3 SQL::Abstract::Manual::Specification
7 This discusses the specification for the AST provided by L<SQL::Abstract>. It is
8 meant to describe how the AST is structured, various components provided by
9 L<SQL::Abstract> for use with this AST, how to manipulate the AST, and various
10 uses for the AST once it is generated.
14 L<SQL::Abstract> has been in use for many years. Originally created to handle
15 the where-clause formation found in L<DBIx::Abstract>, it was generalized to
16 manage the creation of any SQL statement through the use of Perl structures.
17 Through the beating it received as the SQL generation syntax for L<DBIx::Class>,
18 various deficiencies were found and a generalized SQL AST was designed. This
19 document describes that AST.
23 The goals for this AST are as follows:
25 =head2 SQL-specific semantics
27 Instead of attempting to be an AST to handle any form of query, this will
28 instead be specialized to manage SQL queries (and queries that map to SQL
29 queries). This means that there will be support for SQL-specific features, such
32 =head2 Perl-specific semantics
34 This AST is meant to be used from within Perl5 only. So, it will take advantage
35 of as many Perl-specific features that make sense to use. No attempt whatosever
36 will be made to make this AST work within any other language, including Perl6.
38 =head2 Whole-lifecycle management
40 Whether a query is built out of whole cloth in one shot or cobbled together from
41 several snippets over the lifetime of a process, this AST will support any way
42 to construct the query. Queries can also be built from other queries, so an
43 UPDATE statement could be used as the basis for a SELECT statement, DELETE
44 statement, or even a DDL statement of some kind.
46 =head2 Dialect-agnostic usage
48 Even though SQL itself has several ANSI specifications (SQL-92 and SQL-99 among
49 them), this only serves as a basis for what a given RDBMS will expect. However,
50 every engine has its own specific extensions and specific ways of handling
51 common features. The AST will provide ways of expressing common functionality in
52 a common language. The emitters (objects that follow the Visitor pattern) will
53 be responsible for converting that common language into RDBMS-specific SQL.
57 The following are the restrictions upon the AST:
61 The AST will only support DML (Data Modelling Language). It will not (currently)
62 support DDL (Data Definition Language). Practically, this means that the only
63 statements supported will be:
77 Additional DML statements may be supported by specific Visitors (such as a
78 MySQL visitor supporting REPLACE INTO). q.v. the relevant sections of this
79 specification for details.
81 =head2 Dialect-agnostic construction
83 The AST will not attempt to be immediately readable to a human as SQL. In fact,
84 due to the dialect differences, particularly in terms of which use operators and
85 which use functions for a given action, the AST will provide simple units. It is
86 the responsibility of the Visitor to provide the appropriate SQL. Furthermore,
87 the AST will be very generic and only provide hints for a subset of SQL. If a
88 Visitor is sufficiently intelligent, pretty SQL may be emitted, but that is not
93 There are two major components to SQL::Abstract v2.
99 This is the Abstract Syntax Tree. It is a data structure that represents
100 everything necessary to construct the SQL statement in whatever dialect the
105 This object conforms to the Visitor pattern and is used to generate the SQL
106 represented by the AST. Each dialect will have a different Visitor object. In
107 addition, there will be visitors for at least one of the ANSI specifications.
111 The division of duties between the two components will focus on what the AST
112 can and cannot assume. For example, identifiers do not have 20 components in
113 any dialect, so the AST can validate that. However, determining what
114 constitutes a legal identifier can only be determined by the Visitor object
115 enforcing that dialect's rules.
119 The AST will be a HoHo..oH (hash of hash of ... of hashes). The keys to the
120 outermost hash will be the various clauses of a SQL statement, plus some
125 These are the additional metadata keys that the AST provides for.
129 This denotes what kind of query this AST should be interpreted as. Different
130 Visitors may accept additional values for type. For example, a MySQL Visitor
131 may choose to accept 'replace' for REPLACE INTO. If a type value is
132 unrecognized by the Visitor, the Visitor is expected to throw an error.
134 All Visitors are expected to handle the following values for type:
140 This is a SELECT statement.
144 This is an INSERT statement.
148 This is an UPDATE statement.
152 This is a DELETE statement.
158 This denotes the version of the AST. Different versions will indicate different
159 capabilities provided. Visitors will choose to respect the ast_version as needed
162 =head2 Structural units
164 All structural units will be hashes. These hashes will have, at minimum, the
171 This indicates the structural unit that this hash is representing. While this
172 specification provides for standard structural units, different Visitors may
173 choose to accept additional units as desired. If a Visitor encounters a unit it
174 doesn't know how to handle, it is expected to throw an exception.
178 Structural units in the AST are supported by loaded components. L<SQL::Abstract>
179 provides for the following structural units by default:
183 This is a (potentially) fully canonicalized identifier for a elemnt in the
184 query. This element could be a schema, table, or column. The Visitor will
185 determine validity within the context of that SQL dialect. The AST is only
186 responsible for validating that the elements are non-empty Strings.
188 The hash will be structured as follows:
191 type => 'Identifier',
192 elements => [ Scalar ],
195 All values in elements must be defined.
197 Visitors are expected to, by default, quote all identifiers according to the SQL
198 dialect's quoting scheme.
200 Any of the elements may be '*', as in SELECT * or SELECT COUNT(*). Visitors must
201 be careful to I<not> quote asterisks.
205 A Value is a Perl scalar. Depending on the subtype, a Visitor may be able to
206 make certain decisions. The following are the minimally-valid subtypes:
212 A String is a quoted series of characters. The Visitor is expected to ensure
213 that embedded quotes are properly handled per the SQL dialect's quoting scheme.
217 A Number is an unquoted number in some numeric format.
221 Null is SQL's NULL and corresponds to Perl's C<undef>.
223 =item * BindParameter
225 This corresponds to a value that will be passed in. This value is normally
226 quoted in such a fashion so as to protect against SQL injection attacks. (q.v.
227 L<DBI/quote()> for an example.)
229 BindParameters are normally represented by a '?'.
233 The hash will be structured as follows:
237 subtype => [ 'String' | 'Number' | 'Null' | 'BindParameter' ]
241 The provided subtypes are the ones that all Visitors are expected to support.
242 Visitors may choose to support additional subtypes. Visitors are expected to
243 throw an exception upon encountering an unknown subtype.
247 An Operator would be, in SQL dialect terms, a unary operator, a binary operator,
248 a trinary operator, or a function. Since different dialects may have a given
249 functionality as an operator or a function (such as CONCAT in MySQl vs. || in
250 Oracle for string concatenation), they will be represented in the AST as generic
253 The hash will be structured as follows:
263 Operators have a cardinality, or expected number of arguments. Some operators,
264 such as MAX(), have a cardinality of 1. Others, such as IF(), have a cardinality
265 of N, meaning they can have any number of arguments greater than 0. Others, such
266 as NOW(), have a cardinality of 0. Several operators with the same meaning may
267 have a different cardinality in different SQL dialects as different engines may
268 allow different behaviors. As cardinality may differ between dialects, enforcing
269 cardinality is necessarily left to the Visitor.
271 Operators also have restrictions on the types of arguments they will accept. The
272 first argument may or may not restricted in the same fashion as the other
273 arguments. As with cardinality, this restriction will need to be managed by the
276 The operator name needs to take into account the possibility that the RDBMS may
277 allow UDFs (User-Defined Functions) that have the same name as an operator, such
278 as 'AND'. This will have to be managed by the Visitor.
282 A Subquery is another AST whose type metadata parameter is set to "SELECT".
284 Most places that a Subquery can be used would require a single value to be
285 returned (single column, single row), but that is not something that the AST can
286 easily enforce. The single-column restriction may possibly be enforced, but the
287 single-row restriction is much more difficult and, in most cases, probably
290 Subqueries, when expressed in SQL, must be bounded by parentheses.
294 An Alias is any place where the construct "X as Y" appears. While the "as Y" is
295 often optional, the AST will make it required.
297 The hash will be structured as follows:
307 An Expression can be any one of the following:
323 An Expression is a meta-syntactic unit. An "Expression" unit will never appear
324 within the AST. It acts as a junction.
328 There is no specific operator or nodetype for nesting. Instead, nesting is
329 explicitly specified by node descent in the AST.
333 These are all the legal and acceptable clauses within the AST that would
334 correpsond to clauses in a SQL statement. Not all clauses are legal within a
335 given RDBMS engine's SQL dialect and some clauses may be required in one and
336 optional in another. Detecting and enforcing those engine-specific restrictions
337 is the responsibility of the Visitor object.
339 The following clauses are expected to be handled by Visitors for each statement:
369 There are RDBMS-specific variations of the INSERT statement, such the one in
396 The expected clauses are (name and structure):
400 This corresponds to the SELECT clause of a SELECT statement.
402 A select clause unit is an array of one or more Expressions.
406 This is a list of tables that this clause is affecting. It corresponds to the
407 FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in
408 those respective statements. Depending on the type metadata entry, the
409 appropriate clause name will be used.
411 The tables clause has several RDBMS-specific variations. The AST will support
412 all of them and it is up to the Visitor object constructing the actual SQL to
413 validate and/or use what is provided as appropriate.
415 A tables clause is an Expression.
417 The hash for an Operator within a tables clause will be composed as follows:
422 op => '< LEFT|RIGHT|FULL [ OUTER ] > | INNER | CROSS',
424 args => [ Expression ],
427 A USING clause is syntactic sugar for an ON clause and, as such, is not provided
428 for by the AST. A join of a comma is identical to a CROSS JOIN and, as such, is
429 not provided for by the AST. The on clause is optional.
433 This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.
435 A where clause is composed of an Expression.
439 This corresponds to the SET clause in an INSERT or UPDATE statement.
441 The hash for an set clause will be composed as follows:
451 The args is an array that is organized as follows: The first element is an array of
452 Identifiers for the columns being set. The following arrays are Expressions describing
453 the values. The various arrays should be the same length. The array of Identifiers can
458 This corresponds to the ORDER BY clause in a SELECT statement.
460 A orderby clause unit is an array of one or more OrderbyComponent units.
462 The hash for a OrderbyComponent unit is composed as follows:
465 type => 'OrderbyComponent',
467 dir => '< ASC | DESC >',
470 The value should either be an Identifier or a Number. The dir element, if
471 omitted, will be defaulted to ASC by the AST. The number corresponds to a column
472 in the select clause.
476 This corresponds to the GROUP BY clause in a SELECT statement.
478 A groupby clause unit is an array of one or more GroupbyComponent units.
480 The hash for a GroupbyComponent unit is composed as follows:
483 type => 'GroupbyComponent',
487 The value should either be an Identifier or a Number. The number corresponds to
488 a column in the select clause.
490 =head2 Possible RDBMS-specific clauses
492 The following clauses are provided as examples for RDBMS-specific elements. They
493 are B<not> expected to be supported by all Visitors. Visitors may choose whether
494 or not to throw on an unexpected clause, though it is strongly recommended.
498 This corresponds to the clause that is used in some RDBMS engines to limit the
499 number of rows returned by a SELECT statement. In MySQL, this would be the LIMIT
502 The hash for a rows clause is composed as follows:
509 The start attribute, if ommitted, will default to 0. The count attribute is
514 This corresponds to the clause that is used in some RDBMS engines to indicate
515 what locks are to be taken by this SELECT statement.
517 The hash for a for clause is composed as follows:
520 value => '< UPDATE | DELETE >',
525 This corresponds to the clause that is used in some RDBMS engines to provide for
526 an adjacency-list query.
528 The hash for a for clause is composed as follows:
535 option => '< PRIOR | NOCYCLE >'
540 order_siblings => orderby-clause,
543 Both the start_with and order_siblings clauses are optional.
551 =item * UNION, UNION ALL, and MINUS
555 Convert INSERT and UPDATE into ->populate form.
559 robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >>
563 You may distribute this code under the same terms as Perl itself.