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 ...
87 XXX FILL ME IN LATER XXX
91 There are two major components to SQL::Abstract v2.
97 This is the Abstract Syntax Tree. It is a data structure that represents
98 everything necessary to construct the SQL statement in whatever dialect the
103 This object conforms to the Visitor pattern and is used to generate the SQL
104 represented by the AST. Each dialect will have a different Visitor object. In
105 addition, there will be visitors for at least one of the ANSI specifications.
109 The division of duties between the two components will focus on what the AST
110 can and cannot assume. For example, identifiers do not have 20 components in
111 any dialect, so the AST can validate that. However, determining what
112 constitutes a legal identifier can only be determined by the Visitor object
113 enforcing that dialect's rules.
117 The AST will be a HoHo..oH (hash of hash of ... of hashes). The keys to the
118 outermost hash will be the various clauses of a SQL statement, plus some
119 metadata keys. All metadata keys will be identifiable as such by being prefixed
120 with an underscore. All keys will be in lowercase.
124 These are the additional metadata keys that the AST provides for.
128 This denotes what kind of query this AST should be interpreted as. Different
129 Visitors may accept additional values for _query. For example, a MySQL Visitor
130 may choose to accept 'replace' for REPLACE INTO. If a _query value is
131 unrecognized by the Visitor, the Visitor is expected to throw an error.
133 All Visitors are expected to handle the following values for _query:
139 This is a SELECT statement.
143 This is an INSERT statement.
147 This is an UPDATE statement.
151 This is a DELETE statement.
157 This denotes the version of the AST. Different versions will indicate different
158 capabilities provided. Visitors will choose to respect the _version as needed
161 =head2 Structural units
163 All structural units will be hashes. These hashes will have, at minimum, the
170 This indicates the structural unit that this hash is representing. While this
171 specification provides for standard structural units, different Visitors may
172 choose to accept additional units as desired. If a Visitor encounters a unit it
173 doesn't know how to handle, it is expected to throw an exception.
177 Structural units in the AST are supported by loaded components. L<SQL::Abstract>
178 provides for the following structural units by default:
182 This is a (potentially) fully canonicalized identifier for a elemnt in the
183 query. This element could be a schema, table, or column. The Visitor will
184 determine validity within the context of that SQL dialect. The AST is only
185 responsible for validating that the elements are non-empty Strings.
187 The hash will be structured as follows:
190 type => 'Identifier',
196 If element3 exists, then element2 must exist. element1 must always exist. If a
197 given element exists, then it must be defined and of non-zero length.
199 Visitors are expected to, by default, quote all identifiers according to the SQL
200 dialect's quoting scheme.
204 A Value is a Perl scalar. Depending on the type, a Visitor may be able to make
211 A String is a quoted series of characters. The Visitor is expected to ensure
212 that embedded quotes are properly handled per the SQL dialect's quoting scheme.
216 A Number is an unquoted number in some numeric format.
220 Null is SQL's NULL and corresponds to Perl's C<undef>.
222 =item * BindParameter
224 This corresponds to a value that will be passed in. This value is normally
225 quoted in such a fashion so as to protect against SQL injection attacks. (q.v.
226 L<DBI/quote()> for an example.)
228 BindParameters are normally represented by a '?'.
232 The hash will be structured as follows:
236 subtype => [ 'String' | 'Number' | 'Null' | 'BindParameter' ]
240 The provided subtypes are the ones that all Visitors are expected to support.
241 Visitors may choose to support additional subtypes. Visitors are expected to
242 throw an exception upon encountering an unknown subtype.
246 An Operator would be, in SQL dialect terms, a unary operator, a binary operator,
247 a trinary operator, or a function. Since different dialects may have a given
248 functionality as an operator or a function (such as CONCAT in MySQl vs. || in
249 Oracle for string concatenation), they will be represented in the AST as generic
252 The hash will be structured as follows:
257 args => ExpressionList,
260 Operators have a cardinality, or expected number of arguments. Some operators,
261 such as MAX(), have a cardinality of 1. Others, such as IF(), have a cardinality
262 of N, meaning they can have any number of arguments greater than 0. Others, such
263 as NOW(), have a cardinality of 0. Several operators with the same meaning may
264 have a different cardinality in different SQL dialects as different engines may
265 allow different behaviors. As cardinality may differ between dialects, enforcing
266 cardinality is necessarily left to the Visitor.
268 Operators also have restrictions on the types of arguments they will accept. The
269 first argument may or may not restricted in the same fashion as the other
270 arguments. As with cardinality, this restriction will need to be managed by the
273 The operator name needs to take into account the possibility that the RDBMS may
274 allow UDFs (User-Defined Functions) that have the same name as an operator, such
275 as 'AND'. This will have to be managed by the Visitor.
279 A Subquery is another AST whose _query metadata parameter is set to "SELECT".
281 Most places that a Subquery can be used would require a single value to be
282 returned (single column, single row), but that is not something that the AST can
283 easily enforce. The single-column restriction may possibly be enforced, but the
284 single-row restriction is much more difficult and, in most cases, probably
287 Subqueries, when expressed in SQL, must be bounded by parentheses.
291 An Expression can be any one of the following:
305 An Expression is a meta-syntactic unit. An "Expression" unit will never appear
306 within the AST. It acts as a junction.
308 =head3 ExpressionList
310 An ExpressionList is a list of Expressions, generally separated by commas
311 (though other separators may be appropriate at times or for different SQL
314 The hash for an ExpressionList is as follows:
317 type => 'ExpressionList',
319 elements => Array of Expressions,
322 An ExpressionList is always rendered in SQL with parentheses around it.
326 These are all the legal and acceptable clauses within the AST that would
327 correpsond to clauses in a SQL statement. Not all clauses are legal within a
328 given RDBMS engine's SQL dialect and some clauses may be required in one and
329 optional in another. Detecting and enforcing those engine-specific restrictions
330 is the responsibility of the Visitor object.
332 The clauses are defined with a yacc-like syntax. The various parts are:
338 This means "defined" and is used to create a new term to be used below.
342 This means optional and indicates that the items within it are optional.
346 This means optional and repeating as many times as desired.
350 This means alternation. It is a binary operator and indicates that either the
351 left or right hand sides may be used, but not both.
355 This is a grouping construct. It means that all elements within this construct
356 are treated together for the purposes of optional, repeating, alternation, etc.
360 The expected clauses are (name and structure):
364 This corresponds to the SELECT clause of a SELECT statement.
366 A select clause unit is an array of one or more SelectComponent units.
368 The hash for a SelectComponent unit is composed as follows:
371 type => 'SelectComponent',
373 [ as => Identifier, ]
376 The 'as' component is optional. Visitors may choose to make it required in
381 This is a list of tables that this clause is affecting. It corresponds to the
382 FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in
383 those respective statements. Depending on the _query metadata entry, the
384 appropriate clause name will be used.
386 A tables clause unit is an array of one or more TableComponent units.
388 The tables clause has several RDBMS-specific variations. The AST will support
389 all of them and it is up to the Visitor object constructing the actual SQL to
390 validate and/or use what is provided as appropriate.
392 The hash for a TableJoin will be composed as follows:
397 join => < LEFT|RIGHT [ OUTER ] > | INNER | CROSS | ',',
398 [ using => IdentifierList, ]
399 [ on => ExpressionList, ]
402 A TableJoin may not have both a 'using' element and an 'on' element. It may
403 have one of them if the 'join' element is not equal to ',' but doesn't have to.
404 If the 'join' element is equal to ',', then it may not have either a 'using' or
407 The hash for a TableIdentifier will be composed as follows:
411 type => 'TableIdentifier',
412 value => Identifier | SubQuery
413 [ join => TableJoin, ]
414 [ as => Identifier, ]
417 The first TableComponent in a tables clause may not have a join element. All
418 other TableComponent elements that do not have a join element will have a
419 default join element of:
426 The 'as' component is optional. Visitors may choose to make it required in
427 certain situations (such as MySQL requiring an alias for subqueries).
429 Additionally, where aliases are provided for in the TableIdentifier, those
430 aliases must be used as the tablename in subsequent Identifiers that identify a
431 column of that table. This may be enforceable by the AST or the Visitor. But, it
432 is more likely that it will not be.
436 This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.
438 A where clause is composed as follows:
440 WhereOperator := AND | OR
441 WhereExpression := Expression | Expression WhereOperator Expression
447 This corresponds to the SET clause in an INSERT or UPDATE statement.
449 A set clause is composed as follows:
451 SetComponent := Identifier = Expression
453 SetComponent [ , SetComponent ]*
457 This corresponds to the optional list of columns in an INSERT statement.
459 A columns clause is an IdentifierList and the unit is composed as follows:
468 This corresponds to the VALUES clause in an INSERT statement.
470 A values clause is an ExpressionList and the unit is composed as follows.
477 If there is a columns clause, the number of entries in the values clause must be
478 equal to the number of entries in the columns clause.
482 This corresponds to the ORDER BY clause in a SELECT statement.
484 An orderby clause is composed as follows:
486 OrderByComponent := XXX-TODO-XXX
487 OrderByDirection := ASC | DESC
489 OrderByComponent [ OrderByDirection ]
490 [ , OrderByComponent [ OrderByDirection ] ]*
494 This corresponds to the GROUP BY clause in a SELECT statement.
496 An groupby clause is composed as follows:
498 GroupByComponent := XXX-TODO-XXX
500 GroupByComponent [ , GroupByComponent ]*
504 This corresponds to the clause that is used in some RDBMS engines to limit the
505 number of rows returned by a query. In MySQL, this would be the LIMIT clause.
507 A rows clause is composed as follows:
513 This corresponds to the clause that is used in some RDBMS engines to indicate
514 what locks are to be taken by this SELECT statement.
516 A for clause is composed as follows:
522 This corresponds to the clause that is used in some RDBMS engines to provide for
523 an adjacency-list query.
525 A connectby clause is composed as follows:
527 Identifier, WhereExpression
531 The following are example SQL statements and a possible AST for each one.
539 _ast_version => 0.0001,
542 type => 'SelectComponent',
552 =item * SELECT NOW() AS time FROM dual AS duality
556 _ast_version => 0.0001,
559 type => 'SelectComponent',
565 type => 'Identifier',
572 type => 'TablesComponent',
574 type => 'Identifier',
578 type => 'Identifier',
579 element1 => 'duality',
589 robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >>
593 You may distribute this code under the same terms as Perl itself.