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.
83 There are two major components to SQL::Abstract v2.
89 This is the Abstract Syntax Tree. It is a data structure that represents
90 everything necessary to construct the SQL statement in whatever dialect the
95 This object conforms to the Visitor pattern and is used to generate the SQL
96 represented by the AST. Each dialect will have a different Visitor object. In
97 addition, there will be visitors for at least one of the ANSI specifications.
101 The division of duties between the two components will focus on what the AST
102 can and cannot assume. For example, identifiers do not have 20 components in
103 any dialect, so the AST can validate that. However, determining what
104 constitutes a legal identifier can only be determined by the Visitor object
105 enforcing that dialect's rules.
109 The AST will be a HoHo..oH (hash of hash of ... of hashes). The keys to the
110 outermost hash will be the various clauses of a SQL statement, plus some
111 metadata keys. All metadata keys will be identifiable as such by being prefixed
112 with an underscore. All keys will be in lowercase.
116 These are the additional metadata keys that the AST provides for.
120 This denotes what kind of query this AST should be interpreted as. Different
121 Visitors may accept additional values for _query. For example, a MySQL Visitor
122 may choose to accept 'replace'. If a _query value is unrecognized by the
123 Visitor, the Visitor is expected to throw an error.
125 All Visitors are expected to handle the following values for _query:
131 This is a SELECT statement.
135 This is an INSERT statement.
139 This is an UPDATE statement.
143 This is a DELETE statement.
149 This denotes the version of the AST. Different versions will indicate different
150 capabilities provided. Visitors will choose to respect the _version as needed
153 =head2 Structural units
155 All structural units will be hashes. These hashes will have, at minimum, the
162 This indicates the structural unit that this hash is representing. While this
163 specification provides for standard structural units, different Visitors may
164 choose to accept additional units as desired. If a Visitor encounters a unit it
165 doesn't know how to handle, it is expected to throw an exception.
169 Structural units in the AST are supported by loaded components. L<SQL::Abstract>
170 provides for the following structural units by default:
174 This is a (potentially) fully canonicalized identifier for a elemnt in the
175 query. This element could be a schema, table, or column. The Visitor will
176 determine validity within the context of that SQL dialect. The AST is only
177 responsible for validating that the elements are non-empty Strings.
179 The hash will be structured as follows:
182 _name => 'Identifier',
186 Visitors are expected to, by default, quote all identifiers according to the SQL
187 dialect's quoting scheme.
191 A Value is a Perl scalar. It may either be a:
197 A String is a quoted series of characters
201 A Number is an unquoted number in some numeric format
205 Null is SQL's NULL and corresponds to Perl's C<undef>.
207 =item * BindParameter
209 This corresponds to a value that will be passed in. This value is normally
210 quoted in such a fashion so as to protect against SQL injection attacks. (q.v.
211 L<DBI/quote()> for an example.)
215 The hash will be structured as follows:
219 _subtype => [ 'String' | 'Number' | 'Null' | 'BindParameter' ]
223 The provided subtypes are the ones that all Visitors are expected to support.
224 Visitors may choose to support additional subtypes. Visitors are expected to
225 throw an exception upon encountering an unknown subtype.
229 A Function is anything of the form C< name( arglist ) > where C<name> is a
230 string and C<arglist> is a comma-separated list of Expressions.
232 Yes, a Subquery is legal as an argument for many functions. Some example
247 Functions have a cardinality, or expected number of arguments. Some functions,
248 such as MAX(), have a cardinality of 1. Others, such as IF(), have a cardinality
249 of N, meaning they can have any number of arguments greater than 0. Others, such
250 as NOW(), have a cardinality of 0. Several functions with the same meaning may
251 have a different cardinality in different SQL dialects as different engines may
252 allow different behaviors.
254 As cardinality may differ between dialects, enforcing cardinality is necessarily
259 A Subquery is another AST whose _query metadata parameter is set to "SELECT".
261 Most places that a Subquery can be used would require a single value to be
262 returned (single column, single row), but that is not something that the AST can
263 easily enforce. The single-column restriction may possibly be enforced, but the
264 single-row restriction is much more difficult and, in most cases, probably
267 Subqueries, when expressed in SQL, must bounded by parentheses.
269 =head3 Unary Operator
271 A UnaryOperator takes a single argument on the RHS. The argument for a
272 UnaryOperator is an Expression.
274 Visitors are expected to support, at minimum, the following operators:
288 The hash for a UnaryOperator is as follows:
291 _name => 'UnaryOperator'
292 _operator => [ .... ],
293 argument1 => Expression,
296 Visitors may choose to support additional operators. Visitors are expected to
297 throw an exception upon encountering an unknown operator.
299 =head3 BinaryOperator
301 A BinaryOperator takes two arguments (one on the LHS and one on the RHS). The
302 arguments for a BinaryOperator are all Expressions.
304 Visitors are expected to support, at minimum, the following operators:
332 (Note that an operator can comprise of what would be multiple tokens in a normal
335 Visitors may choose to support additional operators. Visitors are expected to
336 throw an exception upon encountering an unknown operator.
338 The hash for a BinaryOperator is as follows:
341 _name => 'BinaryOperator'
342 _operator => [ .... ],
343 argument1 => Expression,
344 argument2 => Expression,
347 =head3 TrinaryOperator
349 A TrinaryOperator takes three arguments. It generally is composed of two
350 elements with one argument to the LHS, one to the RHS, and a third in the middle
351 of the elements. The arguments for a TrinaryOperator are all Expressions.
353 Visitors are expected to support, at minimum, the following operators:
357 =item * X BETWEEN Y AND Z
361 Visitors may choose to support additional operators. Visitors are expected to
362 throw an exception upon encountering an unknown operator.
364 The hash for a TrinaryOperator is as follows:
367 _name => 'TrinaryOperator'
368 _operator => [ .... ],
369 argument1 => Expression,
370 argument2 => Expression,
371 argument3 => Expression,
376 An expression can be any one of the following:
386 =item * UnaryOperator
388 =item * BinaryOperator
390 =item * TrinaryOperator
392 =item * ( Expression )
396 Parentheses indicate precedence and, in some situations, are necessary for
399 The hash for an Expression is as follows:
402 _name => 'Expression',
403 _subtype => [ 'Value' | 'Function' | 'SubQuery' | . . . ],
408 These are all the legal and acceptable clauses within the AST that would
409 correpsond to clauses in a SQL statement. Not all clauses are legal within a
410 given RDBMS engine's SQL dialect and some clauses may be required in one and
411 optional in another. Detecting and enforcing those engine-specific restrictions
412 is the responsibility of the Visitor object.
414 The clauses are defined with a yacc-like syntax. The various parts are:
420 This means "defined" and is used to create a new term to be used below.
424 This means optional and indicates that the items within it are optional.
428 This means optional and repeating as many times as desired.
432 This means alternation. It is a binary operator and indicates that either the
433 left or right hand sides may be used, but not both.
437 This is a grouping construct. It means that all elements within this construct
438 are treated together for the purposes of optional, repeating, alternation, etc.
442 The expected clauses are (name and structure):
446 This corresponds to the SELECT clause of a SELECT statement.
448 A select clause is composed as follows:
450 SelectComponent := Expression [ [ AS ] String ]
453 [ , SelectComponent ]*
457 This is a list of tables that this clause is affecting. It corresponds to the
458 FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in
459 those respective statements. Depending on the _query metadata entry, the
460 appropriate clause name will be used.
462 The tables clause has several RDBMS-specific variations. The AST will support
463 all of them and it is up to the Visitor object constructing the actual SQL to
464 validate and/or use what is provided as appropriate.
466 A table clause is composed as follows:
468 TableIdentifier := Identifier [ [ AS ] String ]
469 JoinType := < LEFT|RIGHT [ OUTER ] > | INNER | CROSS
473 < , TableIdentifier >
475 [ JoinType ] JOIN TableIdentifier
477 < USING ( Identifier [ , Identifier ] ) >
478 | < ON [ ( ] Expression [ , Expression ] [ ) ] >
483 Additionally, where aliases are provided for in the TableIdentifier, those
484 aliases must be used as the tablename in subsequent Identifiers that identify a
485 column of that table.
489 This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.
491 A where clause is composed as follows:
493 WhereOperator := AND | OR
494 WhereExpression := Expression | Expression WhereOperator Expression
500 This corresponds to the SET clause in an INSERT or UPDATE statement.
502 A set clause is composed as follows:
504 SetComponent := Identifier = Expression
506 SetComponent [ , SetComponent ]*
510 This corresponds to the optional list of columns in an INSERT statement.
512 A columns clause is composed as follows:
514 ( Identifier [ , Identifier ]* )
518 This corresponds to the VALUES clause in an INSERT statement.
520 A values clause is composed as follows:
522 ( Expression [ , Expression ]* )
524 If there is a columns clause, the number of entries in the values clause must be
525 equal to the number of entries in the columns clause.
529 This corresponds to the ORDER BY clause in a SELECT statement.
531 An orderby clause is composed as follows:
533 OrderByComponent := XXX-TODO-XXX
534 OrderByDirection := ASC | DESC
536 OrderByComponent [ OrderByDirection ]
537 [ , OrderByComponent [ OrderByDirection ] ]*
541 This corresponds to the GROUP BY clause in a SELECT statement.
543 An groupby clause is composed as follows:
545 GroupByComponent := XXX-TODO-XXX
547 GroupByComponent [ , GroupByComponent ]*
551 This corresponds to the clause that is used in some RDBMS engines to limit the
552 number of rows returned by a query. In MySQL, this would be the LIMIT clause.
554 A rows clause is composed as follows:
560 This corresponds to the clause that is used in some RDBMS engines to indicate
561 what locks are to be taken by this SELECT statement.
563 A for clause is composed as follows:
569 This corresponds to the clause that is used in some RDBMS engines to provide for
570 an adjacency-list query.
572 A connectby clause is composed as follows:
574 Identifier, WhereExpression
578 robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >>
582 You may distribute this code under the same terms as Perl itself.