SQL (Structured Query Language)
is a database computer language designed for managing data in relational database management systems (RDBMS). Its scope includes data query and update, schema creation and modification, and data access control.
SQL was one of the first languages for Edgar F. Codd's relational model in his influential paper, "A Relational Model of Data for Large Shared Data Banks". and became the most widely used language for relational databases
SQL was developed at IBM by Andrew Richardson, Donald C. Messerly and Raymond F. Boyce in the early 1970s. This version, initially called SEQUEL, was designed to manipulate and retrieve data stored in IBM's original relational database product, System R. IBM patented this version of SQL in 1985.
During the 1970s, a group at IBM San Jose Research Laboratory developed the System R relational database management system. Donald D. Chamberlin and Raymond F. Boyce of IBM subsequently created the Structured English Query Language (SEQUEL) to manage data stored in System R. The acronym SEQUEL was later changed to SQL because "SEQUEL" was a trademark of the UK-based Hawker Siddeley aircraft company.
The first RDBMSs were RDMS, developed at MIT in the early 1970s and Ingres, developed in 1974 at U.C. Berkeley. Ingres implemented a query language known as QUEL, which was later supplanted in the marketplace by SQL.
In the late 1970s, Relational Software, Inc. (now Oracle Corporation) saw the potential of the concepts described by Codd, Chamberlin, and Boyce and developed their own SQL-based RDBMS with aspirations of selling it to the U.S. Navy, Central Intelligence Agency, and other U.S. government agencies. In the summer of 1979, Relational Software, Inc. introduced the first commercially available implementation of SQL, Oracle V2 (Version2) for VAX computers. Oracle V2 beat IBM's release of the System/38 RDBMS to market by a few weeks.
After testing SQL at customer test sites to determine the usefulness and practicality of the system, IBM began developing commercial products based on their System R prototype including System/38, SQL/DS, and DB2, which were commercially available in 1979, 1981, and 1983, respectively.
Common criticisms of SQL include a perceived lack of cross-platform portability between vendors, inappropriate handling of missing data (see Null (SQL)), and unnecessarily complex and occasionally ambiguous language grammar and semantics.
Queries
The most common operation in SQL is the query, which is performed with the declarative SELECT statement. SELECT retrieves data from one or more tables, or expressions. Standard SQL statements have no persistent effects on the database. Some non-standard implementations of SELECT can have persistent effects, such as the SELECT INTO syntax that exists in some databases.
Queries allow the user to describe desired data, leaving the database management system (DBMS) responsible for planning, optimizing, and performing the physical operations necessary to produce that result as it chooses.
A query includes a list of columns to be included in the final result immediately following the SELECT keyword. An asterisk ("*") can also be used to specify that the query should all return columns of the queried tables. SELECT is the most complex statement in SQL, with optional keywords and clauses that include:
* The FROM clause which indicates the table(s) from which data is to be retrieved. The FROM clause can include optional JOIN subclauses to specify the rules for joining tables.
* The WHERE clause includes a comparison predicate, which restricts the rows returned by the query. The WHERE clause eliminates all rows from the result set for which the comparison predicate does not evaluate to True.
* The GROUP BY clause is used to project rows having common values into a smaller set of rows. GROUP BY is often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result set. The WHERE clause is applied before the GROUP BY clause.
* The HAVING clause includes a predicate used to filter rows resulting from the GROUP BY clause. Because it acts on the results of the GROUP BY clause, aggregation functions can be used in the HAVING clause predicate.
* The ORDER BY clause identifies which columns are used to sort the resulting data, and in which direction they should be sorted (options are ascending or descending). Without an ORDER BY clasue, the order of rows returned by an SQL query is undefined.
The following is an example of a SELECT query that returns a list of expensive books. The query retrieves all rows from the Book table in which the price column contains a value greater than 100.00. The result is sorted in ascending order by title. The asterisk (*) in the select list indicates that all columns of the Book table should be included in the result set.
SELECT *
FROM Book
WHERE price > 100.00
ORDER BY title;
The example below demonstrates a query of multiple tables, grouping, and aggregation, by returning a list of books and the number of authors associated with each book.
SELECT Book.title,
count(*) AS Authors
FROM Book
JOIN Book_author ON Book.isbn = Book_author.isbn
GROUP BY Book.title;
Example output might resemble the following:
Title Authors
---------------------- -------
SQL Examples and Guide 3
The Joy of SQL 1
How to use Wikipedia 2
Pitfalls of SQL 1
Under the precondition that isbn is the only common column name of the two tables and that a column named title only exists in the Books table, the above query could be rewritten in the following form:
SELECT title,
count(*) AS Authors
FROM Book
NATURAL JOIN Book_author
GROUP BY title;
However, many vendors either do not support this approach, or require column naming conventions.
SQL includes operators and functions for calculating values on stored values. SQL allows the use of expressions in the select list to project data, as in the following example which returns a list of books that cost more than 100.00 with an additional sales_tax column containing a sales tax figure calculated at 6% of the price.
SELECT isbn,
title,
price,
price * 0.06 AS sales_tax
FROM Book
WHERE price > 100.00
ORDER BY title;
Data manipulation
The Data Manipulation Language (DML) is the subset of SQL used to add, update and delete data:
* INSERT adds rows (formally tuples) to an existing table, e.g.,:
INSERT INTO My_table
(field1, field2, field3)
VALUES
('test', 'N', NULL);
* UPDATE modifies a set of existing table rows, e.g.,:
UPDATE My_table
SET field1 = 'updated value'
WHERE field2 = 'N';
* DELETE removes existing rows from a table, e.g.,:
DELETE FROM My_table
WHERE field2 = 'N';
* TRUNCATE deletes all data from a table in a very fast way. It usually implies a subsequent COMMIT operation.
* MERGE is used to combine the data of multiple tables. It combines the INSERT and UPDATE elements. It is defined in the SQL:2003 standard; prior to that, some databases provided similar functionality via different syntax, sometimes called "upsert".
is a database computer language designed for managing data in relational database management systems (RDBMS). Its scope includes data query and update, schema creation and modification, and data access control.
SQL was one of the first languages for Edgar F. Codd's relational model in his influential paper, "A Relational Model of Data for Large Shared Data Banks". and became the most widely used language for relational databases
SQL was developed at IBM by Andrew Richardson, Donald C. Messerly and Raymond F. Boyce in the early 1970s. This version, initially called SEQUEL, was designed to manipulate and retrieve data stored in IBM's original relational database product, System R. IBM patented this version of SQL in 1985.
During the 1970s, a group at IBM San Jose Research Laboratory developed the System R relational database management system. Donald D. Chamberlin and Raymond F. Boyce of IBM subsequently created the Structured English Query Language (SEQUEL) to manage data stored in System R. The acronym SEQUEL was later changed to SQL because "SEQUEL" was a trademark of the UK-based Hawker Siddeley aircraft company.
The first RDBMSs were RDMS, developed at MIT in the early 1970s and Ingres, developed in 1974 at U.C. Berkeley. Ingres implemented a query language known as QUEL, which was later supplanted in the marketplace by SQL.
In the late 1970s, Relational Software, Inc. (now Oracle Corporation) saw the potential of the concepts described by Codd, Chamberlin, and Boyce and developed their own SQL-based RDBMS with aspirations of selling it to the U.S. Navy, Central Intelligence Agency, and other U.S. government agencies. In the summer of 1979, Relational Software, Inc. introduced the first commercially available implementation of SQL, Oracle V2 (Version2) for VAX computers. Oracle V2 beat IBM's release of the System/38 RDBMS to market by a few weeks.
After testing SQL at customer test sites to determine the usefulness and practicality of the system, IBM began developing commercial products based on their System R prototype including System/38, SQL/DS, and DB2, which were commercially available in 1979, 1981, and 1983, respectively.
Common criticisms of SQL include a perceived lack of cross-platform portability between vendors, inappropriate handling of missing data (see Null (SQL)), and unnecessarily complex and occasionally ambiguous language grammar and semantics.
Queries
The most common operation in SQL is the query, which is performed with the declarative SELECT statement. SELECT retrieves data from one or more tables, or expressions. Standard SQL statements have no persistent effects on the database. Some non-standard implementations of SELECT can have persistent effects, such as the SELECT INTO syntax that exists in some databases.
Queries allow the user to describe desired data, leaving the database management system (DBMS) responsible for planning, optimizing, and performing the physical operations necessary to produce that result as it chooses.
A query includes a list of columns to be included in the final result immediately following the SELECT keyword. An asterisk ("*") can also be used to specify that the query should all return columns of the queried tables. SELECT is the most complex statement in SQL, with optional keywords and clauses that include:
* The FROM clause which indicates the table(s) from which data is to be retrieved. The FROM clause can include optional JOIN subclauses to specify the rules for joining tables.
* The WHERE clause includes a comparison predicate, which restricts the rows returned by the query. The WHERE clause eliminates all rows from the result set for which the comparison predicate does not evaluate to True.
* The GROUP BY clause is used to project rows having common values into a smaller set of rows. GROUP BY is often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result set. The WHERE clause is applied before the GROUP BY clause.
* The HAVING clause includes a predicate used to filter rows resulting from the GROUP BY clause. Because it acts on the results of the GROUP BY clause, aggregation functions can be used in the HAVING clause predicate.
* The ORDER BY clause identifies which columns are used to sort the resulting data, and in which direction they should be sorted (options are ascending or descending). Without an ORDER BY clasue, the order of rows returned by an SQL query is undefined.
The following is an example of a SELECT query that returns a list of expensive books. The query retrieves all rows from the Book table in which the price column contains a value greater than 100.00. The result is sorted in ascending order by title. The asterisk (*) in the select list indicates that all columns of the Book table should be included in the result set.
SELECT *
FROM Book
WHERE price > 100.00
ORDER BY title;
The example below demonstrates a query of multiple tables, grouping, and aggregation, by returning a list of books and the number of authors associated with each book.
SELECT Book.title,
count(*) AS Authors
FROM Book
JOIN Book_author ON Book.isbn = Book_author.isbn
GROUP BY Book.title;
Example output might resemble the following:
Title Authors
---------------------- -------
SQL Examples and Guide 3
The Joy of SQL 1
How to use Wikipedia 2
Pitfalls of SQL 1
Under the precondition that isbn is the only common column name of the two tables and that a column named title only exists in the Books table, the above query could be rewritten in the following form:
SELECT title,
count(*) AS Authors
FROM Book
NATURAL JOIN Book_author
GROUP BY title;
However, many vendors either do not support this approach, or require column naming conventions.
SQL includes operators and functions for calculating values on stored values. SQL allows the use of expressions in the select list to project data, as in the following example which returns a list of books that cost more than 100.00 with an additional sales_tax column containing a sales tax figure calculated at 6% of the price.
SELECT isbn,
title,
price,
price * 0.06 AS sales_tax
FROM Book
WHERE price > 100.00
ORDER BY title;
Data manipulation
The Data Manipulation Language (DML) is the subset of SQL used to add, update and delete data:
* INSERT adds rows (formally tuples) to an existing table, e.g.,:
INSERT INTO My_table
(field1, field2, field3)
VALUES
('test', 'N', NULL);
* UPDATE modifies a set of existing table rows, e.g.,:
UPDATE My_table
SET field1 = 'updated value'
WHERE field2 = 'N';
* DELETE removes existing rows from a table, e.g.,:
DELETE FROM My_table
WHERE field2 = 'N';
* TRUNCATE deletes all data from a table in a very fast way. It usually implies a subsequent COMMIT operation.
* MERGE is used to combine the data of multiple tables. It combines the INSERT and UPDATE elements. It is defined in the SQL:2003 standard; prior to that, some databases provided similar functionality via different syntax, sometimes called "upsert".
