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.
77 The AST will be a HoHo..oH (hash of hash of ... of hashes). The keys to the
78 outermost hash will be the various clauses of a SQL statement, plus some
79 metadata keys. All metadata keys will be identifiable as such by being prefixed
80 with an underscore. All keys will be in lowercase.
84 These are the additional metadata keys that the AST provides for.
90 This denotes what kind of query this AST should be interpreted as.
94 This denotes the version of the AST.
98 =head2 Structural units
100 Structural units in the AST are supported by loaded components. L<SQL::Abstract>
101 provides for the following structural units by default:
105 This is a (potentially) fully canonicalized identifier for a table or column. Is
106 is of the structure C< [schema][sep][table][sep]column > or
107 C< [schema][sep]table >.
109 In the case of a two-element identifier which could be C< table[sep]column > or
110 C< schema[sep]table >, context will determine which it is. However, the AST
111 doesn't care which it is, only that it properly parses.
115 A Value is a Perl scalar. It may either be a:
121 A String is a quoted series of characters
125 A Number is an unquoted number in some numeric format
129 NULL is SQL's NULL and corresponds to Perl's C<undef>.
131 =item * BindParameter
133 This corresponds to a value that will be passed in. This value is normally
134 quoted in such a fashion so as to protect against SQL injection attacks. (q.v.
135 L<DBI/quote()> for an example.)
143 A Function is anything of the form C< name( arglist ) > where C<name> is a
144 string and C<arglist> is a comma-separated list of Expressions.
146 Yes, a Subquery is legal as an argument for many functions. Some example
163 A Subquery is another AST whose _query metadata parameter is set to "SELECT".
165 Most places that a Subquery can be used would require a single value to be
166 returned (single column, single row), but that is not something that the AST can
167 easily enforce. The single-column restriction can possibly be enforced, but the
168 single-row restriction is much more difficult and, in most cases, probably
171 Subqueries, when expressed in SQL, must bounded by parentheses.
173 =head3 Unary Operator
175 A UnaryOperator takes a single argument on the RHS and is one of the following:
183 =head3 BinaryOperator
185 A BinaryOperator takes two arguments (one on the LHS and one on the RHS) and is
186 one of the following:
204 =item * C<< IS NOT >>
208 Note that an operator can comprise of what would be multiple tokens in a normal
213 An expression can be any one of the following:
223 =item * UnaryOperator Expression
225 =item * Expression BinaryOperator Expression
227 =item * ( Expression )
231 Parentheses indicate precedence and, in some situations, are necessary for
236 These are all the legal and acceptable clauses within the AST that would
237 correpsond to clauses in a SQL statement. Not all clauses are legal within a
238 given RDBMS engine's SQL dialect and some clauses may be required in one and
239 optional in another. Detecting and enforcing those engine-specific restrictions
240 is the responsibility of the Visitor object.
242 The clauses are defined with a yacc-like syntax. The various parts are:
248 This means "defined" and is used to create a new term to be used below.
252 This means optional and indicates that the items within it are optional.
256 This means optional and repeating as many times as desired.
260 This means alternation. It is a binary operator and indicates that either the
261 left or right hand sides may be used, but not both.
265 This is a grouping construct. It means that all elements within this construct
266 are treated together for the purposes of optional, repeating, alternation, etc.
270 The expected clauses are (name and structure):
274 This corresponds to the SELECT clause of a SELECT statement.
276 A select clause is composed as follows:
278 SelectComponent := Expression [ [ AS ] String ]
281 [ , SelectComponent ]*
285 This is a list of tables that this clause is affecting. It corresponds to the
286 FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in
287 those respective statements. Depending on the _query metadata entry, the
288 appropriate clause name will be used.
290 The tables clause has several RDBMS-specific variations. The AST will support
291 all of them and it is up to the Visitor object constructing the actual SQL to
292 validate and/or use what is provided as appropriate.
294 A table clause is composed as follows:
296 TableIdentifier := Identifier [ [ AS ] String ]
297 JoinType := < LEFT|RIGHT [ OUTER ] > | INNER | CROSS
301 < , TableIdentifier >
303 [ JoinType ] JOIN TableIdentifier
305 < USING ( Identifier [ , Identifier ] ) >
306 | < ON [ ( ] Expression [ , Expression ] [ ) ] >
311 Additionally, where aliases are provided for in the TableIdentifier, those
312 aliases must be used as the tablename in subsequent Identifiers that identify a
313 column of that table.
317 This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.
319 A where clause is composed as follows:
321 WhereOperator := AND | OR
322 WhereExpression := Expression | Expression WhereOperator Expression
328 This corresponds to the SET clause in an INSERT or UPDATE statement.
330 A set clause is composed as follows:
332 SetComponent := Identifier = Expression
334 SetComponent [ , SetComponent ]*
338 This corresponds to the optional list of columns in an INSERT statement.
340 A columns clause is composed as follows:
342 ( Identifier [ , Identifier ]* )
346 This corresponds to the VALUES clause in an INSERT statement.
348 A values clause is composed as follows:
350 ( Expression [ , Expression ]* )
352 If there is a columns clause, the number of entries in the values clause must be
353 equal to the number of entries in the columns clause.
357 This corresponds to the ORDER BY clause in a SELECT statement.
359 An orderby clause is composed as follows:
361 OrderByComponent := XXX-TODO-XXX
362 OrderByDirection := ASC | DESC
364 OrderByComponent [ OrderByDirection ]
365 [ , OrderByComponent [ OrderByDirection ] ]*
369 This corresponds to the GROUP BY clause in a SELECT statement.
371 An groupby clause is composed as follows:
373 GroupByComponent := XXX-TODO-XXX
375 GroupByComponent [ , GroupByComponent ]*
379 This corresponds to the clause that is used in some RDBMS engines to limit the
380 number of rows returned by a query. In MySQL, this would be the LIMIT clause.
382 A rows clause is composed as follows:
388 This corresponds to the clause that is used in some RDBMS engines to indicate
389 what locks are to be taken by this SELECT statement.
391 A for clause is composed as follows:
397 This corresponds to the clause that is used in some RDBMS engines to provide for
398 an adjacency-list query.
400 A connectby clause is composed as follows:
402 Identifier, WhereExpression
406 robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >>
410 You may distribute this code under the same terms as Perl itself.