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.)
- 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.
The most common operation in SQL is the query, which is performed with the declarative
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.
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:
FROMclause which indicates the table(s) from which data is to be retrieved. The
FROMclause can include optional
JOINsubclauses to specify the rules for joining tables.
WHEREclause includes a comparison predicate, which restricts the rows returned by the query. The
WHEREclause eliminates all rows from the result set for which the comparison predicate does not evaluate to True.
GROUP BYclause is used to project rows having common values into a smaller set of rows.
GROUP BYis often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result set. The
WHEREclause is applied before the
HAVINGclause includes a predicate used to filter rows resulting from the
GROUP BYclause. Because it acts on the results of the
GROUP BYclause, aggregation functions can be used in the
ORDER BYclause 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 BYclause, 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;
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)
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).
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
INTERSECT operators, which do identify nulls with each other. 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.” 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.”
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. Universal quantification is not explicitly supported by SQL, and must be worked out as a negated existential quantification. 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.
The Data Manipulation Language (DML) is the subset of SQL used to add, update and delete data:
INSERT INTO My_table (field1, field2, field3) VALUES ('test', 'N', NULL);
UPDATEmodifies a set of existing table rows, e.g.:
UPDATE My_table SET field1 = 'updated value' WHERE field2 = 'N';
DELETEremoves existing rows from a table, e.g.:
DELETE FROM My_table WHERE field2 = 'N';
MERGEis used to combine the data of multiple tables. It combines the
UPDATEelements. 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 ...
Transactions, if available, wrap DML operations:
BEGIN WORK, or
BEGIN TRANSACTION, depending on SQL dialect) marks the start of a database transaction, which either completes entirely or not at all.
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;
COMMITcauses all data changes in a transaction to be made permanent.
ROLLBACKcauses all data changes since the last
ROLLBACKto be discarded, leaving the state of the data as it was prior to those changes.
COMMIT statement completes, the transaction’s changes cannot be rolled back.
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;
The Data Definition Language (DDL) manages table and index structure. The most basic items of DDL are the
CREATEcreates 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) );
ALTERmodifies 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;
TRUNCATEdeletes 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;
DROPdeletes an object in the database, usually irretrievably, i.e., it cannot be rolled back, e.g.:
DROP TABLE My_table;
Each column in an SQL table declares the type(s) that column may contain. ANSI SQL includes the following data types.
CHAR(n)— fixed-width n-character string, padded with spaces as needed
VARCHAR(n)— variable-width string with a maximum size of n characters
NCHAR(n)— fixed width string supporting an international character set
NATIONAL CHARACTER VARYING(n)or
BIT(n)— an array of n bits
BIT VARYING(n)— an array of up to n bits
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)
Date and time
DATE— for date values (e.g.,
TIME— for time values (e.g.,
15:51:36). The granularity of the time value is usually a tick (100 nanoseconds).
TIME WITH TIME ZONEor
TIMETZ— the same as
TIME, but including details about the time zone in question.
TIMESTAMP— This is a
TIMEput together in one variable (e.g.,
TIMESTAMP WITH TIME ZONEor
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_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
The Data Control Language (DCL) authorizes users to access and manipulate data. Its two main statements are:
GRANTauthorizes one or more users to perform an operation or a set of operations on an object.
REVOKEeliminates a grant, which may be the default grant.
GRANT SELECT, UPDATE ON My_table TO some_user, another_user; REVOKE SELECT, UPDATE ON My_table FROM some_user, another_user;
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:
|ANSI/ISO Standard||SQL/PSM||SQL/Persistent Stored Modules|
|IBM DB2||SQL PL||SQL Procedural Language (implements SQL/PSM)|
|IBM Informix||SPL||Stored Procedural Language|
|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.
|<> or !=||Not equal|
|>=||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|