SQL syntax


The syntax of the SQL programming language is defined and maintained by ISO/IEC SC 32 as part of ISO/IEC 9075. This standard is not freely available. Despite the existence of the standard, SQL code is not completely portable among different database systems without adjustments.

Language elements

The SQL language is subdivided into several language elements, including:
OperatorDescriptionExample
=Equal to
<>Not equal to
>Greater than
<Less than
>=Greater than or equal
<=Less than or equal
Where_#BETWEEN|Between an inclusive range
Like |Begins with a character pattern
Like |Contains a character pattern
Where_#IN|Equal to one of multiple possible values
SQL_syntax#Null_or_three-valued_logic_|Compare to null
or Boolean truth value test
SQL_syntax#Null_or_three-valued_logic_|Is equal to value or both are nulls
Used to change a column name when viewing results

Other operators have at times been suggested or implemented, such as the skyline operator.
SQL has the case expression, which was introduced in SQL-92. In its most general form, which is called a "searched case" in the SQL standard:

CASE WHEN n > 0
THEN 'positive'
WHEN n < 0
THEN 'negative'
ELSE 'zero'
END

SQL tests WHEN conditions in the order they appear in the source. If the source does not specify an ELSE expression, SQL defaults to ELSE NULL. An abbreviated syntax called "simple case" can also be used:

CASE n WHEN 1
THEN 'One'
WHEN 2
THEN 'Two'
ELSE 'I cannot count that high'
END

This syntax uses implicit equality comparisons, with the usual caveats for comparing with NULL.
There are two short forms for special CASE expressions: COALESCE and NULLIF.
The COALESCE expression returns the value of the first non-NULL operand, found by working from left to right, or NULL if all the operands equal NULL.

COALESCE
is equivalent to:

CASE WHEN x1 IS NOT NULL THEN x1
ELSE x2
END

The NULLIF expression has two operands and returns NULL if the operands have the same value, otherwise it has the value of the first operand.

NULLIF

is equivalent to

CASE WHEN x1 = x2 THEN NULL ELSE x1 END

Comments

Standard SQL allows two formats for comments: -- comment, which is ended by the first newline, and /* comment */, which can span multiple lines.

Queries

The most common operation in SQL, the query, makes use of 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 provided in some databases.
Queries allow the user to describe desired data, leaving the database management system to carry out planning, optimizing, and performing the physical operations necessary to produce that result as it chooses.
A query includes a list of columns to include in the final result, normally immediately following the SELECT keyword. An asterisk can 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 clauses of a query have a particular order of execution, which is denoted by the number on the right hand side. It is as follows:
The following example of a SELECT query 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 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 the Book table, one could re-write the query above 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 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 alternatives in many cases, the use of subqueries introduces a hierarchy in execution that 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 <
ORDER BY title;

A subquery can use values from the outer query, in which case it is known as a correlated subquery.
Since 1999 the SQL standard allows WITH clauses for subqueries, i.e. named subqueries, usually called common table expressions. CTEs can also be recursive by referring to themselves; the resulting mechanism allows tree or graph traversals, and more generally fixpoint computations.

Derived table

A derived table is the use of referencing an SQL subquery in a FROM clause. Essentially, the derived table is a subquery that can be selected from or joined to. The derived table functionality allows the user to reference the subquery as a table. The inline view is also referred to as an inline view or a subselect.
In the following example, the SQL statement involves a join from the initial "Book" table to the derived table "sales". This derived table captures associated book sales information using the ISBN to join to the "Book" table. As a result, the derived table provides the result set with additional columns :

SELECT b.isbn, b.title, b.price, sales.items_sold, sales.company_nm
FROM Book b
JOIN sales
ON sales.isbn = b.isbn

Null or three-valued logic (3VL)

The concept of Null allows SQL to deal with 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.
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 return a truth value of Unknown. This is in line with the interpretation that Null does not have a value 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. Consequently, these set operations in SQL may produce results not representing sure information, unlike operations involving explicit comparisons with NULL. In Codd's 1979 proposal 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, because 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. SQL does not explicitly support universal quantification, and must work it 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 ". also introduced BOOLEAN type variables, which according to the standard can also hold Unknown values if it is nullable. In practice, a number of systems implement the BOOLEAN Unknown as a BOOLEAN NULL, which the standard says that the NULL BOOLEAN and UNKNOWN "may be used interchangeably to mean exactly the same thing".

Data manipulation

The Data Manipulation Language is the subset of SQL used to add, update and delete data:
  • INSERT adds rows to an existing table, e.g.:

INSERT INTO example

VALUES
;

  • UPDATE modifies a set of existing table rows, e.g.:

UPDATE example
SET column1 = 'updated value'
WHERE column2 = 'N';

  • DELETE removes existing rows from a table, e.g.:

DELETE FROM example
WHERE column2 = '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
WHEN MATCHED THEN
UPDATE SET column1 = value1
WHEN NOT MATCHED THEN
INSERT VALUES

Transaction controls

Transactions, if available, wrap DML operations:
  • START TRANSACTION marks the start of a database transaction, which either completes entirely or not at all.
  • SAVE TRANSACTION saves the state of the database at the current point in transaction

CREATE TABLE tbl_1;
INSERT INTO tbl_1 VALUES;
INSERT INTO tbl_1 VALUES;
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 makes all data changes in a transaction permanent.
  • ROLLBACK discards all data changes since the last COMMIT or ROLLBACK, leaving 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 manages table and index structure. The most basic items of DDL are the CREATE, ALTER, RENAME, DROP and TRUNCATE statements:
  • CREATE creates an object in the database, e.g.:

CREATE TABLE example,
column3 DATE NOT NULL,
PRIMARY KEY
);

  • 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 example ADD column4 INTEGER DEFAULT 25 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.

TRUNCATE TABLE example;

  • DROP deletes an object in the database, usually irretrievably, i.e., it cannot be rolled back, e.g.:

DROP TABLE example;

Data types

Each column in an SQL table declares the type that column may contain. ANSI SQL includes the following data types.
; Character strings and national character strings
  • CHARACTER : fixed-width n-character string, padded with spaces as needed
  • CHARACTER VARYING : variable-width string with a maximum size of n characters
  • CHARACTER LARGE OBJECT : character large object with a maximum size of n characters
  • NATIONAL CHARACTER : fixed width string supporting an international character set
  • NATIONAL CHARACTER VARYING : variable-width NCHAR string
  • NATIONAL CHARACTER LARGE OBJECT : national character large object with a maximum size of n characters
For the CHARACTER LARGE OBJECT and NATIONAL CHARACTER LARGE OBJECT data types, the multipliers K, M, G and T can be optionally used when specifying the length.
; Binary
  • BINARY: Fixed length binary string, maximum length n.
  • BINARY VARYING : Variable length binary string, maximum length n.
  • BINARY LARGE OBJECT : binary large object with a maximum length n .
For the BINARY LARGE OBJECT data type, the multipliers K, M, G and T can be optionally used when specifying the length.
; Boolean
  • BOOLEAN
The BOOLEAN data type can store the values TRUE and FALSE.
; Numerical
  • INTEGER, SMALLINT and BIGINT
  • FLOAT, REAL and DOUBLE PRECISION
  • NUMERIC or DECIMAL
  • DECFLOAT
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. 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 the functions CEILING and FLOOR to round numerical values. and ROUND )
; Temporal
  • DATE: for date values
  • TIME: for time values.
  • TIME WITH TIME ZONE: 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.
  • TIMESTAMP WITH TIME ZONE: the same as TIMESTAMP, but including details about the time zone in question.
The SQL function EXTRACT can be used for extracting a single field of a datetime or interval value. The current system date / time of the database server can be called by using functions like CURRENT_DATE, CURRENT_TIMESTAMP, LOCALTIME, or LOCALTIMESTAMP.
; Interval
  • YEAR: a number of years
  • YEAR TO MONTH: a number of years and months
  • MONTH: a number of months
  • DAY: a number of days
  • DAY TO HOUR: a number of days and hours
  • DAY TO MINUTE: a number of days, hours and minutes
  • DAY TO SECOND: a number of days, hours, minutes and seconds
  • HOUR: a number of hours
  • HOUR TO MINUTE: a number of hours and minutes
  • HOUR TO SECOND: a number of hours, minutes and seconds
  • MINUTE: a number of minutes
  • MINUTE TO SECOND: a number of minutes and seconds

    Data control

The Data Control Language 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 example
TO some_user, another_user;
REVOKE SELECT, UPDATE
ON example
FROM some_user, another_user;