SQL

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                                          SQL

SQL (play /ˈɛs kjuː ˈɛl/ “S-Q-L”;[3] or Structured Query Language) is a special-purposeprogramming language designed for managing data in relational database management systems (RDBMS).

Originally based upon relational algebra and tuple relational calculus, its scope includes data insert, query, update and delete, schema creation and modification, and data access control.

Language elements

This chart shows several of the SQL language elements that compose a single statement.

The SQL language is subdivided into several language elements, including:

  • Clauses, which are constituent components of statements and queries. (In some cases, these are optional.)[10]
  • Expressions, which can produce either scalar values or tables consisting ofcolumns and rows of data.
  • Predicates, which specify conditions that can be evaluated to SQL three-valued logic (3VL) or Boolean (true/false/unknown) truth values and which are used to limit the effects of statements and queries, or to change program flow.
  • Queries, which retrieve the data based on specific criteria. This is the most important element of SQL.
  • Statements, which may have a persistent effect on schemata and data, or which may control transactions, program flow, connections, sessions, or diagnostics.
    • SQL statements also include the semicolon (“;”) statement terminator. Though not required on every platform, it is defined as a standard part of the SQL grammar.
  • Insignificant whitespace is generally ignored in SQL statements and queries, making it easier to format SQL code for readability.

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 SELECT 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.[12]

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 return all 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 JOINsubclauses 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 theGROUP 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 clause, 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 theselect 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 AS 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     4
The Joy of SQL             1
An Introduction to SQL     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 theBooks 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 certain column naming conventions in order for natural joins to work effectively.

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;

Subqueries

Queries can be nested so that the results of one query can be used in another query via a relational operator or aggregation function. A nested query is also known as a subquery. While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases, the use of subqueries introduces a hierarchy in execution which can be useful or necessary. In the following example, the aggregation function AVG receives as input the result of a subquery:

SELECT isbn, title, price
    FROM Book
    WHERE price < AVG(SELECT price FROM Book)
    ORDER BY title;

Since 1999 the SQL standard allows named subqueries called common table expression (named and designed after the IBM DB2 version 2 implementation; Oracle calls these subquery factoring). CTEs can be also be recursive by referring to themselves; the resulting mechanism allows tree or graph traversals (when represented as relations), and more generally fixpoint computations.

Null and three-valued logic (3VL)

Main article: Null (SQL)

The concept of Null was introduced into SQL to handle missing information in the relational model. The word NULL is a reserved keyword in SQL, used to identify the Null special marker. Comparisons with Null, for instance equality (=) in WHERE clauses, results in an Unknown truth value. In SELECT statements SQL returns only results for which the WHERE clause returns a value of True; i.e. it excludes results with values of False and also excludes those whose value is Unknown.

Along with True and False, the Unknown resulting from direct comparisons with Null thus brings a fragment of three-valued logic to SQL. The truth tables SQL uses for AND, OR, and NOT correspond to a common fragment of the Kleene and Lukasiewicz three-valued logic (which differ in their definition of implication, however SQL defines no such operation).[13]

p AND q p
True False Unknown
q True True False Unknown
False False False False
Unknown Unknown False Unknown
p OR q p
True False Unknown
q True True True True
False True False Unknown
Unknown True Unknown Unknown
p NOT p
True False
False True
Unknown Unknown
p = q p
True False Unknown
q True True False Unknown
False False True Unknown
Unknown Unknown Unknown Unknown

There are however disputes about the semantic interpretation of Nulls in SQL because of its treatment outside direct comparisons. As seen in the table above direct equality comparisons between two NULLs in SQL (e.g. NULL = NULL) returns a truth value of Unknown. This is in line with the interpretation that Null does not have a value (and is not a member of any data domain) but is rather a placeholder or “mark” for missing information. However, the principle that two Nulls aren’t equal to each other is effectively violated in the SQL specification for the UNION and INTERSECT operators, which do identify nulls with each other.[14] Consequently, these set operations in SQL, may produce results not representing sure information, unlike operations involving explicit comparisons with NULL (e.g. those in a WHERE clause discussed above). In Codd’s 1979 proposal (which was basically adopted by SQL92) this semantic inconsistency is rationalized by arguing that removal of duplicates in set operations happens “at a lower level of detail than equality testing in the evaluation of retrieval operations.”[13] However, computer science professor Ron van der Meyden concluded that “The inconsistencies in the SQL standard mean that it is not possible to ascribe any intuitive logical semantics to the treatment of nulls in SQL.”[14]

Additionally, since SQL operators return Unknown when comparing anything with Null directly, SQL provides two Null-specific comparison predicates: IS NULL and IS NOT NULL test whether data is or is not Null.[15] Universal quantification is not explicitly supported by SQL, and must be worked out as a negated existential quantification.[16][17][18] There is also the ” IS DISTINCT FROM ” infixed comparison operator which returns TRUE unless both operands are equal or both are NULL. Likewise, IS NOT DISTINCT FROM is defined as “NOT ( IS DISTINCT FROM )”. SQL:1999 also introduced BOOLEAN type variables, which according to the standard can also hold Unknown values. In practice, a number of systems (e.g. PostgreSQL) implement the BOOLEAN Unknown as a BOOLEAN NULL.

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';
  • 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“.
 MERGE INTO TABLE_NAME USING table_reference ON (condition)
   WHEN MATCHED THEN
   UPDATE SET column1 = value1 [, column2 = value2 ...]
   WHEN NOT MATCHED THEN
   INSERT (column1 [, column2 ...]) VALUES (value1 [, value2 ...

Transaction controls

Transactions, if available, wrap DML operations:

  • START TRANSACTION (or BEGIN WORK, or BEGIN TRANSACTION, depending on SQL dialect) marks the start of a database transaction, which either completes entirely or not at all.
  • SAVE TRANSACTION (or SAVEPOINT) saves the state of the database at the current point in transaction
CREATE TABLE tbl_1(id INT);
  INSERT INTO tbl_1(id) VALUES(1);
  INSERT INTO tbl_1(id) VALUES(2);
COMMIT;
  UPDATE tbl_1 SET id=200 WHERE id=1;
SAVEPOINT id_1upd;
  UPDATE tbl_1 SET id=1000 WHERE id=2;
ROLLBACK TO id_1upd;
  SELECT id FROM tbl_1;
  • 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, leaving the state of the data as it was prior to those changes.

Once the COMMIT statement completes, the transaction’s changes cannot be rolled back.

COMMIT and ROLLBACK terminate the current transaction and release data locks. In the absence of a START TRANSACTION or similar statement, the semantics of SQL are implementation-dependent. The following example shows a classic transfer of funds transaction, where money is removed from one account and added to another. If either the removal or the addition fails, the entire transaction is rolled back.

START TRANSACTION;
  UPDATE Account SET amount=amount-200 WHERE account_number=1234;
  UPDATE Account SET amount=amount+200 WHERE account_number=2345;

IF ERRORS=0 COMMIT;
IF ERRORS<>0 ROLLBACK;

Data definition

The Data Definition Language (DDL) manages table and index structure. The most basic items of DDL are the CREATE, ALTER,RENAME, DROP and TRUNCATE statements:

  • CREATE creates an object (a table, for example) in the database, e.g.:
CREATE TABLE My_table(
    my_field1   INT,
    my_field2   VARCHAR(50),
    my_field3   DATE         NOT NULL,
    PRIMARY KEY (my_field1, my_field2)
);
  • ALTER modifies the structure of an existing object in various ways, for example, adding a column to an existing table or a constraint, e.g.:
ALTER TABLE My_table ADD my_field4 NUMBER(3) NOT NULL;
  • TRUNCATE deletes all data from a table in a very fast way, deleting the data inside the table and not the table itself. It usually implies a subsequent COMMIT operation, i.e., it cannot be rolled back (data is not written to the logs for rollback later, unlike DELETE).
TRUNCATE TABLE My_table;
  • DROP deletes an object in the database, usually irretrievably, i.e., it cannot be rolled back, e.g.:
DROP TABLE My_table;

Data types

Each column in an SQL table declares the type(s) that column may contain. ANSI SQL includes the following data types.[19]

Character strings

  • CHARACTER(n) or CHAR(n) — fixed-width n-character string, padded with spaces as needed
  • CHARACTER VARYING(n) or VARCHAR(n) — variable-width string with a maximum size of n characters
  • NATIONAL CHARACTER(n) or NCHAR(n) — fixed width string supporting an international character set
  • NATIONAL CHARACTER VARYING(n) or NVARCHAR(n) — variable-width NCHAR string

Bit strings

  • BIT(n) — an array of n bits
  • BIT VARYING(n) — an array of up to n bits

Numbers

  • INTEGER and SMALLINT
  • FLOAT, REAL and DOUBLE PRECISION
  • NUMERIC(precision, scale) or DECIMAL(precision, scale)

For example, the number 123.45 has a precision of 5 and a scale of 2. The precision is a positive integer that determines the number of significant digits in a particular radix (binary or decimal). The scale is a non-negative integer. A scale of 0 indicates that the number is an integer. For a decimal number with scale S, the exact numeric value is the integer value of the significant digits divided by 10S.

SQL provides a function to round numerics or dates, called TRUNC (in Informix, DB2, PostgreSQL, Oracle and MySQL) or ROUND (in Informix, Sybase, Oracle, PostgreSQL and Microsoft SQL Server)[20]

Date and time

  • DATE — for date values (e.g., 2011-05-03)
  • TIME — for time values (e.g., 15:51:36). The granularity of the time value is usually a tick (100 nanoseconds).
  • TIME WITH TIME ZONE or TIMETZ — the same as TIME, but including details about the time zone in question.
  • TIMESTAMP — This is a DATE and a TIME put together in one variable (e.g., 2011-05-03 15:51:36).
  • TIMESTAMP WITH TIME ZONE or TIMESTAMPTZ — the same as TIMESTAMP, but including details about the time zone in question.

SQL provides several functions for generating a date / time variable out of a date / time string (TO_DATE, TO_TIME, TO_TIMESTAMP), as well as for extracting the respective members (seconds, for instance) of such variables. The current system date / time of the database server can be called by using functions like NOW.

Data control

The Data Control Language (DCL) authorizes users to access and manipulate data. Its two main statements are:

  • GRANT authorizes one or more users to perform an operation or a set of operations on an object.
  • REVOKE eliminates a grant, which may be the default grant.

Example:

GRANT SELECT, UPDATE
    ON My_table
    TO some_user, another_user;

REVOKE SELECT, UPDATE
    ON My_table
    FROM some_user, another_user;

Procedural extensions

SQL is designed for a specific purpose: to query data contained in a relational database. SQL is a set-based, declarative query language, not an imperative language like C or BASIC. However, there are extensions to Standard SQL which add procedural programming language functionality, such as control-of-flow constructs. These include:

Source Common
Name		 Full Name
ANSI/ISO Standard SQL/PSM SQL/Persistent Stored Modules
Interbase/
Firebird		 PSQL Procedural SQL
IBM DB2 SQL PL SQL Procedural Language (implements SQL/PSM)
IBM Informix SPL Stored Procedural Language
Microsoft/
Sybase		 T-SQL Transact-SQL
Mimer SQL SQL/PSM SQL/Persistent Stored Module (implements SQL/PSM)
MySQL SQL/PSM SQL/Persistent Stored Module (implements SQL/PSM)
Oracle PL/SQL Procedural Language/SQL (based on Ada)
PostgreSQL PL/pgSQL Procedural Language/PostgreSQL Structured Query Language (based on Oracle PL/SQL)
PostgreSQL PL/PSM Procedural Language/Persistent Stored Modules (implements SQL/PSM)
Sybase Watcom-SQL SQL Anywhere Watcom-Structured Query Language Dialect

In addition to the standard SQL/PSM extensions and proprietary SQL extensions, procedural and object-oriented programmability is available on many SQL platforms via DBMS integration with other languages. The SQL standard defines SQL/JRT extensions (SQL Routines and Types for the Java Programming Language) to support Java code in SQL databases. SQL Server 2005 uses the SQLCLR(SQL Server Common Language Runtime) to host managed .NET assemblies in the database, while prior versions of SQL Server were restricted to using unmanaged extended stored procedures that were primarily written in C. PostgreSQL allows functions to be written in a wide variety of languages including Perl, Python, Tcl, and C.[21]

SQL Operators

Operator Description
= Equal
<> or != Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern
IN To specify multiple possible values for a column

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