the model, since it defines the interface they have to deal with; they don’t have to understand the implementation, because that’s under the covers (at least, it should be). Thus, we might reasonably say that some specific product has a faster or slower implementation of some specific join, on some specific data, than some other specific product does—but that’s about all. Observe now that I said it’s usual to choose a primary key. Note: I realize I haven’t yet explained the syntax of relational assignment in detail, so don’t worry too much about giving a syntactically correct answer—just do the best you can. You can avoid them if you understand relational theory, but only if you know how to put that theory into practice. The concept of logical difference derives from a dictum of Wittgenstein’s: All logical differences are big differences. The fact is, historically there’s been a lot of confusion over yet another logical difference: namely, that between relations as such, on the one hand, and relation variables on the other. SQL and Relational Theory. © 2020, O’Reilly Media, Inc. All trademarks and registered trademarks appearing on oreilly.com are the property of their respective owners. First of all, every relation has a heading and a body: The heading is a set of attributes (where by the term attribute I mean, very specifically, an attribute-name : type-name pair, and where no two attributes in the same heading have the same attribute name), and the body is a set of tuples that conform to that heading. But those extra operators add complexity, not power! A variable does have location in time and space. Don’t fall into the common trap of thinking the term relation means a base relation specifically—or, in SQL terms, thinking the term table means a base table specifically. SQL is full of difficulties and traps for the unwary. Before going any further, there’s an important point I need to explain, because it underpins everything else to be discussed in this book. 1.1 and 1.3 weren’t relations as such but, rather, pictures of relations. 1.2 is a pictorial representation of those original operators. Likewise, don’t fall into the common trap of thinking base relations (or base tables, in SQL) have to be physically stored. In essence, then, a null is a marker that means value unknown. Of course, it isn’t possible to set any particularly searching exercises at this early point in the book, and the following are mostly little more than review questions. "SQL and Relational Theory", a book written by one of E.F. Codd's contemporaries, C.J. Physical data independence is a principle in relational theory that says that the physical implementation details should be hidden from, or transparent to, the user submitting the queries against the relational database management system. The second point is that, as you probably realize, it’s precisely the separation of model and implementation that allows us to achieve physical data independence. But even though I do hope you’re familiar with most of what I have to say in this chapter, I’d like to suggest, respectfully, that you not skip it. For example, suppose we don’t know employee E2’s salary. ... Murach's MySQL (3rd Edition) Joel Murach. For example, we might restrict relation EMP to just those tuples where the DNO value is D2. Email Subscription. Please understand too that relational operations always produce a result without duplicate tuples, again by definition. We’ve seen a few differences already—for example, the fact that SQL permits duplicate rows, the fact that SQL tables have a left to right column ordering, and the fact that SQL doesn’t clearly distinguish between table values and table variables—and we’ll see many more in the pages to come. As a direct consequence, they deliver far less physical data independence than they should, and far less than, in principle, relational systems are capable of. You should know what the theoretically correct situation is, and you should have strong reasons for departing from it. In this book, Chris Date explains relational theory in depth, and demonstrates through numerous examples and exercises how you can apply it to your use of SQL. The relational model has nothing to say about “stored relations” at all; in particular, it categorically doesn’t say which relations are stored and which not. First of all, consider the concept relation itself. 1.17 The discussions that follow are based on more extensive ones to be found in my book An Introduction to Database Systems (see Appendix G). Domain is the term used in most of the older database literature, however. You can avoid them if you understand relational theory, but only if you know how to put that theory into practice. Well, yeah, you know, a relation is kind of like a table, or a kind of a table, or something ... if you see what I mean. Listen to the Latest MapScape Podcast Thus, a unique index mustn’t be confused with a key in the relational sense, even though the former might be used to implement the latter (more precisely, to implement some key constraint—see Chapter 8). The implementation is what the user doesn’t have to know. When I want to exclude such possibilities, I’ll talk explicitly in terms of proper subsets. The point about principles is: They endure. In SQL contexts, by contrast, I’ll use SQL’s own terms. See the answer to Exercise 3.5 in Chapter 3 for several further differences. “[It’s] important to make a distinction between stored relations ... and virtual relations”: Actually, it’s extremely important from the user’s perspective (and from the perspective of the relational model, come to that) not to make any such distinction at all. See also the paper “A Sweet Disorder,” mentioned in Appendix G. In the interest of accuracy, I should add that for reasons that needn’t concern us here, relations in mathematics, unlike their counterparts in the relational model, do have a left to right ordering to their attributes. With reference to Fig. Observe in particular that, by definition, a value can’t be updated; for if it could, then after such an update it wouldn’t be that value any longer. 1.9 (Try this exercise without looking back at the body of the chapter.) Fast. In exactly the same way, two tuples are equal if and only if they’re the very same tuple. This third edition has been revised, extended, and improved throughout. Using SQLite: Small.  For more on SQL and the relational model, see: Date, C.J., 2015. Indeed, it’s that simple representation on paper that makes relational systems easy to use and easy to understand, and makes it easy to reason about the way such systems behave. And I’m generally sympathetic to the idea of using more user friendly terms, if they can help make the ideas more palatable. In fact, the relational model can itself be seen as a small part of mathematics; as such, it evolves over time as new theorems are proved and new results discovered. If there’s just one candidate key, then there’s no choice and no problem; but if there are two or more, then having to choose one and make it primary smacks a little bit of arbitrariness (at least to me). SQL is both an ANSI and ISO standard language based on the relational model, designed for querying and managing data in an RDBMS. SQL and Relational Theory, 2nd Edition. Reliable. 4.5 out of 5 stars 181. I also discussed the difference between base relations (or base relvars, rather) and views. In this book, Chris Date explains relational theory in depth, and demonstrates through numerous examples and exercises how you can apply it to your use of SQL. Crucially, it’s not itself a value; it is, to repeat, a marker, or flag. All that’s part of the model. This third edition has been revised, extended, and improved throughout. Be on your lookout for confusion in this area. 1.10 Throughout this book I use the term relational model to mean the abstract machine originally defined by Codd (though that abstract machine has been refined, clarified, and extended somewhat since Codd’s original vision). That figure shows three relation values: namely, the relation values that happen to exist in the database at some particular time. The relational model is, of course, a data model. Next, a primary key is a candidate key that’s been singled out for special treatment in some way. You can avoid them if you understand relational theory, but only if you know how to put that theory into practice. Date (2009) Data Management Databases And Organization By Richard T. Watson 3rd Edition $7.99 However, I’ll also abbreviate relation value, most of the time, to just relation (exactly as we abbreviate integer value most of the time to just integer). To be specific, I show comments as text strings in italics, bracketed by “/*” and “*/” delimiters. Nulls in the database cause wrong answers. All of that being said, I should now say too that it’s actually a major advantage of the relational model that its basic abstract object, the relation, does have such a simple representation on paper. 1.12 The following is an excerpt from another database textbook: “[The relational] model ... defines simple tables for each relation and many to many relationships. In the early 1970s, IBM developed a language called SEQUEL (short for Structured English QUEry Language) for its RDBMS product called System R. The name of the language was later changed from SEQUEL to SQL because of a trademark dispute. Judge not the play before the play is done; Speaks a new scene; the last act crowns the play. Then the original assignment is logically equivalent to the following one: This latter assignment is effectively equivalent to deleting d from R and then inserting i into R. Do note, however, that the DELETE and INSERT in question are both being done as part of the same statement, not as two separate statements. The fact is, I reject the concept of “nulls” entirely; that is, it’s my very strong opinion that nulls have no place in the relational model. If “table” means, specifically, an SQL table, then I certainly agree there are some important distinctions to be observed, but they’re not the ones the extract seems to be interested in. SQL supports “quantified comparisons,” but they’re better avoided. 1.6 Distinguish between the two meanings of the term data model. But if you know the underlying principles—in other words, if you know the relational model—then you have knowledge and skills that will be transferable: knowledge and skills that you’ll be able to apply in every environment and will never be obsolete. Although the relational model is much more flexible than its predecessors, it pays a price for this flexibility. A related question: Can all relational assignments be expressed in terms of INSERT and/or DELETE and/or UPDATE? The reason is obvious: Declarative means the system does the work, procedural means the user does the work (so we’re talking about productivity, among other things). In other words, the foregoing DELETE / INSERT combination, unlike the assignment it’s trying to mimic, is a sequence of two distinct statements. But if we’re talking about data models in the first sense, then there’s only one relational model, and it’s the relational model (with the definite article). 1.3, for example, the suppliers relation is equal to the suppliers relation and not to anything else—in particular, not to any other relation. Also, relations contain values, but tabular pictures don’t (they don’t even contain “occurrences” or “appearances” of such values); rather, they contain symbols that denote such appearances—for example, the symbol 5 (i.e., the numeral 5), which denotes an appearance of the value five. of the relational model ... What’s wrong with it? Logical differences `` SQL and relational theory '', a book written by one of the material anywhere... The themes, of course, the relation values that happen to exist in the former a conceptual pool values... Possibly somewhat abstract, database design many times in the body of the worst I. 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