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 There are two major components to SQL::Abstract v2.
63 This is the Abstract Syntax Tree. It is a data structure that represents
64 everything necessary to construct the SQL statement in whatever dialect the
69 This object conforms to the Visitor pattern and is used to generate the SQL
70 represented by the AST. Each dialect will have a different Visitor object. In
71 addition, there will be visitors for at least one of the ANSI specifications.
75 The division of duties between the two components will focus on what the AST
76 can and cannot assume. For example, identifiers do not have 20 components in
77 any dialect, so the AST can validate that. However, determining what
78 constitutes a legal identifier can only be determined by the Visitor object
79 enforcing that dialect's rules.
83 The AST will be a HoHo..oH (hash of hash of ... of hashes). The keys to the
84 outermost hash will be the various clauses of a SQL statement, plus some
85 metadata keys. All metadata keys will be identifiable as such by being prefixed
86 with an underscore. All keys will be in lowercase.
90 These are the additional metadata keys that the AST provides for.
94 This denotes what kind of query this AST should be interpreted as. Different
95 Visitors may accept additional values for _query. For example, a MySQL Visitor
96 may choose to accept 'replace'. If a _query value is unrecognized by the
97 Visitor, the Visitor is expected to throw an error.
99 All Visitors are expected to handle the following values for _query:
105 This is a SELECT statement.
109 This is an INSERT statement.
113 This is an UPDATE statement.
117 This is a DELETE statement.
123 This denotes the version of the AST. Different versions will indicate different
124 capabilities provided. Visitors will choose to respect the _version as needed
127 =head2 Structural units
129 All structural units will be hashes. These hashes will have, at minimum, the
136 This indicates the structural unit that this hash is representing. While this
137 specification provides for standard structural units, different Visitors may
138 choose to accept additional units as desired. If a Visitor encounters a unit it
139 doesn't know how to handle, it is expected to throw an exception.
143 Structural units in the AST are supported by loaded components. L<SQL::Abstract>
144 provides for the following structural units by default:
148 This is a (potentially) fully canonicalized identifier for a elemnt in the
149 query. This element could be a schema, table, or column. The Visitor will
150 determine validity within the context of that SQL dialect. The AST is only
151 responsible for validating that the elements are non-empty Strings.
153 The hash will be structured as follows:
156 _name => 'identifier',
160 The items will always be quoted per the SQL dialect's quoting scheme. It is the
161 responsibility of the Visitor to do this.
165 A Value is a Perl scalar. It may either be a:
171 A String is a quoted series of characters
175 A Number is an unquoted number in some numeric format
179 NULL is SQL's NULL and corresponds to Perl's C<undef>.
181 =item * BindParameter
183 This corresponds to a value that will be passed in. This value is normally
184 quoted in such a fashion so as to protect against SQL injection attacks. (q.v.
185 L<DBI/quote()> for an example.)
193 A Function is anything of the form C< name( arglist ) > where C<name> is a
194 string and C<arglist> is a comma-separated list of Expressions.
196 Yes, a Subquery is legal as an argument for many functions. Some example
213 A Subquery is another AST whose _query metadata parameter is set to "SELECT".
215 Most places that a Subquery can be used would require a single value to be
216 returned (single column, single row), but that is not something that the AST can
217 easily enforce. The single-column restriction can possibly be enforced, but the
218 single-row restriction is much more difficult and, in most cases, probably
221 Subqueries, when expressed in SQL, must bounded by parentheses.
223 =head3 Unary Operator
225 A UnaryOperator takes a single argument on the RHS and is one of the following:
233 =head3 BinaryOperator
235 A BinaryOperator takes two arguments (one on the LHS and one on the RHS) and is
236 one of the following:
254 =item * C<< IS NOT >>
258 Note that an operator can comprise of what would be multiple tokens in a normal
263 An expression can be any one of the following:
273 =item * UnaryOperator Expression
275 =item * Expression BinaryOperator Expression
277 =item * ( Expression )
281 Parentheses indicate precedence and, in some situations, are necessary for
286 These are all the legal and acceptable clauses within the AST that would
287 correpsond to clauses in a SQL statement. Not all clauses are legal within a
288 given RDBMS engine's SQL dialect and some clauses may be required in one and
289 optional in another. Detecting and enforcing those engine-specific restrictions
290 is the responsibility of the Visitor object.
292 The clauses are defined with a yacc-like syntax. The various parts are:
298 This means "defined" and is used to create a new term to be used below.
302 This means optional and indicates that the items within it are optional.
306 This means optional and repeating as many times as desired.
310 This means alternation. It is a binary operator and indicates that either the
311 left or right hand sides may be used, but not both.
315 This is a grouping construct. It means that all elements within this construct
316 are treated together for the purposes of optional, repeating, alternation, etc.
320 The expected clauses are (name and structure):
324 This corresponds to the SELECT clause of a SELECT statement.
326 A select clause is composed as follows:
328 SelectComponent := Expression [ [ AS ] String ]
331 [ , SelectComponent ]*
335 This is a list of tables that this clause is affecting. It corresponds to the
336 FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in
337 those respective statements. Depending on the _query metadata entry, the
338 appropriate clause name will be used.
340 The tables clause has several RDBMS-specific variations. The AST will support
341 all of them and it is up to the Visitor object constructing the actual SQL to
342 validate and/or use what is provided as appropriate.
344 A table clause is composed as follows:
346 TableIdentifier := Identifier [ [ AS ] String ]
347 JoinType := < LEFT|RIGHT [ OUTER ] > | INNER | CROSS
351 < , TableIdentifier >
353 [ JoinType ] JOIN TableIdentifier
355 < USING ( Identifier [ , Identifier ] ) >
356 | < ON [ ( ] Expression [ , Expression ] [ ) ] >
361 Additionally, where aliases are provided for in the TableIdentifier, those
362 aliases must be used as the tablename in subsequent Identifiers that identify a
363 column of that table.
367 This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.
369 A where clause is composed as follows:
371 WhereOperator := AND | OR
372 WhereExpression := Expression | Expression WhereOperator Expression
378 This corresponds to the SET clause in an INSERT or UPDATE statement.
380 A set clause is composed as follows:
382 SetComponent := Identifier = Expression
384 SetComponent [ , SetComponent ]*
388 This corresponds to the optional list of columns in an INSERT statement.
390 A columns clause is composed as follows:
392 ( Identifier [ , Identifier ]* )
396 This corresponds to the VALUES clause in an INSERT statement.
398 A values clause is composed as follows:
400 ( Expression [ , Expression ]* )
402 If there is a columns clause, the number of entries in the values clause must be
403 equal to the number of entries in the columns clause.
407 This corresponds to the ORDER BY clause in a SELECT statement.
409 An orderby clause is composed as follows:
411 OrderByComponent := XXX-TODO-XXX
412 OrderByDirection := ASC | DESC
414 OrderByComponent [ OrderByDirection ]
415 [ , OrderByComponent [ OrderByDirection ] ]*
419 This corresponds to the GROUP BY clause in a SELECT statement.
421 An groupby clause is composed as follows:
423 GroupByComponent := XXX-TODO-XXX
425 GroupByComponent [ , GroupByComponent ]*
429 This corresponds to the clause that is used in some RDBMS engines to limit the
430 number of rows returned by a query. In MySQL, this would be the LIMIT clause.
432 A rows clause is composed as follows:
438 This corresponds to the clause that is used in some RDBMS engines to indicate
439 what locks are to be taken by this SELECT statement.
441 A for clause is composed as follows:
447 This corresponds to the clause that is used in some RDBMS engines to provide for
448 an adjacency-list query.
450 A connectby clause is composed as follows:
452 Identifier, WhereExpression
456 robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >>
460 You may distribute this code under the same terms as Perl itself.