As an exercise, you might like to go back over the text of this chapter so far and see exactly where I used the term relation when I really ought to have been using the term relvar instead (or as well). Aside: Perhaps I should also state here for the record that throughout this book—in accordance with standard scientific practice—I take expressions of the form “B is a subset of A” to include the possibility that A and B might be equal. Reproduction of site books is authorized only for informative purposes and strictly for personal, private use. Now, I don’t want to give the wrong impression here. Finally, what about the operators? But it’s convenient to overlook this nicety in informal contexts. Every subset of a body is a body. “physical” than views are, so far as the relational model is concerned. Date, is intended to give database practitioners an introduction to relational theory and its relationship with the SQL standard and syntax. Then if it also contained a duplicate of that tuple (if that were possible), it would simply be informing us of that same “true fact” a second time. Caveat lector. 1.3 shows one supplier, supplier S5, with no shipments at all. ), Following on from the previous point, I should add that not all table expressions—in either my sense or the standard’s—are legal in SQL in all contexts where they might be expected to be. In such situations, I generally prefer to use terminology of my own. For the remainder of this book I’ll use the term data model, or more usually just model for short, exclusively in its first sense. For example, consider the body of the suppliers relation in Fig. This updated edition of a classic on relational theory has added NoSQL to the mix. Find books What you can do about it? There’s nothing useful that can be done if n > 1 that can’t be done if n = 1. But if that relation has two or more candidate keys, then it’s usual to choose one of them as primary, meaning it’s somehow “more equal than the others.” Suppose, for example, that every department always has both a unique department number and a unique department name—not a very realistic example, perhaps, but good enough for present purposes—so that {DNO} and {DNAME} are both candidate keys for DEPT. A similar remark applies to tuples also. The TransRelational™ Approach to DBMS Implementation, discussed briefly in Appendix G.). 1.3. Judge not the play before the play is done; Speaks a new scene; the last act crowns the play. (As a matter of fact the standard does use the term table expression, but this term too it uses quite inappropriately; to be specific, it uses it to refer to what comes after the SELECT clause in a SELECT - FROM - WHERE - GROUP BY - HAVING expression. ), Finally, a foreign key is a combination, or set, of attributes FK in some relation r2 such that each FK value is required to be equal to some value of some key K in some relation r1 (r1 and r2 not necessarily distinct).6 With reference to Fig. By the way, it follows from all of the above that if we’re talking about data models in the second sense, then we might reasonably speak of “relational models” in the plural, or “a” relational model (with an indefinite article). Note the braces, by the way; to repeat, candidate keys are always combinations, or sets, of attributes (even when the set in question contains just one attribute), and the conventional representation of a set on paper is as a commalist of elements enclosed in braces. Note: Many other negative consequences of attribute ordering (or column ordering, rather, in SQL) are discussed in subsequent chapters (especially Chapters 6 and 7). (Note that access paths as such are nowhere mentioned in the relational model.) To repeat, DELETE is shorthand for a certain relational assignment—and, of course, an analogous remark applies to INSERT and UPDATE also: They too are basically just shorthand for certain relational assignments. Topics whose treatment has been expanded include data types and domains, table comparisons, image relations, aggregate operators and summarization, view updating, and subqueries. 1.3, for example, I could have shown the columns in any left to right order—say STATUS, SNAME, CITY, SNO— and the picture would still represent the same relation in the relational model. (The purpose of this exercise is to get you thinking about what’s involved. SQL first became an ANSI standard in 1986, and then an ISO standard in 1987. Recommendation: Don’t do this! Date, is intended to give database practitioners an introduction to relational theory and its relationship with the SQL standard and syntax. You can avoid them if you understand relational theory, but only if you know how to put that theory into practice. 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). An integrity constraint (constraint for short) is basically just a boolean expression that must evaluate to TRUE. Next, relations never contain duplicate tuples. For example, projecting the suppliers relation of Fig. 1.13 Here are some possible CREATE TABLE statements. Certainly the model isn’t well understood in the database community at large. Certainly there are situations where there don’t seem to be any good reasons for making such a choice. So there’s a logical difference between relation values and relation variables. Returns a relation containing all tuples that appear in the first and not the second of two specified relations. By required to match here, I mean that if, for example, EMP contains a tuple in which the DNO attribute has the value D2, then DEPT must also contain a tuple in which the DNO attribute has the value D2—for otherwise EMP would show some employee as being in a nonexistent department, and the database wouldn’t be “a faithful model of reality.”. End of aside. 1.13 Write CREATE TABLE statements for an SQL version of the suppliers-and-parts database. Also, be aware that some older writings (including certain very early ones by myself) unfortunately and mistakenly use the term domain when what they really mean is attribute. For example, these two pictures represent the same relation but different SQL tables: ┌─────┬────────┐           ┌────────┬─────┐│ SNO │ CITY   │           │ CITY   │ SNO │├═════┼────────┤           ├────────┼═════┤│ S1  │ London │           │ London │ S1  ││ S2  │ Paris  │           │ Paris  │ S2  ││ S3  │ Paris  │           │ Paris  │ S3  ││ S4  │ London │           │ London │ S4  ││ S5  │ Athens │           │ Athens │ S5  │└─────┴────────┘           └────────┴─────┘, (The corresponding SQL queries are SELECT SNO, CITY FROM S and SELECT CITY, SNO FROM S, respectively. 1.5 The terms heading, body, attribute, tuple, cardinality, and degree, defined in the body of the chapter for relation values, can all be interpreted in the obvious way to apply to relvars as well. 1.7 Explain in your own words (a) physical data independence, (b) the difference between model and implementation. Using SQLite: Small. End of aside. Aside: I’d like to elaborate for a moment on this matter of performance. Again, all that’s part of the model. I’ll have more to say about the calculus later, mostly in Chapters 10 and 11. Note: These CREATE TABLE statements, along with their Tutorial D counterparts, are repeated in Chapter 5, where further pertinent discussion can be found. Very importantly, therefore, physical data independence means protecting investment in user training and applications (investment in logical database designs also, I might add). 1.14 Tutorial D (I can’t show this in SQL, because SQL doesn’t support relational assignment): SP := SP UNION RELATION { TUPLE { SNO 'S5' , PNO 'P6' , QTY 250 } } ; The text between the keyword UNION and the closing semicolon is a relation selector invocation (see Chapter 3), and it denotes the relation that contains just the tuple to be inserted. As for the relational algebra, it consists of a set of operators that—speaking very loosely— allow us to derive “new” relations from “old” ones. But if you insist on, in effect, hand coding that join yourself, perhaps like this (pseudocode)—. I’ll come back to this issue in the next section. As a result, the scope for errors and obscure programming is reduced. O’Reilly Media, Inc, Sebastopol, CA. And I’m generally sympathetic to the idea of using more user friendly terms, if they can help make the ideas more palatable. SQL and Relational Theory: How to Write Accurate SQL Code by Date, C. J. and a great selection of related books, art and collectibles available now at AbeBooks.com. Title: SQL and Relational Theory, 3rd Edition; Author(s): Release date: November 2015; Publisher(s): O'Reilly Media, Inc. ISBN: 9781491941171 In SQL contexts, by contrast, I’ll use SQL’s own terms. (It’s true that you might drive better if you do have some understanding of what goes on under the hood, but you don’t have to know. Before going any further, there’s an important point I need to explain, because it underpins everything else to be discussed in this book. 1.3 has degree 4 and cardinality 5; likewise, relation P in that figure has degree 5 and cardinality 6, and relation SP in that figure has degree 3 and cardinality 12. Each shipment has one supplier number (SNO), one part number (PNO), and one quantity (QTY). Returns a relation containing all (sub)tuples that remain in a specified relation after specified attributes have been removed. Be aware, however, that all SQL products include features that aren’t part of the standard at all. The value n can be any nonnegative integer. More formally, it means that every tuple in every relation contains just a single value, of the appropriate type, in every attribute position. Nevertheless, I’d like to recommend that you have a go at them before going on to read the answers in the next section. In the case of the suppliers relation as depicted in Fig. First of all, observe that everything to do with performance is fundamentally an implementation issue, not a model issue. For example, suppose we don’t know employee E2’s salary. What does it mean to “define tables” for such things? In particular, an explicit JOIN invocation, although it certainly does denote a table, can’t appear as a “stand alone” table expression (i.e., at the outermost level of nesting), nor can it appear as the table expression in parentheses that constitutes a subquery (see Chapter 12).3 Please note that these remarks apply to many of the individual discussions in the body of the book; it would be very tedious to keep on repeating them, however, and I won’t. I’ll be appealing to it many times in the pages ahead. Even if we agree that the term “stored relation” might make some kind of sense (meaning a user visible relation that’s represented in storage in some direct and efficient manner, without getting too specific on just what direct and efficient might mean), which relations are “stored” should be of no significance whatsoever at the relational (i.e., user) level of the system. Unfortunately, the technical documentatio, Functional Programming Patterns in Scala and Clojure, Security and Privacy in Internet of Things (IoTs), HTML5 and JavaScript Projects, 2nd Edition, Building a RESTful Web Service with Spring, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. I’d also like to say something about the crucial notion of equality—especially as that notion applies to tuples and relations specifically. Download books for free. This third edition has been revised, extended, and improved throughout. Make Offer - SQL and Relational Theory : How to Write Accurate SQL Code by C. J. (An expanded version of that tribute appears in my book Date on Database: Writings 2000-2006, Apress, 2006. (Actually there’s nothing in the standard that does, so far as I’m aware, but I know the same isn’t true of certain SQL products.). Again, when we draw a relation as a table on paper, we do have to show the columns in some left to right order, but that ordering doesn’t correspond to anything relational. Although I’ll generally have little to say about performance in this book, therefore, I will occasionally point out certain performance implications of what I’m recommending. That concept is part of the model: Users have to know what relations are, they have to know they’re made up of tuples and attributes, they have to know how to interpret them, and so on. A relational approach to SQL: That’s the theme, or one of the themes, of this book. 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. SQL and Relational Theory (3rd Edition) Author: C J Date Publisher: O'Reilly Pages: 582 ISBN: 978-1491941171 Print:1491941170 Kindle: B017S0YPOG Audience:SQL Developers Rating: 4 Reviewer:Kay Ewbank. With reference to Fig. One widespread misconception about the relational model is that it’s a totally static thing. 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). If not, why not? 1.1 represent relations: n-ary relations, to be precise. 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). 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