The history of SQL begins in an IBM laboratory in San Jose, California, where SQL was developed in the late 1970s. The initials stand for Structured Query Language, and the language itself is often referred to as "sequel." It was originally developed for IBM's DB2 product (a relational database management system, or RDBMS, that can still be bought toChapter for various platforms and environments). In fact, SQL makes an RDBMS possible. SQL is a nonprocedural language, in contrast to the procedural or third-generation languages (3GLs) such as COBOL and C that had been created up to that time.
NOTE: Nonprocedural means what rather than how. For example, SQL describes what data to retrieve, delete, or insert, rather than how to perform the operation.
The characteristic that differentiates a DBMS from an RDBMS is that the RDBMS provides a set-oriented database language. For most RDBMSs, this set-oriented database language is SQL. Set oriented means that SQL processes sets of data in groups.
Two standards organizations, the American National Standards Institute (ANSI) and the International Standards Organization (ISO), currently promote SQL standards to industry. The ANSI-92 standard is the standard for the SQL used throughout this guide. Although these standard-making bodies prepare standards for database system designers to follow, all database products differ from the ANSI standard to some degree. In addition, most systems provide some proprietary extensions to SQL that extend the language into a true procedural language. We have used various RDBMSs to prepare the examples in this guide to give you an idea of what to expect from the common database systems. (We discuss procedural SQL--known as PL/SQL--on Chapter 18, "PL/SQL: An Introduction," and Transact-SQL on Chapter 19, "Transact-SQL: An Introduction.")
A little background on the evolution of databases and database theory will help you understand the workings of SQL. Database systems store information in every conceivable business environment. From large tracking databases such as airline reservation systems to a child's baseball card collection, database systems store and distribute the data that we depend on. Until the last few years, large database systems could be run only on large mainframe computers. These machines have traditionally been expensive to design, purchase, and maintain. However, today's generation of powerful, inexpensive workstation computers enables programmers to design software that maintains and distributes data quickly and inexpensively.
The most popular data storage model is the relational database, which grew from the seminal paper "A Relational Model of Data for Large Shared Data Banks," written by Dr. E. F. Codd in 1970. SQL evolved to service the concepts of the relational database model. Dr. Codd defined 13 rules, oddly enough referred to as Codd's 12 Rules, for the relational model:
Most databases have had a "parent/child" relationship; that is, a parent node would contain file pointers to its children. (See Figure 1.1.)
Codd's relational database management system.
This method has several advantages and many disadvantages. In its favor is the fact that the physical structure of data on a disk becomes unimportant. The programmer simply stores pointers to the next location, so data can be accessed in this manner. Also, data can be added and deleted easily. However, different groups of information could not be easily joined to form new information. The format of the data on the disk could not be arbitrarily changed after the database was created. Doing so would require the creation of a new database structure.
Codd's idea for an RDBMS uses the mathematical concepts of relational algebra to break down data into sets and related common subsets.
Because information can naturally be grouped into distinct sets, Dr. Codd organized his database system around this concept. Under the relational model, data is separated into sets that resemble a table structure. This table structure consists of individual data elements called columns or fields. A single set of a group of fields is known as a record or row. For instance, to create a relational database consisting of employee data, you might start with a table called EMPLOYEE that contains the following pieces of information: Name, Age, and Occupation. These three pieces of data make up the fields in the EMPLOYEE table, shown in Table 1.1.
|Will Williams||25||Electrical engineer|
|Dave Davidson||34||Museum curator|
|Don DeMarco||32||Game programmer|
The six rows are the records in the EMPLOYEE table. To retrieve a specific record from this table, for example, Dave Davidson, a user would instruct the database management system to retrieve the records where the NAME field was equal to Dave Davidson. If the DBMS had been instructed to retrieve all the fields in the record, the employee's name, age, and occupation would be returned to the user. SQL is the language that tells the database to retrieve this data. A sample SQL statement that makes this query is
SELECT * FROM EMPLOYEE
Remember that the exact syntax is not important at this point. We cover this topic in much greater detail beginning tomorrow.
Because the various data items can be grouped according to obvious relationships (such as the relationship of Employee Name to Employee Age), the relational database model gives the database designer a great deal of flexibility to describe the relationships between the data elements. Through the mathematical concepts of join and union, relational databases can quickly retrieve pieces of data from different sets (tables) and return them to the user or program as one "joined" collection of data. (See Figure 1.2.) The join feature enables the designer to store sets of information in separate tables to reduce repetition.
The join feature.
Figure 1.3 shows a union. The union would return only data common to both sources.
The union feature.
Here's a simple example that shows how data can be logically divided between two tables. Table 1.2 is called RESPONSIBILITIES and contains two fields: NAME and DUTIES.
|Bill Jackson||Interview for jobs|
It would be improper to duplicate the employee's AGE and OCCUPATION fields for each record. Over time, unnecessary duplication of data would waste a great deal of hard disk space and increase access time for the RDBMS. However, if NAME and DUTIES were stored in a separate table named RESPONSIBILITIES, the user could join the RESPONSIBILITIES and EMPLOYEE tables on the NAME field. Instructing the RDBMS to retrieve all fields from the RESPONSIBILITIES and EMPLOYEE tables where the NAME field equals Becky Boudreaux would return Table 1.3.
More detailed examples of joins begin on Chapter 6, "Joining Tables."
The most important decision for a database designer, after the hardware platform and the RDBMS have been chosen, is the structure of the tables. Decisions made at this stage of the design can affect performance and programming later during the development process. The process of separating data into distinct, unique sets is called normalization.
Computing technology has made a permanent change in the ways businesses work around the world. Information that was at one time stored in warehouses full of filing cabinets can now be accessed instantaneously at the click of a mouse button. Orders placed by customers in foreign countries can now be instantly processed on the floor of a manufacturing facility. Although 20 years ago much of this information had been transported onto corporate mainframe databases, offices still operated in a batch-processing environment. If a query needed to be performed, someone notified the management information systems (MIS) department; the requested data was delivered as soon as possible (though often not soon enough).
In addition to the development of the relational database model, two technologies led to the rapid growth of what are now called client/server database systems. The first important technology was the personal computer. Inexpensive, easy-to-use applications such as Lotus 1-2-3 and Word Perfect enabled employees (and home computer users) to create documents and manage data quickly and accurately. Users became accustomed to continually upgrading systems because the rate of change was so rapid, even as the price of the more advanced systems continued to fall.
The second important technology was the local area network (LAN) and its integration into offices across the world. Although users were accustomed to terminal connections to a corporate mainframe, now word processing files could be stored locally within an office and accessed from any computer attached to the network. After the Apple Macintosh introduced a friendly graphical user interface, computers were not only inexpensive and powerful but also easy to use. In addition, they could be accessed from remote sites, and large amounts of data could be off-loaded to departmental data servers.
During this time of rapid change and advancement, a new type of system appeared. Called client/server development because processing is split between client computers and a database server, this new breed of application was a radical change from mainframe-based application programming. Among the many advantages of this type of architecture are
In Implementing Client/Server Computing, Bernard H. Boar defines client/server computing as follows:
This type of application development requires an entirely new set of programming skills. User interface programming is now written for graphical user interfaces, whether it be MS Windows, IBM OS/2, Apple Macintosh, or the UNIX X-Window system. Using SQL and a network connection, the application can interface to a database residing on a remote server. The increased power of personal computer hardware enables critical database information to be stored on a relatively inexpensive standalone server. In addition, this server can be replaced later with little or no change to the client applications.
You can apply the basic concepts introduced in this guide in many environments--for example, Microsoft Access running on a single-user Windows application or SQL Server running with 100 user connections. One of SQL's greatest benefits is that it is truly a cross-platform language and a cross-product language. Because it is also what programmers refer to as a high-level or fourth-generation language (4GL), a large amount of work can be donehigher-level language 4GL (fourth-generation) language fourth-generation (4GL) language in fewer lines of code.
Oracle Corporation released the first commercial RDBMS that used SQL. Although the original versions were developed for VAX/VMS systems, Oracle was one of the first vendors to release a DOS version of its RDBMS. (Oracle is now available on more than 70 platforms.) In the mid-1980s Sybase released its RDBMS, SQL Server. With client libraries for database access, support for stored procedures (discussed on Chapter 14, "Dynamic Uses of SQL"), and interoperability with various networks, SQL Server became a successful product, particularly in client/server environments. One of the strongest points for both of theseSQL Server powerful database systems is their scalability across platforms. C language code (combined with SQL) written for Oracle on a PC is virtually identical to its counterpart written for an Oracle database running on a VAX system.
The common thread that runs throughout client/server application development is the use client/server computing of SQL and relational databases. Also, using this database technology in a single-user business application positions the application for future growth.
SQL is the de facto standard language used to manipulate and retrieve data from these relational databases. SQL enables a programmer or database administrator to do the following:
NOTE: The term SQL can be confusing. The S, for Structured, and the L, for Language, are straightforward enough, but the Q is a little misleading. Q, of course, stands for "Query," which--if taken literally--would restrict you to asking the database questions. But SQL does much more than ask questions. With SQL you can also create tables, add data, delete data, splice data together, trigger actions based on changes to the database, and store your queries within your program or database.
Unfortunately, there is no good substitute for Query. Obviously, Structured Add Modify Delete Join Store Trigger and Query Language (SAMDJSTQL) is a bit cumbersome. In the interest of harmony, we will stay with SQL. However, you now know that its function is bigger than its name.
The most commonly used statement in SQL is the SELECT statement (see Chapter 2, "Introduction to the Query: The SELECT Statement"), which retrieves data from the database and returns the data to the user. The EMPLOYEE table example illustrates a typical example of a SELECT statement situation. In addition to the SELECT statement, SQL provides statements for creating new databases, tables, fields, and indexes, as well as statements for inserting and deleting records. ANSI SQL also recommends a core group of data manipulation functions. As you will find out, many database systems also have tools for ensuring data integrity and enforcing security (see Chapter 11, "Controlling Transactions") that enable programmers to stop the execution of a group of commands if a certain condition occurs.
This section introduces some of the more popular implementations of SQL, each of which has its own strengths and weaknesses. Where some implementations of SQL have been developed for PC use and easy user interactivity, others have been developed to accommodate very large databases (VLDB). This sections introduces selected key features of some implementations.
NOTE: In addition to serving as an SQL reference, this guide also contains many practical software development examples. SQL is useful only when it solves your real-world problems, which occur inside your code.
We use Microsoft Access, a PC-based DBMS, to illustrate some of the examples in this text. Access is very easy to use. You can use GUI tools or manually enter your SQL statements.
We use Personal Oracle7, which represents the larger corporate database world, to demonstrate command-line SQL and database management techniques. (These techniques are important because the days of the standalone machine are drawing to an end, as are the days when knowing one database or one operating system was enough.) In command-line RÊl, simple stand+[cedilla]one SQL statements are entered into Oracle's SQL*Plus tool. This tool then returns data to the screen for the user to see, or it performs the appropriate action on the database.
Most examples are directed toward the beginning programmer or first-time user of SQL. We begin with the simplest of SQL statements and advance to the topics of transaction management and stored procedure programming. The Oracle RDBMS is distributed with a full complement of development tools. It includes a C++ and Visual Basic language library (Oracle Objects for OLE) that can link an application to a Personal Oracle database. It also comes with graphical tools for database, user, and object administration, as well as the SQL*Loader utility, which is used to import and export data to and from Oracle.
NOTE: Personal Oracle7 is a scaled-down version of the full-blown Oracle7 server product. Personal Oracle7 allows only single-user connections (as the name implies). However, the SQL syntax used on this product is identical to that used on the larger, more expensive versions of Oracle. In addition, the tools used in Personal Oracle7 have much in common with the Oracle7 product.
We chose the Personal Oracle7 RDBMS for several reasons:
Figure 1.4 shows SQL*Plus from this suite of tools.
TIP: Keep in mind that nearly all the SQL code given in this guide is portable to other database management systems. In cases where syntax differs greatly among different vendors' products, examples are given to illustrate these differences.
Microsoft Query (see Figure 1.5) is a useful query tool that comes packaged with Microsoft's Windows development tools, Visual C++, and Visual Basic. It uses the ODBC standard to communicate with underlying databases. Microsoft Query passes SQL statements to a driver, which processes the statements before passing them to a database system.
ODBC is a functional library designed to provide a common Application Programming Interface (API) to underlying database systems. It communicates with the database through a library driver, just as Windows communicates with a printer via a printer driver. Depending on the database being used, a networking driver may be required to connect to a remote database. The architecture of ODBC is illustrated in Figure 1.6.
The unique feature of ODBC (as compared to the Oracle or Sybase libraries) is that none of its functions are database-vendor specific. For instance, you can use the same code to perform queries against a Microsoft Access table or an Informix database with little or no modification. Once again, it should be noted that most vendors add some proprietary extensions to the SQL standard, such as Microsoft's and Sybase's Transact-SQL and Oracle's PL/SQL.
You should always consult the documentation before beginning to work with a new data source. ODBC has developed into a standard adopted into many products, including Visual Basic, Visual C++, FoxPro, Borland Delphi, and PowerBuilder. As always, application developers need to weigh the benefit of using the emerging ODBC standard, which enables you to design code without regard for a specific database, versus the speed gained by using a database specific function library. In other words, using ODBC will be more portable but slower than using the Oracle7 or Sybase libraries.
SQL was originally made an ANSI standard in 1986. The ANSI 1989 standard (often called SQL-89) defines three types of interfacing to SQL within an application program:
Before the concept of dynamic SQL evolved, embedded SQL was the most popular way to use SQL within a program. Embedded SQL, which is still used, uses static SQL--meaning that the SQL statement is compiled into the application and cannot be changed at runtime. The principle is much the same as a compiler versus an interpreter. The performance for this type of SQL is good; however, it is not flexible--and cannot always meet the needs of today's changing business environments. Dynamic SQL is discussed shortly.
The ANSI 1992 standard (SQL-92) extended the language and became an international standard. It defines three levels of SQL compliance: entry, intermediate, and full. The new features introduced include the following:
This guide covers not only all these extensions but also some proprietary extensions used by RDBMS vendors. Dynamic SQL allows you to prepare the SQL statement at runtime. Although the performance for this type of SQL is not as good as that of embedded SQL, it provides the application developer (and user) with a great degree of flexibility. A call-level interface, such as ODBC or Sybase's DB-Library, is an example of dynamic SQL.
Call-level interfaces should not be a new concept to application programmers. When using ODBC, for instance, you simply fill a variable with your SQL statement and call the function to send the SQL statement to the database. Errors or results can be returned to the program through the use of other function calls designed for those purposes. Results are returned through a process known as the binding of variables.
Chapter 1 covers some of the history and structure behind SQL. Because SQL and relational databases are so closely linked, Chapter 1 also covers (albeit briefly) the history and function of relational databases. Tomorrow is devoted to the most important component of SQL: the query.
The Workshop provides quiz questions to help solidify your understanding of the material covered, as well as exercises to provide you with experience in using what you have learned. Try to answer the quiz and exercise questions before checking the answers in Appendix F, "Answers to Quizzes and Exercises."