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1 database concepts

So what I plan to do in this book is this: First, I’ll explain the relational model without using SQL at all (that’s Part I of the book). Try this amazing Database Concepts Exam Quiz: Trivia! Likewise, an attempt to update the status for supplier S1 to 200 must also be rejected, if status values are supposed never to exceed 100. A DBMS is a software system that manages databases and access to those databases. Statements: The code overall consists of nine statements. In order to illustrate the concepts I’m going to be discussing, I obviously need to show coding examples; thus, I clearly need some formal language in which to express those examples. A relation has the following properties: Tuple: The rows of tables in a relationship are generally termed as Tuples. The database can control the inconsistency of data to a large extent. Unfortunately, there’s a problem. From everything I’ve said so far, it should be quite clear that there’s a logical difference between a database, which is a repository for data, and a DBMS, which is the software that manages such a repository. 1.1 What do you think the following Tutorial D expressions represent? Now, the reason we might want to change the way the database is physically stored and accessed is almost always performance; and the fact that we can make such changes without having to change the way the database looks to the user means that existing application programs, queries, and the like can all still work after the change. Logical data independence (For the rest of this book, I’ll take the term database, unqualified, to mean a logical database specifically, unless the context demands otherwise—which in fact it almost never will.) (Actually they’re relations, as we’ll see in Chapter 2, but for the purposes of this introductory chapter “files” or “tables” will do.) We might also say, somewhat glibly, that it protects the data from users!—by which I mean it provides certain security, concurrency, integrity, and recovery controls. The innkeepers sometimes access the files directly. Assignment: Assignment (denoted “:=” in Figure 1-3) is an operator for updating a variable—that is, assigning a value, probably different from the previous value, to the variable in question. The column of a relation are assigned distinct names, and the ordering of these columns is immaterial. Note: Of these two operators (“:=” and “=”), equality in particular is absolutely fundamental—for without it, we couldn’t even tell, given some value v and some set of values S, whether v appears in S (i.e., whether v is an element of S). [6] I do not follow that usage in this book. International Islamic University By the way, notice the use of the keyword UPDATE (in upper case) in this exercise. This level is used by users. One immediate and significant implication of the foregoing is this: Assuming the optimizer makes a reasonably good job of things, users shouldn’t have to get involved in performance issues at all. For example, in a geometric application, we might have user defined types called POINT, LINE, RECTANGLE, CIRCLE, and so on. Oil and Gas Forms are computer programs that manage the data you use to fill in regulatory forms. Very importantly, therefore, data independence means protecting existing investment—investment in user training, in existing applications, and in existing database designs (among other things). This document introduces concepts, principles, terminology, and architecture of near-zero downtime database migration for cloud architects who are migrating databases to Google Cloud from on-premises or other cloud environments. To repeat, then, a relational DBMS is a DBMS that supports a user interface that’s a faithful implementation of the relational model—meaning, as far as the user is concerned, that: Relational operators (i.e., operators that operate on data in relational form) are available to serve as a basis for formulating retrieval and update requests. See also www.thethirdmanifesto.com. The code fragment in Figure 1-3 involves three literals—0, 1, and ‘The sum is’ (the first two of these are of type INTEGER and the third is of type CHAR). RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM DB2, Oracle, MyS For example, in the case of type INTEGER, which for simplicity I take to be system defined, the agency responsible for defining the type—in other words, the system, by my assumption—must define: Operators “:=”, “=”, “<”, etc., for assigning and comparing integers, Operators “+”, “*”, etc., for performing arithmetic on integers, Perhaps a CAST operator, for converting integers to character strings, But not character string operators such as “| |” (“concatenate”), SUBSTR (“substring”), etc., because—let’s agree, for the sake of the example at least—these operators make no sense for integers. Now it’s your turn. Note: I believe Tutorial D is pretty much self-explanatory; however, a comprehensive description can be found if needed in the book Databases, Types, and the Relational Model: The Third Manifesto, by Hugh Darwen and myself (3rd edition, Addison-Wesley, 2007).[8]. Table P represents kinds of parts. The point is this: Most requests (indeed, very likely all requests) are capable of being implemented in a variety—typically a very large variety—of different ways. Since the database keeps on growing, then there may be frequent changes at times. Geodatabase Concepts Image vs. The code fragment in Figure 1-3 didn’t illustrate the point, but types in general can be either system defined (i.e., built in) or user defined. In other words, the DBMS provides users with a perception of the database that’s more abstract, and thus more user friendly, than the way the database looks physically—i.e., the way it’s physically stored inside the system.[4]. Variables: A variable is a container for a value (different values at different times, in general). 1.6 Explain the following in your own words: 1.7 (Try this exercise without looking back at the body of the chapter.) Figure 1-1 below shows sample values for a typical database, having to do with suppliers, parts, and shipments (of parts by suppliers). Note: In the rest of this book I’ll abbreviate “kinds of parts,” most of the time, to just parts. EXTERNAL LEVEL OR VIEW LEVEL It defines a number of simplified domain-specific views. The prescriptions of the relational model aren’t a straitjacket—rather, they’re a discipline, a discipline that makes life much easier for the user. I’ve therefore decided to use both forms in this book, letting context be my guide in any given situation (and I won’t pretend I’ve been all that consistent in this regard, either). As for the INSERT and DELETE operators, however, where no ambiguity arises, it can be a little tedious always to set them in all caps—especially when they’re being used as qualifiers, as in, e.g., “INSERT statement” (“insert statement”?). [7] Much simpler too than their counterparts in the various proposals—which crop up all too frequently, I’m sorry to say—for replacing the relational model (see, e.g., XML, NoSQL, the Actor Model, and on and on). Parts with weight less than 12.5. Each kind of part has one part number (PNO), which is unique; one name (PNAME); one color (COLOR); one weight (WEIGHT); and one location where parts of that kind are stored (CITY). Integrity can be managed using the database. In a nutshell, users must be limited to performing only those operations they’re allowed to perform. Now, in practice, those user requests to the DBMS can be formulated in a variety of different ways (e.g., by pointing and clicking with a mouse). Finally, the reason for rehearsing all of this extremely familiar material is that (as I hope you were expecting) all of the foregoing concepts are directly relevant to databases, as we’ll see in the chapters to come. Perhaps a little confusingly, it has become standard practice in the database world to use the uppercase term UPDATE to refer to the specific operator that changes existing data (as opposed to DELETE, which deletes existing data, and INSERT, which inserts new data), and the lowercase term update to refer to the INSERT, DELETE, and UPDATE operators considered generically. Equality: The comparison v1 = v2 must give TRUE if and only if v1 and v2 are the very same value (implying, incidentally, that they must certainly be of the same type). Cardinality: The number of tuples or rows in a relation is termed as cardinality. Take O’Reilly online learning with you and learn anywhere, anytime on your phone and tablet. Get Relational Theory for Computer Professionals now with O’Reilly online learning. One last point to close this section: You might or might not know (but I hope you do) that the relational model was originally the invention of E. F. Codd, when he was employed as a researcher at IBM (E for Edgar and F for Frank—but he always signed with his initials; to his friends, among whom I was proud to count myself, he was Ted). Welcome to the home page of Database System Concepts, Sixth Edition. It follows that, logically speaking, assignment is the only update operator we need. It describes only part of databases. So now we know what a DBMS is. UPDATE S WHERE STATUS > 10 : { STATUS := STATUS + 5 } ; 1.4 What do you understand by the following terms: (a) security controls; (b) integrity controls; (c) concurrency controls; (d) recovery controls? This part of the exercise was definitely unfair, but you probably guessed the answer anyway—just as restriction picks out certain rows, so the operation of projection, illustrated by this example, picks out certain columns. Note: The term update, in lower case,[5] is used to refer generically to requests that insert new data or delete or change existing data. Last updated 1/2016 English English [Auto] Cyber Week Sale. As the database may be viewed through three levels of abstraction, any change at any level can affect other levels' schemas. Of course, it isn’t possible to set any particularly searching exercises at this early point in the book, and the following are little more than review questions. To repeat, the DBMS acts as an intermediary between the logical and physical databases; in other words, it supports the user interface to the database. Concurrency controls have to do with the possibility that several users might be using the database at the same time. In other words, the relational model can be thought of as a kind of recipe for what the user interface is supposed to look like in such a DBMS. Users can insert new information into the database, and delete, change, or retrieve existing information in the database, by issuing requests or commands to the software that manages the database—which is to say, the database management system (DBMS for short). It’s not so important to remember the actual data values in detail—though it certainly wouldn’t hurt if you did. Increase the status by 5 for all suppliers with status greater than 10. Different application programmers and various end-users have different views upon data, which must be derived from a common overall data structure. This example involves a relational restriction operation. Database System Concepts Sixth Edition Avi Silberschatz Henry F. Korth S. Sudarshan McGraw-Hill ISBN 0-07-352332-1 Face The Real World of Database Systems Fully Equipped. See the answer to Q: at the end of the chapter. O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers. A database is a structured collection of information on a defined subject. A database intends to have a collection of data stored together to serve multiple applications as possible. quiz which has been attempted 5340 times by avid quiz takers. Recovery controls protect against data loss. The optimizer is the DBMS component that’s responsible for deciding exactly how to implement user requests. Before going any further, I’d like to stress the fact that that recipe (viz., the relational model) is very simple! Also, there’s at most one shipment at any given time for a given supplier and given part, and so the combination of supplier number and part number is unique to any given shipment. Since the database keeps on growing, then there may be frequent changes at times. What’s more, the semantics of these operators must be such that the following requirements are satisfied: Assignment: After assignment of value v to variable V, the comparison V = v must give TRUE. Note: Throughout this book, I take the term user to mean either an application programmer or an interactive user[2] or both, as the context demands. The “current value” of a given variable—meaning the value the variable contains at some particular time—can be changed in one and only one way: namely, by executing an assignment statement in which the variable in question serves as the target. As that figure is meant to suggest, the DBMS—the software that manages the database—effectively serves as a kind of intermediary between the logical and physical levels of the system: User requests for access to the database are expressed in terms of the logical database, and they’re implemented by the DBMS (“executed”) in terms of the corresponding physical database. Database Concepts gives undergraduate database management students and business professionals alike a firm understanding of the concepts behind the software, using Access 2016 to illustrate the concepts and techniques. As a matter of fact—to jump ahead of myself for a moment—let me state here for the record that it is and always was a major objective of the relational model that it should be the system, not the user, that has to worry about performance issues. Now, it’s true that the recipe might sometimes look a little complicated, but that’s because the relational model is, above all, precise, and precision requires precise terminology, and precise terminology can sometimes be a little daunting. Values: A value is an “individual constant”; it’s what’s denoted by an expression, and it’s what can be assigned to a variable. A value is an “individual constant”—e.g., the integer three—and it can’t be changed. Examples throughout the rest of this book are based for the most part on the foregoing database. It’s deliberately not deep, and if you do already know something about database management you probably won’t find anything here you don’t already know. Read-only operators: A read-only operator is an operator like “+” that “derives new values from old ones”; for example, the expression 2 + 3 derives the “new” value 5 from the “old” values 2 and 3. Second, I’ll show how ideas from the relational model are realized in concrete form in SQL specifically (that’s Part III of the book). one-to-one (1:1 or 1..1) relationship associations among two or more entities that are used by data models. Terms of service • Privacy policy • Editorial independence, Figure 1-1. Hence a database is often conceived of as a repository of information needed for running certain functions in a corporation or organization. 1.2 What do you think the following Tutorial D statements do? Third and last, it’s important to understand that associated with any given type T, there’s a set of operators defined for operating on values and variables of type T (because types without operators are useless). Sync all your devices and never lose your place. 1. Current price $22.99. The fact that the logical and physical databases are distinguished and (ideally, at least) kept rigidly apart is what allows us to achieve the important goal of data independence. For further explanation of this point, see Chapter 2. Chapter 1: Database Concepts Bob Dalesio and Philip Stanley. Degree: The number of attributes in a relation determines the degree of relation. [6] A typical example, found almost at random on the Internet: “MySQL [is] the world’s most popular open source database.” No, it isn’t. RG Edu 91,401 views. Note carefully that a read-only operator, when it’s invoked, returns a result but doesn’t update anything (in particular, it doesn’t update its operands). Of course, it’s true that real databases tend to be much more complicated than this “toy” example; but the trouble with using more realistic examples is that they tend to be too complicated (they make it difficult to see the forest for the trees, as it were). In Figure 1-3, for example, “I := I + 1 ;” is a statement—an assignment statement, as it happens (see further discussion below)—while “I + 1” is an expression. A database can be thought of as a kind of electronic filing cabinet; it contains digitized information (“data”), which is kept in persistent storage of some kind, typically on magnetic disks. Over 5 similar quizzes in this Release for Oracle database Concepts for Beginners a database is a collection information..., denoted by the way, notice the use of the database keeps on growing, then there be... Expressly designed for the purpose the VIEW from 30,000 feet, ” as were..., games, and 1/2016 English English [ Auto ] Cyber Week Sale contains one more!: 29:56, O ’ Reilly Media, Inc. all trademarks and registered trademarks appearing on are! Avid quiz takers Inc. all trademarks and registered trademarks appearing on oreilly.com are the property of their respective owners ll... 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