Information AboutSql |
|
HISTORY A seminal , was published in June, 1970 in the Association For Computing Machinery (ACM) journal, Communications Of The ACM . Codd's Model became widely accepted as the definitive model for ''relational'' database management systems ( RDBMS or RDMS ). During the 1970s, a group at IBM 's San Jose research center developed a database system " System R " based upon, but not strictly faithful to, Codd's model. Structured English Query Language (''"SEQUEL"'') was designed to manipulate and retrieve data stored in System R. The acronym ''SEQUEL'' was later condensed to '''SQL''' because the word 'SEQUEL' was held as a Trademark by the Hawker-Siddeley aircraft company of the UK . Although SQL was influenced by Codd's work, Donald D. Chamberlin and Raymond F. Boyce at IBM were the authors of the SEQUEL language design.Donald D. Chamberlin and Raymond F. Boyce, 1974. " SEQUEL: A structured English query language ", International Conference on Management of Data, Proceedings of the 1974 ACM SIGFIDET (now SIGMOD) workshop on Data description, access and control, Ann Arbor, Michigan, pp. 249-264. Their concepts were published to increase interest in SQL. The first non-commercial, relational, non-SQL database, Ingres , was developed in 1974 at U.C. Berkeley . In 1978, methodical testing commenced at customer test sites. Demonstrating both the usefulness and practicality of the system, this testing proved to be a success for IBM. As a result, IBM began to develop commercial products that implemented SQL based on their System R prototype, including the System/38 (announced in 1978 and commercially available in August 1979), SQL/DS (introduced in 1981), and DB2 (in 1983). {Link without Title} At the same time Relational Software, Inc. (now Oracle Corporation ) saw the potential of the concepts described by Chamberlin and Boyce and developed their own version of a RDBMS for the Navy, CIA and others. In the summer of 1979 Relational Software, Inc. introduced Oracle V2 (Version2) for VAX computers as the first commercially available implementation of SQL. Oracle is often incorrectly cited as beating IBM to market by two years, when in fact they only beat IBM 's release of the System/38 by a few weeks. Considerable public interest then developed; soon many other vendors developed versions, and Oracle's future was ensured. It is often suggested that IBM was slow to develop SQL and relational products, possibly because it wasn't available initially on the mainframe and Unix environments, and that they were afraid it would cut into lucrative sales of their IMS database product, which used Navigational Database models instead of relational. But at the same time as Oracle was being developed, IBM was developing the System/38 , which was intended to be the first relational database system, and was thought by some at the time, because of its advanced design and capabilities, that it might have become a possible replacement for the mainframe and Unix systems. SQL was adopted as a standard by ANSI (American National Standards Institute) in 1986 and ISO (International Organization for Standardization) in 1987 . ANSI has declared that the official pronunciation for SQL is , although many English-speaking database professionals still pronounce it as ''sequel''. The SQL standard has gone through a number of revisions: SCOPE The SQL standard is not freely available. SQL:2003 may be purchased from ISO or ANSI . A late draft is available as a zip archive from Whitemarsh Information Systems Corporation . The zip archive contains a number of PDF files that define the parts of the SQL:2003 specification. Although SQL is defined by both ANSI and ISO, there are many extensions to and variations on the version of the language defined by these standards bodies. Many of these extensions are of a proprietary nature, such as Oracle Corporation's PL/SQL or Sybase , IBM 's SQL PL (SQL Procedural Language) and Microsoft 's Transact-SQL . It is also not uncommon for commercial implementations to omit support for basic features of the standard, such as the DATE or TIME data types, preferring some variant of their own. As a result, in contrast to ANSI C or ANSI Fortran , which can usually be ported from platform to platform without major structural changes, SQL code can rarely be ported between database systems without major modifications. There are several reasons for this lack of portability between database systems:
SQL is designed for a specific, limited purpose — querying data contained in a relational database. As such, it is a Set-based , Declarative Computer Language rather than an Imperative Language such as C or BASIC which, being Programming Language s, are designed to solve a much broader set of problems. Language extensions such as PL/SQL are designed to address this by turning SQL into a full-fledged programming language while maintaining the advantages of SQL. Another approach is to allow programming language code to be embedded in and interact with the database. For example, Oracle and others include Java in the database, while PostgreSQL allows functions to be written in a wide variety of languages, including Perl , Tcl , and C . One joke about SQL is that "SQL is neither structured, nor is it limited to queries, nor is it a language." This is founded on the notion that ''pure'' SQL is not a classic programming language since it is not '' Turing-complete ''. On the other hand, however, it is a programming language because it has a grammar, syntax, and programmatic purpose and intent. The joke recalls Voltaire 's remark that the Holy Roman Empire was "neither holy, nor Roman, nor an empire." SQL contrasts with the more powerful Database-oriented Fourth-generation Programming Languages such as Focus or SAS in its relative functional simplicity and simpler command set. This greatly reduces the degree of difficulty involved in maintaining SQL source code, but it also makes programming such questions as 'Who had the top ten scores?' more difficult, leading to the development of procedural extensions, discussed Above . However, it also makes it possible for SQL source code to be produced (and optimized) by software, leading to the development of a number of natural language database query languages, as well as ' Drag And Drop ' database programming packages with ' Object Oriented ' interfaces. Often these allow the resultant SQL source code to be examined, for educational purposes, further enhancement, or to be used in a different environment. SQL KEYWORDS SQL keywords fall into several groups. Data retrieval The most frequently used operation in transactional databases is the data retrieval operation. When restricted to data retrieval commands, SQL acts as a Functional Language
''Example 1:''
ORDER BY title
''Example 2:''
FROM books AS bk, book_authors AS ba WHERE bk.book_number = ba.book_number GROUP BY bk.title Example 2 shows both the use of multiple tables (called a "join"), and aggregation (grouping). "bk" and "ba" are known as "table aliases" and help avoid ambiguity when performing joins. ("AS" is optional in many dialects.) This example shows how many authors there are per book. Example output may resemble: Title Authors -- --- SQL Examples and Guide 3 The Joy of SQL 1 How to use Wikipedia 2 Pitfalls of SQL 1 How SQL Saved my Dog 1 Data manipulation First there are the standard Data Manipulation Language (DML) elements. DML is the subset of the language used to add, update and delete data.
Example: INSERT INTO my_table (field1, field2, field3) VALUES ('test', 'N', NULL); UPDATE my_table SET field1 = 'updated value' WHERE field2 = 'N'; DELETE FROM my_table WHERE field2 = 'N'; Data transaction Transaction, if available, can be used to wrap around the DML operations.
COMMIT and ROLLBACK interact with areas such as transaction control and locking. Strictly, both terminate any open transaction and release any locks held on data. In the absence of a START TRANSACTION or similar statement, the semantics of SQL are implementation-dependent.Example: START TRANSACTION; UPDATE inventory SET quantity = quantity - 3 WHERE item = 'pants'; COMMIT; Data definition The second group of keywords is the Data Definition Language (DDL). DDL allows the user to define new tables and associated elements. Most commercial SQL databases have proprietary extensions in their DDL, which allow control over nonstandard features of the database system. The most basic items of DDL are the CREATE and DROP commands.
Some database systems also have an ALTER command, which permits the user to modify an existing object in various ways -- for example, adding a column to an existing table.Example: CREATE TABLE my_table ( my_field1 INT UNSIGNED, my_field2 VARCHAR (50), my_field3 DATE NOT NULL, PRIMARY KEY (my_field1, my_field2) ) Data control The third group of SQL keywords is the Data Control Language (DCL). DCL handles the authorization aspects of data and permits the user to control who has access to see or manipulate data within the database. Its two main keywords are:
Example: GRANT SELECT, UPDATE ON my_table TO some_user, another_user Other
Example:
DATABASE SYSTEMS USING SQL
CRITICISMS OF SQL Technically, SQL is a declarative computer language for use with " SQL Database s". Theorists and some practitioners note that many of the original SQL features were inspired by, but in violation of, the Relational Model for database management and its Tuple Calculus realisation. Recent extensions to SQL achieved relational completeness, but have worsened the violations, as documented in '' The Third Manifesto ''. In addition, there are also some criticisms about the practical use of SQL:
ALTERNATIVES TO SQL A distinction should be made between alternatives to relational and alternatives to SQL. The list below are proposed alternatives to SQL, but are still (nominally) relational. See Navigational Database for alternatives to relational.
SEE ALSO Wikibook EXTERNAL LINKS
BOOKS REFERENCES # Discussion on alleged SQL flaws (C2 wiki) # Web page about FSQL : References and links. # Galindo J., Urrutia A., Piattini M., "Fuzzy Databases: Modeling, Design and Implementation" . Idea Group Publishing Hershey, USA, 2005. |
|
|