Constant -> Value with some enhancements

[dbsrgits/SQL-Abstract-2.0-ish.git] / lib / SQL / Abstract / Manual / Specification.pod

=head1 NAME

SQL::Abstract::Manual::Specification

=head1 SYNOPSIS

This discusses the specification for the AST provided by L<SQL::Abstract>. It is
meant to describe how the AST is structured, various components provided by
L<SQL::Abstract> for use with this AST, how to manipulate the AST, and various
uses for the AST once it is generated.

=head1 MOTIVATIONS

L<SQL::Abstract> has been in use for many years. Originally created to handle
the where-clause formation found in L<DBIx::Abstract>, it was generalized to
manage the creation of any SQL statement through the use of Perl structures.
Through the beating it received as the SQL generation syntax for L<DBIx::Class>,
various deficiencies were found and a generalized SQL AST was designed. This
document describes that AST.

=head1 GOALS

The goals for this AST are as follows:

=head2 SQL-specific semantics

Instead of attempting to be an AST to handle any form of query, this will
instead be specialized to manage SQL queries (and queries that map to SQL
queries). This means that there will be support for SQL-specific features, such
as placeholders.

=head2 Perl-specific semantics

This AST is meant to be used from within Perl5 only. So, it will take advantage
of as many Perl-specific features that make sense to use. No attempt whatosever
will be made to make this AST work within any other language, including Perl6.

=head2 Whole-lifecycle management

Whether a query is built out of whole cloth in one shot or cobbled together from
several snippets over the lifetime of a process, this AST will support any way
to construct the query. Queries can also be built from other queries, so an
UPDATE statement could be used as the basis for a SELECT statement, DELETE
statement, or even a DDL statement of some kind.

=head2 Dialect-agnostic usage

Even though SQL itself has several ANSI specifications (SQL-92 and SQL-99 among
them), this only serves as a basis for what a given RDBMS will expect. However,
every engine has its own specific extensions and specific ways of handling
common features. The API to the AST will provide ways of expressing common
functionality in a common language.  The emitters (objects that follow the
Visitor pattern) will be responsible for converting that common language into
RDBMS-specific SQL.

=head1 AST STRUCTURE

The AST will be a HoA (hash of arrays). The keys to the hash will be the various
clauses of a SQL statement, plus some metadata keys. All metadata keys will be
identifiable as such by being prefixed with an underscore. All keys will be in
lowercase.

=head2 Metadata keys

These are the additional metadata keys that the AST provides for.

=over 4

=item * _query

This denotes what kind of query this AST should be interpreted as.

=item *

=back

=head2 Structural units

Structural units in the AST are supported by loaded components. L<SQL::Abstract>
provides for the following structural units by default:

=head3 Identifier

This is a (potentially) fully canonicalized identifier for a table or column. Is
is of the structure C< [schema][sep][table][sep]column > or
C< [schema][sep]table >.

In the case of a two-element identifier which could be C< table[sep]column > or
C< schema[sep]table >, context will determine which it is. However, the AST
doesn't care which it is, only that it properly parses.

=head3 Value

A Value is a Perl scalar. It may either be a:

=over 4

=item * String

A String is a quoted series of characters

=item * Number

A Number is an unquoted number in some numeric format

=item * NULL

NULL corresponds to Perl's C<undef>

=item * BindParameter

This corresponds to a value that will be passed in. This value is normally
quoted in such a fashion so as to protect against SQL injection attacks. (q.v.
L<DBI/quote()> for an example.)

=back

=head3 

=head3 Function

A Function is anything of the form C< name( arglist ) > where C<name> is a
string and C<arglist> is a comma-separated list of Expressions.

Yes, a Subquery is legal as an argument for many functions. Some example
functions are:

=over 4

=item * C<< IN >>

=item * C<< MAX >>

=item * C<< MIN >>

=item * C<< SUM >>

=back

=head3 Subquery

A Subquery is another AST whose _query metadata parameter is set to "SELECT".

Most places that a Subquery can be used would require a single value to be
returned (single column, single row), but that is not something that the AST can
easily enforce. The single-column restriction can possibly be enforced, but the
single-row restriction is much more difficult and, in most cases, probably
impossible.

Subqueries, when expressed in SQL, must bounded by parentheses.

=head3 Unary Operator

A UnaryOperator takes a single argument on the RHS and is one of the following:

=over 4

=item * C<< NOT >>

=back

=head3 BinaryOperator

A BinaryOperator takes two arguments (one on the LHS and one on the RHS) and is
one of the following:

=over 4

=item * C<< = >>

=item * C<< != >>

=item * C<< > >>

=item * C<< < >>

=item * C<< >= >>

=item * C<< <= >>

=item * C<< IS >>

=item * C<< IS NOT >>

=back

Note that an operator can comprise of what would be multiple tokens in a normal
parsing effort.

=head3 Expression

An expression can be any one of the following:

=over 4

=item * Value

=item * Function

=item * Subquery

=item * UnaryOperator Expression

=item * Expression BinaryOperator Expression

=item * ( Expression )

=back

Parentheses indicate precedence and, in some situations, are necessary for
certain operators.

=head2 SQL clauses

These are all the legal and acceptable clauses within the AST that would
correpsond to clauses in a SQL statement. Not all clauses are legal within a
given RDBMS engine's SQL dialect and some clauses may be required in one and
optional in another. Detecting and enforcing those engine-specific restrictions
is the responsibility of the Visitor object.

The clauses are defined with a yacc-like syntax. The various parts are:

=over 4

=item * :=

This means "defined" and is used to create a new term to be used below.

=item * []

This means optional and indicates that the items within it are optional.

=item * []*

This means optional and repeating as many times as desired.

=item * |

This means alternation. It is a binary operator and indicates that either the
left or right hand sides may be used, but not both.

=item * C<< <> >>

This is a grouping construct. It means that all elements within this construct
are treated together for the purposes of optional, repeating, alternation, etc.

=back

The expected clauses are (name and structure):

=head3 select

This corresponds to the SELECT clause of a SELECT statement.

A select clause is composed as follows:

  SelectComponent := Expression [ [ AS ] String ]

  SelectComponent
  [ , SelectComponent ]*

=head3 tables

This is a list of tables that this clause is affecting. It corresponds to the
FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in
those respective statements. Depending on the _query metadata entry, the
appropriate clause name will be used.

The tables clause has several RDBMS-specific variations. The AST will support
all of them and it is up to the Visitor object constructing the actual SQL to
validate and/or use what is provided as appropriate.

A table clause is composed as follows:

  TableIdentifier := Identifier [ [ AS ] String ]
  JoinType := < LEFT|RIGHT [ OUTER ] > | INNER | CROSS

  TableIdentifier
  [
      < , TableIdentifier >
    | <
        [ JoinType ] JOIN TableIdentifier
        [
            < USING ( Identifier [ , Identifier ] ) >
          | < ON [ ( ] Expression [ , Expression ] [ ) ] >
        ]
      >
  ]*

Additionally, where aliases are provided for in the TableIdentifier, those
aliases must be used as the tablename in subsequent Identifiers that identify a
column of that table.

=head3 where

This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.

A where clause is composed as follows:

  WhereOperator := AND | OR
  WhereExpression := Expression | Expression WhereOperator Expression

  WhereExpression

=head3 set

This corresponds to the SET clause in an INSERT or UPDATE statement.

A set clause is composed as follows:

  SetComponent := Identifier = Expression

  SetComponent [ , SetComponent ]*

=head3 columns

This corresponds to the optional list of columns in an INSERT statement.

A columns clause is composed as follows:

  ( Identifier [ , Identifier ]* )

=head3 values

This corresponds to the VALUES clause in an INSERT statement.

A values clause is composed as follows:

  ( Expression [ , Expression ]* )

If there is a columns clause, the number of entries in the values clause must be
equal to the number of entries in the columns clause.

=head3 orderby

This corresponds to the ORDER BY clause in a SELECT statement.

An orderby clause is composed as follows:

  OrderByComponent := XXX-TODO-XXX
  OrderByDirection := ASC | DESC

  OrderByComponent [ OrderByDirection ]
  [ , OrderByComponent [ OrderByDirection ] ]*

=head3 groupby

This corresponds to the GROUP BY clause in a SELECT statement.

An groupby clause is composed as follows:

  GroupByComponent := XXX-TODO-XXX

  GroupByComponent [ , GroupByComponent ]*

=head3 rows

This corresponds to the clause that is used in some RDBMS engines to limit the
number of rows returned by a query. In MySQL, this would be the LIMIT clause.

A rows clause is composed as follows:

  Number [, Number ]

=head3 for

This corresponds to the clause that is used in some RDBMS engines to indicate
what locks are to be taken by this SELECT statement.

A for clause is composed as follows:

  UPDATE | DELETE

=head3 connectby

This corresponds to the clause that is used in some RDBMS engines to provide for
an adjacency-list query.

A connectby clause is composed as follows:

  Identifier, WhereExpression

=head1 AUTHORS

robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >>

=head1 LICENSE

You may distribute this code under the same terms as Perl itself.

=cut