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.
+common features. 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 RESTRICTIONS
+
+The following are the restrictions upon the AST:
+
+=head2 DML-only
+
+The AST will only support DML (Data Modelling Language). It will not (currently)
+support DDL (Data Definition Language). Practically, this means that the only
+statements supported will be:
+
+=over 4
+
+=item * SELECT
+
+=item * INSERT INTO
+
+=item * UPDATE
+
+=item * DELETE
+
+=back
+
+Additional DML statements may be supported by specific Visitors (such as a
+MySQL visitor supporting REPLACE INTO). q.v. the relevant sections of this
+specification for details.
+
+=head2 Dialect-agnostic construction
+
+The AST will not attempt to be immediately readable to a human as SQL. In fact,
+due to the dialect differences, particularly in terms of which use operators and
+which use functions for a given action, the AST will provide simple units. It is
+the responsibility of the Visitor to provide the appropriate SQL. Furthermore,
+the AST will be very generic and only provide hints for a subset of SQL. If a
+Visitor is sufficiently intelligent, pretty SQL may be emitted, but that is not
+the goal of this AST.
+
+=head1 COMPONENTS
+
+There are two major components to SQL::Abstract v2.
+
+=over 4
+
+=item * AST
+
+This is the Abstract Syntax Tree. It is a data structure that represents
+everything necessary to construct the SQL statement in whatever dialect the
+user requires.
+
+=item * Visitor
+
+This object conforms to the Visitor pattern and is used to generate the SQL
+represented by the AST. Each dialect will have a different Visitor object. In
+addition, there will be visitors for at least one of the ANSI specifications.
+
+=back
+
+The division of duties between the two components will focus on what the AST
+can and cannot assume. For example, identifiers do not have 20 components in
+any dialect, so the AST can validate that. However, determining what
+constitutes a legal identifier can only be determined by the Visitor object
+enforcing that dialect's rules.
=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.
+The AST will be a HoHo..oH (hash of hash of ... of hashes). The keys to the
+outermost hash will be the various clauses of a SQL statement, plus some
+metadata keys.
=head2 Metadata keys
These are the additional metadata keys that the AST provides for.
+=head3 type
+
+This denotes what kind of query this AST should be interpreted as. Different
+Visitors may accept additional values for type. For example, a MySQL Visitor
+may choose to accept 'replace' for REPLACE INTO. If a type value is
+unrecognized by the Visitor, the Visitor is expected to throw an error.
+
+All Visitors are expected to handle the following values for type:
+
=over 4
-=item * _query
+=item * select
+
+This is a SELECT statement.
+
+=item * insert
+
+This is an INSERT statement.
-This denotes what kind of query this AST should be interpreted as.
+=item * update
-=item *
+This is an UPDATE statement.
+
+=item * delete
+
+This is a DELETE statement.
=back
+=head3 ast_version
+
+This denotes the version of the AST. Different versions will indicate different
+capabilities provided. Visitors will choose to respect the ast_version as needed
+and desired.
+
=head2 Structural units
+All structural units will be hashes. These hashes will have, at minimum, the
+following keys:
+
+=over 4
+
+=item * type
+
+This indicates the structural unit that this hash is representing. While this
+specification provides for standard structural units, different Visitors may
+choose to accept additional units as desired. If a Visitor encounters a unit it
+doesn't know how to handle, it is expected to throw an exception.
+
+=back
+
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 >.
+This is a (potentially) fully canonicalized identifier for a elemnt in the
+query. This element could be a schema, table, or column. The Visitor will
+determine validity within the context of that SQL dialect. The AST is only
+responsible for validating that the elements are non-empty Strings.
+
+The hash will be structured as follows:
+
+ {
+ type => 'Identifier',
+ elements => [ Scalar ],
+ }
+
+All values in elements must be defined.
+
+Visitors are expected to, by default, quote all identifiers according to the SQL
+dialect's quoting scheme.
+
+Any of the elements may be '*', as in SELECT * or SELECT COUNT(*). Visitors must
+be careful to I<not> quote asterisks.
-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
-=head3 Constant
+A Value is a Perl scalar. Depending on the subtype, a Visitor may be able to
+make certain decisions. The following are the minimally-valid subtypes:
-A Constant is a Perl scalar. It may either be a String (quoted series of
-characters) or a number (unquoted).
+=over 4
+
+=item * String
+
+A String is a quoted series of characters. The Visitor is expected to ensure
+that embedded quotes are properly handled per the SQL dialect's quoting scheme.
+
+=item * Number
+
+A Number is an unquoted number in some numeric format.
+
+=item * Null
+
+A Null is SQL's NULL and corresponds to Perl's C<undef>.
+
+=item * Boolean
-=head3 Function
+A Boolean is a two-value entity - true or false. Some DBMSes provide an explicit
+boolean while others do not.
-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.
+=item * Date
-Yes, a Subquery is legal as an argument for many functions.
+A Date represents (generally) a year, month, and day.
+
+=item * Time
+
+A Time represents (generally) an hour, minute, and second.
+
+=item * DateTime
+
+A DateTime represents the complete description necessary to determine a specific point in
+time. This could correspond to a L<DateTime/> object in Perl.
+
+=back
+
+The hash will be structured as follows:
+
+ {
+ type => 'Value'
+ subtype => [ 'String' | 'Number' | 'Null' | 'Boolean' | 'Date' | 'Time' | 'DateTime' ]
+ value => Scalar
+ is_bind => Boolean
+ }
+
+The provided subtypes are the ones that all Visitors are expected to support.
+Visitors may choose to support additional subtypes. Visitors are expected to
+throw an exception upon encountering an unknown subtype.
+
+C<is_bind> defaults to true. It determines whether or not the Visitor should
+attempt to treat this value as a BindParameter or not.
+
+=head3 Operator
+
+An Operator would be, in SQL dialect terms, a unary operator, a binary operator,
+a trinary operator, or a function. Since different dialects may have a given
+functionality as an operator or a function (such as CONCAT in MySQl vs. || in
+Oracle for string concatenation), they will be represented in the AST as generic
+operators.
+
+The hash will be structured as follows:
+
+ {
+ type => 'Operator',
+ op => String,
+ args => [
+ Expression,
+ ],
+ }
+
+Operators have a cardinality, or expected number of arguments. Some operators,
+such as MAX(), have a cardinality of 1. Others, such as IF(), have a cardinality
+of N, meaning they can have any number of arguments greater than 0. Others, such
+as NOW(), have a cardinality of 0. Several operators with the same meaning may
+have a different cardinality in different SQL dialects as different engines may
+allow different behaviors. As cardinality may differ between dialects, enforcing
+cardinality is necessarily left to the Visitor.
+
+Operators also have restrictions on the types of arguments they will accept. The
+first argument may or may not restricted in the same fashion as the other
+arguments. As with cardinality, this restriction will need to be managed by the
+Visitor.
+
+The operator name needs to take into account the possibility that the RDBMS may
+allow UDFs (User-Defined Functions) that have the same name as an operator, such
+as 'AND'. This will have to be managed by the Visitor.
=head3 Subquery
-A Subquery is another AST whose _query metadata parameter is set to "SELECT".
+A Subquery is another AST whose type 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
+easily enforce. The single-column restriction may possibly be enforced, but the
single-row restriction is much more difficult and, in most cases, probably
impossible.
-=head3 Unary Operator
+Subqueries, when expressed in SQL, must be bounded by parentheses.
+
+=head3 Alias
-A UnaryOperator takes a single argument on the RHS and is one of the following:
+An Alias is any place where the construct "X as Y" appears; it is the "AS Y" part.
+
+The hash will be structured as follows:
+
+ {
+ type => 'Alias',
+ value => Expression,
+ as => Identifier,
+ }
+
+=head3 Expression
+
+An Expression can be any one of the following:
=over 4
-=item * C<< NOT >>
+=item * Identifier
+
+=item * Value
+
+=item * Operator
+
+=item * Subquery
+
+=item * Alias
=back
-=head3 BinaryOperator
+An Expression is a meta-syntactic unit. An "Expression" unit will never appear
+within the AST. It acts as a junction.
-A BinaryOperator takes two arguments (one on the LHS and one on the RHS) and is
-one of the following:
+=head3 Nesting
-=over 4
+There is no specific operator or nodetype for nesting. Instead, nesting is
+explicitly specified by node descent in the AST.
+
+=head2 SQL clauses
-=item * C<< = >>
+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.
-=item * C<< != >>
+The following clauses are expected to be handled by Visitors for each statement:
-=item * C<< > >>
+=over 4
-=item * C<< < >>
+=item * select
-=item * C<< >= >>
+=over 4
-=item * C<< <= >>
+=item * select
-=item * C<< IS >>
+=item * tables
-=item * C<< IS NOT >>
+=item * where
-=item * C<< IN >>
+=item * orderby
-=item * C<< NOT IN >>
+=item * groupby
=back
-Note that an operator can comprise of what would be multiple tokens in a normal
-parsing effort.
+=item * insert
-=head3 Expression
+=over 4
+
+=item * tables
+
+=item * set
+
+=back
+
+There are RDBMS-specific variations of the INSERT statement, such the one in
+MySQL's
-An expression can be any one of the following:
+=item * update
=over 4
-=item * Constant
+=item * tables
-=item * Function
+=item * set
-=item * Subquery
+=item * where
+
+=back
+
+=item * delete
+
+=over 4
-=item * UnaryOperator Expression
+=item * tables
-=item * Expression BinaryOperator Expression
+=item * where
=back
-=head2 SQL clauses
+=back
The expected clauses are (name and structure):
=head3 select
-This corresponds to the SELECT clause of a SELECT statement. It maps to a comma-
-separated list of the following construct C< Expression [ [ AS ] String ] >
-(where the [] indicate optional items).
+This corresponds to the SELECT clause of a SELECT statement.
+
+A select clause unit is an array of one or more Expressions.
=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 UPDATE/DELETE clauses in those
-respective statements. Depending on the _query metadata entry, the appropriate
-clause name will be used.
+FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in
+those respective statements. Depending on the type 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:
+A tables clause is an Expression that also allows for Joins.
- TableIdentifier := Identifier [ [ AS ] String ]
- JoinType := < LEFT|RIGHT [ OUTER ] > | < INNER >
+The hash for an Join within a tables clause will be composed as follows:
- TableIdentifier
- [
- < , TableIdentifier >
- | <
- [ JoinType ] JOIN TableIdentifier
- [
- < USING ( Identifier [ , Identifier ] ) >
- | < ON [ ( ] Expression [ , Expression ] [ ) ] >
- ]
- >
- ]*
+ {
+ type => 'Join',
+ op => '< LEFT|RIGHT|FULL [ OUTER ] > | INNER | CROSS',
+ on => Expression,
+ args => [ 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.
+A USING clause is syntactic sugar for an ON clause and, as such, is not provided
+for by the AST. A join of a comma is identical to a CROSS JOIN and, as such, is
+not provided for by the AST. The on clause is optional.
=head3 where
+This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.
+
+A where clause is composed of an Expression.
+
=head3 set
-=head3 values
+This corresponds to the SET clause in an INSERT or UPDATE statement.
+
+The hash for an set clause will be composed as follows:
+
+ {
+ type => 'Set',
+ args => [
+ [ Identifier ],
+ [ Expresion ],
+ ],
+ }
+
+The args is an array that is organized as follows: The first element is an array of
+Identifiers for the columns being set. The following arrays are Expressions describing
+the values. The various arrays should be the same length. The array of Identifiers can
+be omitted.
=head3 orderby
+This corresponds to the ORDER BY clause in a SELECT statement.
+
+A orderby clause unit is an array of one or more OrderbyComponent units.
+
+The hash for a OrderbyComponent unit is composed as follows:
+
+ {
+ type => 'OrderbyComponent',
+ value => Expression,
+ dir => '< ASC | DESC >',
+ }
+
+The value should either be an Identifier or a Number. The dir element, if
+omitted, will be defaulted to ASC by the AST. The number corresponds to a column
+in the select clause.
+
=head3 groupby
+This corresponds to the GROUP BY clause in a SELECT statement.
+
+A groupby clause unit is an array of one or more GroupbyComponent units.
+
+The hash for a GroupbyComponent unit is composed as follows:
+
+ {
+ type => 'GroupbyComponent',
+ value => Expression,
+ }
+
+The value should either be an Identifier or a Number. The number corresponds to
+a column in the select clause.
+
+=head2 Possible RDBMS-specific clauses
+
+The following clauses are provided as examples for RDBMS-specific elements. They
+are B<not> expected to be supported by all Visitors. Visitors may choose whether
+or not to throw on an unexpected clause, though it is strongly recommended.
+
=head3 rows
+This corresponds to the clause that is used in some RDBMS engines to limit the
+number of rows returned by a SELECT statement. In MySQL, this would be the LIMIT
+clause.
+
+The hash for a rows clause is composed as follows:
+
+ {
+ start => Number,
+ count => Number,
+ }
+
+The start attribute, if ommitted, will default to 0. The count attribute is
+optional.
+
=head3 for
-=head3
+This corresponds to the clause that is used in some RDBMS engines to indicate
+what locks are to be taken by this SELECT statement.
+
+The hash for a for clause is composed as follows:
+
+ {
+ value => '< UPDATE | DELETE >',
+ }
+
+=head1 TODO
+
+=over 4
+
+=item * sproc unit
+
+=item * UNION, UNION ALL, and MINUS
+
+=item * start the API guidelines
+
+=back
+
+Convert INSERT and UPDATE into ->populate form.
=head1 AUTHORS
-robkinyon: Rob Kinyon <rkinyon@cpan.org>
+robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >>
=head1 LICENSE