BNF Conventions

Example: Using the ::= Operator Reference: BNF Symbols

This manual employs a variant of what is known as Backus Naur Form (BNF) notation to describe SQL syntax. In BNF, a language construct is defined as follows:

something ::= its definition

A language construct may be used on the right hand side of such an expression (sometimes called a production) to define something else. When a known construct appears on the right side of a production it is represented by its name enclosed within angled brackets (<...>).

a_new_thing ::= <any_old_thing>

The right hand side of a production may be composed of any sequence of bracketed names and printable characters. Curly brackets ({...}) are used to delimit choices separated by vertical bars and square brackets ([...]) are used to indicate optional phrases. When a bracketed expression is followed by an asterisk ([...]*) it represents zero, one or more of the enclosed language constituents. Printable characters employed outside of brackets in productions are meant to be used verbatim.

The ::= operator is the BNF equivalent of is made up of in English.

Example: Using the ::= Operator

The following productions could be read as computer books are made up of zero, one or more instances of information or examples and syntax.

computer_books ::= [<technical_information>]* and syntax.
technical_information ::= information | examples

Productions amount to formulas for creating a well-formed string in a given language. All of the following conform to the computer_books syntax:

information examples and syntax.
and syntax.
information information information and syntax

Reference: BNF Symbols

The chart below summarizes the BNF symbols and their meanings.

Symbol	Meaning
{ }	Indicates a group of valid options. You must choose at least one of the enclosed options.
[ ]	Indicates a group of valid options. You can select one of the options or none.
::=	Indicates is made up of.
< >	Indicates the command or production qualifier or construct.
,	Indicates that you can select one or more or none of the options.
|	Indicates that you can select one of the options or none.
[]*	Zero or more occurrences of item in brackets.