SQL

SQL (commonly expanded to Structured Query Language — see History for the term's derivation) is the most popular computer language used to create, modify, retrieve and manipulate data from relational database management systems. The language has evolved beyond its original purpose to support object-relational database management systems. It is an ANSI/ISO standard.

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:

• the complexity and size of the SQL standard means that most databases do not implement the entire standard.
• the standard does not specify database behavior in several important areas (e.g. indexes), leaving it up to implementations of the standard to decide how to behave.
• the SQL standard precisely specifies the syntax that a conformant database system must implement. However, the standard's specification of the semantics of language constructs is less well-defined, leading to areas of ambiguity.
• many database vendors have large existing customer bases; where the SQL standard conflicts with the prior behavior of the vendor's database, the vendor may be unwilling to break backward compatibility.
• some believe the lack of compatibility between database systems is intentional in order to ensure vendor lock-in.

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 languages, 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

• SELECT is used to retrieve zero or more rows from one or more tables in a database. In most applications, SELECT is the most commonly used DML command. In specifying a SELECT query, the user specifies a description of the desired result set, but they do not specify what physical operations must be executed to produce that result set. Translating the query into an efficient query plan is left to the database system, more specifically to the query optimizer.
  • Commonly available keywords related to SELECT include:
    • FROM is used to indicate from which tables the data is to be taken, as well as how the tables join to each other.
    • WHERE is used to identify which rows to be retrieved, or applied to GROUP BY. WHERE is evaluated before the GROUP BY.
    • GROUP BY is used to combine rows with related values into elements of a smaller set of rows.
    • HAVING is used to identify which of the "combined rows" (combined rows are produced when the query has a GROUP BY keyword or when the SELECT part contains aggregates), are to be retrieved. HAVING acts much like a WHERE, but it operates on the results of the GROUP BY and hence can use aggregate functions.
    • ORDER BY is used to identify which columns are used to sort the resulting data.

Example 1:
  SELECT * FROM books WHERE price > 100.00
  ORDER BY title

This example shows the records for all books that cost more than 100.00 (units of currency). The result is sorted alphabetically by book title. The asterisk (*) means to show all columns of the referenced table(s). Alternatively, specific columns could be named.

 Example 2:
   SELECT bk.title, count(*) AS Authors
   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.

• INSERT is used to add zero or more rows (formally tuples) to an existing table.
• UPDATE is used to modify the values of a set of existing table rows.
• MERGE is used to combine the data of multiple tables. It is something of a combination of 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 an "upsert".
• TRUNCATE deletes all data from a table (non-standard, but common SQL command).
• DELETE removes zero or more existing rows from a table.

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.

• START TRANSACTION (or BEGIN WORK, depending on SQL dialect) can be used to mark the start of a database transaction, which either completes completely or not at all.
• COMMIT causes all data changes in a transaction to be made permanent.
• ROLLBACK causes all data changes since the last COMMIT or ROLLBACK to be discarded, so that the state of the data is "rolled back" to the way it was prior to those changes being requested.

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.

• CREATE causes an object (a table, for example) to be created within the database.
• DROP causes an existing object within the database to be deleted, usually irretrievably.

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:

• GRANT — authorises one or more users to perform an operation or a set of operations on an object.
• REVOKE — removes or restricts the capability of a user to perform an operation or a set of operations.

Example:
GRANT SELECT, UPDATE ON my_table TO some_user, another_user

Other

ANSI-standard SQL supports -- as a single line comment identifier (some extensions also support curly brackets or C-style /* comments */ for multi-line comments).

Example:
SELECT * FROM inventory -- Retrieve everything from inventory table

Database systems using SQL

• List of relational database management systems
• List of object-relational database management systems

Criticisms of SQL

Technically, SQL is a declarative computer language for use with "SQL databases". 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:

• The language syntax is rather complex (sometimes called "COBOL-like").

• It does not provide a standard way, or at least a commonly-supported way, to split large commands into multiple smaller ones that reference each other by name. This tends to result in "run-on SQL sentences" and may force one into a deep hierarchical nesting when a graph-like (reference-by-name) approach may be more appropriate and better repetition-factoring.

• Implementations are inconsistent and, usually, incompatible between vendors.

• For larger statements, it is often difficult to factor repeated patterns and expressions into one or fewer places to avoid repetition and avoid having to make the same change to different places in a given statement.

• Confusion about the difference between value-to-column assignment in UPDATE and INSERT syntax.

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.

• IBM Business System 12 (IBM BS12)
• Tutorial D
• TQL - Luca Cardelli
• Top's Query Language - A draft language influenced by IBM BS12. Tentatively renamed to SMEQL to avoid confusion with similar projects call TQL.
• Hibernate Query Language (HQL) - A Java-based tool that uses modified SQL
• Quel introduced in 1974 by the U.C. Berkeley Ingres project.
• The Object Data Standard - Object Data Management Group.
• Object Query Language - Object Data Management Group.

See also

Wikibook

==

Books

• Learning SQL
• Database in Depth
• SQL Cookbook