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.
=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 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. All metadata keys will be identifiable as such by being prefixed
-with an underscore. All keys will be in lowercase.
+metadata keys.
=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 * _version
-
-This denotes the version of the AST.
-
-=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
+=head3 type
-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 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.
-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:
+All Visitors are expected to handle the following values for type:
=over 4
-=item * String
+=item * select
-A String is a quoted series of characters
+This is a SELECT statement.
-=item * Number
+=item * insert
-A Number is an unquoted number in some numeric format
+This is an INSERT statement.
-=item * NULL
+=item * update
-NULL is SQL's NULL and corresponds to Perl's C<undef>.
+This is an UPDATE statement.
-=item * BindParameter
+=item * delete
-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.)
+This is a DELETE statement.
=back
-=head3
+=head3 ast_version
-=head3 Function
+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.
-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.
+=head2 Structural units
-Yes, a Subquery is legal as an argument for many functions. Some example
-functions are:
+All structural units will be hashes. These hashes will have, at minimum, the
+following keys:
=over 4
-=item * C<< IN >>
+=item * type
-=item * C<< MAX >>
+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.
-=item * C<< MIN >>
+=back
-=item * C<< SUM >>
+Structural units in the AST are supported by loaded components. L<SQL::Abstract>
+provides for the following structural units by default:
-=back
+=head3 Identifier
-=head3 Subquery
+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.
-A Subquery is another AST whose _query metadata parameter is set to "SELECT".
+The hash will be structured as follows:
-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.
+ {
+ type => 'Identifier',
+ element1 => Scalar,
+ element2 => Scalar,
+ element3 => Scalar,
+ }
+
+If element3 exists, then element2 must exist. element1 must always exist. If a
+given element exists, then it must be defined and of non-zero length.
-Subqueries, when expressed in SQL, must bounded by parentheses.
+Visitors are expected to, by default, quote all identifiers according to the SQL
+dialect's quoting scheme.
-=head3 Unary Operator
+Any of the elements may be '*', as in SELECT * or SELECT COUNT(*). Visitors must
+be careful to I<not> quote asterisks.
-A UnaryOperator takes a single argument on the RHS and is one of the following:
+=head3 Value
+
+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:
=over 4
-=item * C<< NOT >>
+=item * String
-=back
+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.
-=head3 BinaryOperator
+=item * Number
-A BinaryOperator takes two arguments (one on the LHS and one on the RHS) and is
-one of the following:
+A Number is an unquoted number in some numeric format.
-=over 4
+=item * Null
-=item * C<< = >>
+Null is SQL's NULL and corresponds to Perl's C<undef>.
-=item * C<< != >>
+=item * BindParameter
-=item * C<< > >>
+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.)
-=item * C<< < >>
+BindParameters are normally represented by a '?'.
-=item * C<< >= >>
+=back
-=item * C<< <= >>
+The hash will be structured as follows:
+
+ {
+ type => 'Value'
+ subtype => [ 'String' | 'Number' | 'Null' | 'BindParameter' ]
+ value => Scalar
+ }
+
+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.
+
+=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.
-=item * C<< IS >>
+=head3 Subquery
-=item * C<< IS NOT >>
+A Subquery is another AST whose type metadata parameter is set to "SELECT".
-=back
+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 may possibly be enforced, but the
+single-row restriction is much more difficult and, in most cases, probably
+impossible.
-Note that an operator can comprise of what would be multiple tokens in a normal
-parsing effort.
+Subqueries, when expressed in SQL, must be bounded by parentheses.
=head3 Expression
-An expression can be any one of the following:
+An Expression can be any one of the following:
=over 4
+=item * Identifier
+
=item * Value
-=item * Function
+=item * Operator
=item * Subquery
-=item * UnaryOperator Expression
-
-=item * Expression BinaryOperator Expression
+=back
-=item * ( Expression )
+An Expression is a meta-syntactic unit. An "Expression" unit will never appear
+within the AST. It acts as a junction.
-=back
+=head3 Nesting
-Parentheses indicate precedence and, in some situations, are necessary for
-certain operators.
+There is no specific operator or nodetype for nesting. Instead, nesting is
+explicitly specified by node descent in the AST.
=head2 SQL clauses
This corresponds to the SELECT clause of a SELECT statement.
-A select clause is composed as follows:
+A select clause unit is an array of one or more SelectComponent units.
- SelectComponent := Expression [ [ AS ] String ]
+The hash for a SelectComponent unit is composed as follows:
- SelectComponent
- [ , SelectComponent ]*
+ {
+ type => 'SelectComponent',
+ value => Expression,
+ as => String,
+ }
+
+The 'as' component is optional. Visitors may choose to make it required in
+certain situations.
=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
+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 TableJoin is a junction of the following elements:
- TableIdentifier := Identifier [ [ AS ] String ]
- JoinType := < LEFT|RIGHT [ OUTER ] > | INNER | CROSS
+=over 4
- TableIdentifier
- [
- < , TableIdentifier >
- | <
- [ JoinType ] JOIN TableIdentifier
- [
- < USING ( Identifier [ , Identifier ] ) >
- | < ON [ ( ] Expression [ , Expression ] [ ) ] >
- ]
- >
- ]*
+=item * TableIdentifier
-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.
+=item * Operator
-=head3 where
+=back
-This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.
+The hash for a TableIdentifier will be composed as follows:
+
+ # TableIdentifier
+ {
+ type => 'TableIdentifier',
+ value => Expression,
+ as => String,
+ }
-A where clause is composed as follows:
+The value should be either an Identifier or a SubQuery.
- WhereOperator := AND | OR
- WhereExpression := Expression | Expression WhereOperator Expression
+The hash for an Operator within a tables clause will be composed as follows:
+
+ # Operator
+ {
+ type => 'Operator',
+ op => '< LEFT|RIGHT|FULL [ OUTER ] > | INNER | CROSS',
+ on => Expression,
+ }
+
+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.
- WhereExpression
+A where clause is composed of an Expression.
=head3 set
This corresponds to the SET clause in an INSERT or UPDATE statement.
-A set clause is composed as follows:
+A set clause unit is an array of one or more SetComponent units.
- SetComponent := Identifier = Expression
+The hash for SetComponent unit is composed as follows:
- SetComponent [ , SetComponent ]*
+ {
+ type => 'SetComponent',
+ col => Identifier,
+ value => Expression,
+ }
=head3 columns
This corresponds to the optional list of columns in an INSERT statement.
-A columns clause is composed as follows:
-
- ( Identifier [ , Identifier ]* )
+A columns clause unit is an array of one or more Identifier units.
=head3 values
This corresponds to the VALUES clause in an INSERT statement.
-A values clause is composed as follows:
-
- ( Expression [ , Expression ]* )
+A values clause unit is an array of one or more Expression units.
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.
This corresponds to the ORDER BY clause in a SELECT statement.
-An orderby clause is composed as follows:
+A orderby clause unit is an array of one or more OrderbyComponent units.
+
+The hash for a OrderbyComponent unit is composed as follows:
- OrderByComponent := XXX-TODO-XXX
- OrderByDirection := ASC | DESC
+ {
+ type => 'OrderbyComponent',
+ value => < Identifier | Number >
+ dir => '< ASC | DESC >',
+ }
- OrderByComponent [ OrderByDirection ]
- [ , OrderByComponent [ OrderByDirection ] ]*
+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.
-An groupby clause is composed as follows:
+A groupby clause unit is an array of one or more GroupbyComponent units.
- GroupByComponent := XXX-TODO-XXX
+The hash for a GroupbyComponent unit is composed as follows:
- GroupByComponent [ , GroupByComponent ]*
+ {
+ type => 'GroupbyComponent',
+ value => < Identifier | Number >
+ }
+
+The number corresponds to a column in the select clause.
=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:
+The hash for a rows clause is composed as follows:
+
+ {
+ start => Number,
+ count => Number,
+ }
- Number [, Number ]
+The start attribute, if ommitted, will default to 0. The count attribute is
+optional.
=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:
+The hash for a for clause is composed as follows:
- UPDATE | DELETE
+ {
+ value => '< 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:
+The hash for a for clause is composed as follows:
+
+ {
+ start_with => [
+ Expression,
+ ],
+ connect_by => {
+ option => '< PRIOR | NOCYCLE >'
+ cond => [
+ Expression,
+ ],
+ },
+ order_siblings => orderby-clause,
+ }
- Identifier, WhereExpression
+Both the start_with and order_siblings clauses are optional.
+
+=head1 TODO
+
+=over 4
+
+=item * sproc unit
+
+=back
=head1 AUTHORS
projects / dbsrgits/SQL-Abstract-2.0-ish.git / blobdiff