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 API to the AST will provide ways of expressing common
52 functionality in a common language. The emitters (objects that follow the
53 Visitor pattern) will be responsible for converting that common language into
58 The AST will be a HoA (hash of arrays). The keys to the hash will be the various
59 clauses of a SQL statement, plus some metadata keys. All metadata keys will be
60 identifiable as such by being prefixed with an underscore. All keys will be in
65 These are the additional metadata keys that the AST provides for.
71 This denotes what kind of query this AST should be interpreted as.
77 =head2 Structural units
79 Structural units in the AST are supported by loaded components. L<SQL::Abstract>
80 provides for the following structural units by default:
84 This is a (potentially) fully canonicalized identifier for a table or column. Is
85 is of the structure C< [schema][sep][table][sep]column > or
86 C< [schema][sep]table >.
88 In the case of a two-element identifier which could be C< table[sep]column > or
89 C< schema[sep]table >, context will determine which it is. However, the AST
90 doesn't care which it is, only that it properly parses.
94 A Value is a Perl scalar. It may either be a:
100 A String is a quoted series of characters
104 A Number is an unquoted number in some numeric format
108 NULL corresponds to Perl's C<undef>
110 =item * BindParameter
112 This corresponds to a value that will be passed in. This value is normally
113 quoted in such a fashion so as to protect against SQL injection attacks. (q.v.
114 L<DBI/quote()> for an example.)
122 A Function is anything of the form C< name( arglist ) > where C<name> is a
123 string and C<arglist> is a comma-separated list of Expressions.
125 Yes, a Subquery is legal as an argument for many functions. Some example
142 A Subquery is another AST whose _query metadata parameter is set to "SELECT".
144 Most places that a Subquery can be used would require a single value to be
145 returned (single column, single row), but that is not something that the AST can
146 easily enforce. The single-column restriction can possibly be enforced, but the
147 single-row restriction is much more difficult and, in most cases, probably
150 Subqueries, when expressed in SQL, must bounded by parentheses.
152 =head3 Unary Operator
154 A UnaryOperator takes a single argument on the RHS and is one of the following:
162 =head3 BinaryOperator
164 A BinaryOperator takes two arguments (one on the LHS and one on the RHS) and is
165 one of the following:
183 =item * C<< IS NOT >>
187 Note that an operator can comprise of what would be multiple tokens in a normal
192 An expression can be any one of the following:
202 =item * UnaryOperator Expression
204 =item * Expression BinaryOperator Expression
206 =item * ( Expression )
210 Parentheses indicate precedence and, in some situations, are necessary for
215 These are all the legal and acceptable clauses within the AST that would
216 correpsond to clauses in a SQL statement. Not all clauses are legal within a
217 given RDBMS engine's SQL dialect and some clauses may be required in one and
218 optional in another. Detecting and enforcing those engine-specific restrictions
219 is the responsibility of the Visitor object.
221 The clauses are defined with a yacc-like syntax. The various parts are:
227 This means "defined" and is used to create a new term to be used below.
231 This means optional and indicates that the items within it are optional.
235 This means optional and repeating as many times as desired.
239 This means alternation. It is a binary operator and indicates that either the
240 left or right hand sides may be used, but not both.
244 This is a grouping construct. It means that all elements within this construct
245 are treated together for the purposes of optional, repeating, alternation, etc.
249 The expected clauses are (name and structure):
253 This corresponds to the SELECT clause of a SELECT statement.
255 A select clause is composed as follows:
257 SelectComponent := Expression [ [ AS ] String ]
260 [ , SelectComponent ]*
264 This is a list of tables that this clause is affecting. It corresponds to the
265 FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in
266 those respective statements. Depending on the _query metadata entry, the
267 appropriate clause name will be used.
269 The tables clause has several RDBMS-specific variations. The AST will support
270 all of them and it is up to the Visitor object constructing the actual SQL to
271 validate and/or use what is provided as appropriate.
273 A table clause is composed as follows:
275 TableIdentifier := Identifier [ [ AS ] String ]
276 JoinType := < LEFT|RIGHT [ OUTER ] > | INNER | CROSS
280 < , TableIdentifier >
282 [ JoinType ] JOIN TableIdentifier
284 < USING ( Identifier [ , Identifier ] ) >
285 | < ON [ ( ] Expression [ , Expression ] [ ) ] >
290 Additionally, where aliases are provided for in the TableIdentifier, those
291 aliases must be used as the tablename in subsequent Identifiers that identify a
292 column of that table.
296 This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement.
298 A where clause is composed as follows:
300 WhereOperator := AND | OR
301 WhereExpression := Expression | Expression WhereOperator Expression
307 This corresponds to the SET clause in an INSERT or UPDATE statement.
309 A set clause is composed as follows:
311 SetComponent := Identifier = Expression
313 SetComponent [ , SetComponent ]*
317 This corresponds to the optional list of columns in an INSERT statement.
319 A columns clause is composed as follows:
321 ( Identifier [ , Identifier ]* )
325 This corresponds to the VALUES clause in an INSERT statement.
327 A values clause is composed as follows:
329 ( Expression [ , Expression ]* )
331 If there is a columns clause, the number of entries in the values clause must be
332 equal to the number of entries in the columns clause.
336 This corresponds to the ORDER BY clause in a SELECT statement.
338 An orderby clause is composed as follows:
340 OrderByComponent := XXX-TODO-XXX
341 OrderByDirection := ASC | DESC
343 OrderByComponent [ OrderByDirection ]
344 [ , OrderByComponent [ OrderByDirection ] ]*
348 This corresponds to the GROUP BY clause in a SELECT statement.
350 An groupby clause is composed as follows:
352 GroupByComponent := XXX-TODO-XXX
354 GroupByComponent [ , GroupByComponent ]*
358 This corresponds to the clause that is used in some RDBMS engines to limit the
359 number of rows returned by a query. In MySQL, this would be the LIMIT clause.
361 A rows clause is composed as follows:
367 This corresponds to the clause that is used in some RDBMS engines to indicate
368 what locks are to be taken by this SELECT statement.
370 A for clause is composed as follows:
376 This corresponds to the clause that is used in some RDBMS engines to provide for
377 an adjacency-list query.
379 A connectby clause is composed as follows:
381 Identifier, WhereExpression
385 robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >>
389 You may distribute this code under the same terms as Perl itself.