Table of contents
SQL is the abbreviation of Structure Query Language. It is a database application language using the relational model. It was developed by IBM in the 1970s. As the prototype relational language of IBM's relational database prototype System R, it implements the relational database Information retrieval.
SQL (Structured Query Language) is a high-level non-procedural programming language that allows users to work on high-level data structures. SQL is a database query and programming language used to access data and query, update, and manage relational database systems. SQL is also the extension of the database file format.
SQL Overview
- The reason why the SQL language can be accepted by users and the industry and become an international standard is that it is a comprehensive, powerful language that is also easy to learn. The SQL language integrates the functions of data query (Data Query), data manipulation (Data Manipulation), data definition (Data Definition) and data control (Data Control).
- SQL is a high-level, non-procedural programming language that allows users to work on high-level data structures. It does not require users to specify the data storage method, nor does it require users to understand the specific data storage method, so different database systems with completely different underlying structures can use the same SQL language as the interface for data input and management. It uses record collections as operation objects. All SQL statements accept collections as input and return collections as output. This collection feature allows the output of one SQL statement to be used as the input of another SQL statement, so SQL statements can be nested, which gives it Great flexibility and powerful functions. In most cases, functions that require a large program in other languages can be achieved with just one SQL statement. This also means that very complex statements can be written in SQL language. .
- Structured Query Language (Structured Query Language) was first developed by IBM's St. Joseph Research Laboratory for its relational database management system SYSTEM R. Its predecessor was SQUARE language. The SQL language has a simple structure, powerful functions, and is easy to learn. Therefore, since its launch by IBM in 1981, the SQL language has been widely used. Nowadays, whether it is large database management systems such as Oracle, Sybase, DB2, Informix, SQL SERver, or commonly used on PCs such as Visual Foxpro and PowerBuilder, All database development systems support SQL language as a query language.
- The American National Standards Institute (ANSI) and the International Organization for Standardization (ISO) have developed SQL standards. ANSI is an American industry and business group responsible for developing American business and communications standards. ANSI is also a member of ISO and the International Electrotechnical Commission (IEC). ANSI publishes U.S. standards that correspond to international standards organizations. In 1992, ISO and IEC released an international standard for SQL, called SQL-92. The corresponding standard published by ANSI is ANSI SQL-92. ANSI SQL-92 is sometimes called ANSI SQL. Although there are some differences in the SQL versions used by different relational databases, most follow the ANSI SQL standard. SQL Server uses an extension set of ANSI SQL-92, called T-SQL, which follows the SQL-92 standard established by ANSI.
SQL development history
- In the early 1970s, Edgar Codd of the IBM San Jose, California Research Laboratory published the application principles of organizing data into tables (Codd's Relational Algebra). In 1974, D.D. Chamberlin and R.F. Boyce of the same laboratory developed a set of specification language - SEQUEL (Structured English QUEry Language) for Codd's Relational Algebra in developing the relational database management system System R, and in November 1976 A new version of SQL (called SEQUEL/2) was announced in the IBM Journal of R&D in March. It was renamed SQL in 1980.
- In 1979, ORACLE first provided commercial SQL, and IBM also implemented SQL in DB2 and SQL/DS database systems.
- In October 1986, ANSI in the United States adopted SQL as the standard language for relational database management systems (ANSI X3. 135-1986), which was later adopted as an international standard by the International Organization for Standardization (ISO).
- In 1989, the American ANSI adopted the SQL standard language for relational database management systems defined in the ANSI X3.135-1989 report, called ANSI SQL 89. This standard replaced the ANSI X3.135-1986 version. This standard is adopted by the following organizations:
- International Organization for Standardization (ISO), Report "Database Language SQL With Integrity Enhancement" for ISO 9075-1989
- U.S. Federal Government, published in The Federal Information Processing Standard Publication (FIPS PUB) 127
- Currently, all major relational database management systems support some form of SQL, and most databases are intended to comply with the ANSI SQL89 standard.
SQL features
- 1. SQL language integrates data query, data manipulation, data definition and data control functions
- 2. Collection-oriented languages
- 3. Non-procedural languages
- 4. Similar to natural language, simple and easy to use
- 5. Self-contained language is also an embedded language. It can be used independently or embedded into the host language.
SQL basic statements
- There are four basic DML operations in SQL: INSERT, SELECT, UPDATE and DELETE.
- 1. INSERT statement
- Users can use the INSERT statement to insert a row of records into a specified table.
- 2. SELECT statement
- The SELECT statement selects specific rows and columns from one or more tables. Because querying and retrieving data are the most important functions in database management, the SELECT statement is the most workload-intensive part of SQL. In fact, someone who only accesses the database to analyze data and generate reports may know nothing about other SQL statements.
- The result of a SELECT statement is usually another table. During the execution process, the system selects matching rows and columns from the database according to the user's criteria, and places the results into a temporary table. In direct SQL, it displays the results on the terminal's display or sends the results to a printer or file. It can also be combined with other SQL statements to put the results into a table with a known name.
- The SELECT statement is powerful. Although on the surface it seems that it is only used to complete the relational algebra operation "selection" (or "restriction") mentioned in the first part of this article, in fact it can also complete two other relational operations - "projection" and "connection" ", the SELECT statement can also complete aggregation calculations and sort data.
- 3. UPDATE statement
- The UPDATE statement allows users to modify existing rows in a known table.
- 4. DELETE statement
- The DELETE statement is used to delete rows in a known table. As in the UPDATE statement, all rows that meet the conditions in the WHERE clause will be deleted. Since there is no UNDO statement or warning such as "Are you sure to delete?" in SQL, be careful when executing this statement.