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1 | =head1 NAME |
2 | |
3 | SQL::Abstract::Manual::Specification |
4 | |
5 | =head1 SYNOPSIS |
6 | |
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. |
11 | |
12 | =head1 MOTIVATIONS |
13 | |
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. |
20 | |
21 | =head1 GOALS |
22 | |
23 | The goals for this AST are as follows: |
24 | |
25 | =head2 SQL-specific semantics |
26 | |
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 |
30 | as placeholders. |
31 | |
32 | =head2 Perl-specific semantics |
33 | |
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. |
37 | |
38 | =head2 Whole-lifecycle management |
39 | |
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. |
45 | |
46 | =head2 Dialect-agnostic usage |
47 | |
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 |
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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. |
54 | |
ad0f8fa6 |
55 | =head1 RESTRICTIONS |
56 | |
57 | The following are the restrictions upon the AST: |
58 | |
59 | =head2 DML-only |
60 | |
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: |
64 | |
65 | =over 4 |
66 | |
67 | =item * SELECT |
68 | |
69 | =item * INSERT INTO |
70 | |
71 | =item * UPDATE |
72 | |
73 | =item * DELETE |
74 | |
75 | =back |
76 | |
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. |
80 | |
804bd4ab |
81 | =head2 Dialect-agnostic construction |
82 | |
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 |
cca4daf5 |
85 | which use functions for a given action, the AST will provide simple units. It is |
86 | the responsibility of the Visitor to provide the appropriate SQL. Furthermore, |
87 | the AST will be very generic and only provide hints for a subset of SQL. If a |
88 | Visitor is sufficiently intelligent, pretty SQL may be emitted, but that is not |
89 | the goal of this AST. |
804bd4ab |
90 | |
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91 | =head1 COMPONENTS |
92 | |
93 | There are two major components to SQL::Abstract v2. |
94 | |
95 | =over 4 |
96 | |
97 | =item * AST |
98 | |
99 | This is the Abstract Syntax Tree. It is a data structure that represents |
100 | everything necessary to construct the SQL statement in whatever dialect the |
101 | user requires. |
102 | |
103 | =item * Visitor |
104 | |
105 | This object conforms to the Visitor pattern and is used to generate the SQL |
106 | represented by the AST. Each dialect will have a different Visitor object. In |
107 | addition, there will be visitors for at least one of the ANSI specifications. |
108 | |
109 | =back |
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110 | |
df35a525 |
111 | The division of duties between the two components will focus on what the AST |
112 | can and cannot assume. For example, identifiers do not have 20 components in |
113 | any dialect, so the AST can validate that. However, determining what |
114 | constitutes a legal identifier can only be determined by the Visitor object |
115 | enforcing that dialect's rules. |
116 | |
d6e108eb |
117 | =head1 AST STRUCTURE |
118 | |
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119 | The AST will be a HoHo..oH (hash of hash of ... of hashes). The keys to the |
120 | outermost hash will be the various clauses of a SQL statement, plus some |
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121 | metadata keys. |
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122 | |
123 | =head2 Metadata keys |
124 | |
125 | These are the additional metadata keys that the AST provides for. |
126 | |
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127 | =head3 type |
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128 | |
129 | This denotes what kind of query this AST should be interpreted as. Different |
37f2cc3f |
130 | Visitors may accept additional values for type. For example, a MySQL Visitor |
131 | may choose to accept 'replace' for REPLACE INTO. If a type value is |
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132 | unrecognized by the Visitor, the Visitor is expected to throw an error. |
df35a525 |
133 | |
37f2cc3f |
134 | All Visitors are expected to handle the following values for type: |
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135 | |
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136 | =over 4 |
137 | |
df35a525 |
138 | =item * select |
139 | |
140 | This is a SELECT statement. |
d6e108eb |
141 | |
df35a525 |
142 | =item * insert |
d6e108eb |
143 | |
df35a525 |
144 | This is an INSERT statement. |
393a4eb8 |
145 | |
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146 | =item * update |
147 | |
148 | This is an UPDATE statement. |
149 | |
150 | =item * delete |
151 | |
152 | This is a DELETE statement. |
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153 | |
154 | =back |
155 | |
37f2cc3f |
156 | =head3 ast_version |
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157 | |
158 | This denotes the version of the AST. Different versions will indicate different |
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159 | capabilities provided. Visitors will choose to respect the ast_version as needed |
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160 | and desired. |
161 | |
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162 | =head2 Structural units |
163 | |
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164 | All structural units will be hashes. These hashes will have, at minimum, the |
165 | following keys: |
166 | |
167 | =over 4 |
168 | |
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169 | =item * type |
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170 | |
171 | This indicates the structural unit that this hash is representing. While this |
172 | specification provides for standard structural units, different Visitors may |
173 | choose to accept additional units as desired. If a Visitor encounters a unit it |
174 | doesn't know how to handle, it is expected to throw an exception. |
175 | |
176 | =back |
177 | |
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178 | Structural units in the AST are supported by loaded components. L<SQL::Abstract> |
179 | provides for the following structural units by default: |
180 | |
181 | =head3 Identifier |
182 | |
df35a525 |
183 | This is a (potentially) fully canonicalized identifier for a elemnt in the |
184 | query. This element could be a schema, table, or column. The Visitor will |
185 | determine validity within the context of that SQL dialect. The AST is only |
186 | responsible for validating that the elements are non-empty Strings. |
187 | |
188 | The hash will be structured as follows: |
189 | |
190 | { |
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191 | type => 'Identifier', |
7c66a0ab |
192 | element1 => Scalar, |
193 | element2 => Scalar, |
194 | element3 => Scalar, |
df35a525 |
195 | } |
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196 | |
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197 | If element3 exists, then element2 must exist. element1 must always exist. If a |
198 | given element exists, then it must be defined and of non-zero length. |
199 | |
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200 | Visitors are expected to, by default, quote all identifiers according to the SQL |
201 | dialect's quoting scheme. |
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202 | |
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203 | Any of the elements may be '*', as in SELECT * or SELECT COUNT(*). Visitors must |
204 | be careful to I<not> quote asterisks. |
205 | |
10000e9e |
206 | =head3 Value |
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207 | |
da93022e |
208 | A Value is a Perl scalar. Depending on the subtype, a Visitor may be able to |
209 | make certain decisions. The following are the minimally-valid subtypes: |
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210 | |
211 | =over 4 |
212 | |
213 | =item * String |
214 | |
7c66a0ab |
215 | A String is a quoted series of characters. The Visitor is expected to ensure |
216 | that embedded quotes are properly handled per the SQL dialect's quoting scheme. |
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217 | |
218 | =item * Number |
219 | |
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220 | A Number is an unquoted number in some numeric format. |
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221 | |
ad0f8fa6 |
222 | =item * Null |
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223 | |
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224 | Null is SQL's NULL and corresponds to Perl's C<undef>. |
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225 | |
226 | =item * BindParameter |
227 | |
228 | This corresponds to a value that will be passed in. This value is normally |
229 | quoted in such a fashion so as to protect against SQL injection attacks. (q.v. |
230 | L<DBI/quote()> for an example.) |
231 | |
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232 | BindParameters are normally represented by a '?'. |
233 | |
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234 | =back |
235 | |
a3872878 |
236 | The hash will be structured as follows: |
237 | |
238 | { |
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239 | type => 'Value' |
7c66a0ab |
240 | subtype => [ 'String' | 'Number' | 'Null' | 'BindParameter' ] |
241 | value => Scalar |
a3872878 |
242 | } |
243 | |
244 | The provided subtypes are the ones that all Visitors are expected to support. |
245 | Visitors may choose to support additional subtypes. Visitors are expected to |
246 | throw an exception upon encountering an unknown subtype. |
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247 | |
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248 | =head3 Operator |
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249 | |
804bd4ab |
250 | An Operator would be, in SQL dialect terms, a unary operator, a binary operator, |
251 | a trinary operator, or a function. Since different dialects may have a given |
252 | functionality as an operator or a function (such as CONCAT in MySQl vs. || in |
253 | Oracle for string concatenation), they will be represented in the AST as generic |
254 | operators. |
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255 | |
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256 | The hash will be structured as follows: |
257 | |
258 | { |
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259 | type => 'Operator', |
260 | op => String, |
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261 | args => [ |
262 | Expression, |
263 | ], |
7c66a0ab |
264 | } |
265 | |
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266 | Operators have a cardinality, or expected number of arguments. Some operators, |
ad0f8fa6 |
267 | such as MAX(), have a cardinality of 1. Others, such as IF(), have a cardinality |
268 | of N, meaning they can have any number of arguments greater than 0. Others, such |
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269 | as NOW(), have a cardinality of 0. Several operators with the same meaning may |
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270 | have a different cardinality in different SQL dialects as different engines may |
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271 | allow different behaviors. As cardinality may differ between dialects, enforcing |
272 | cardinality is necessarily left to the Visitor. |
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273 | |
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274 | Operators also have restrictions on the types of arguments they will accept. The |
275 | first argument may or may not restricted in the same fashion as the other |
276 | arguments. As with cardinality, this restriction will need to be managed by the |
277 | Visitor. |
278 | |
279 | The operator name needs to take into account the possibility that the RDBMS may |
280 | allow UDFs (User-Defined Functions) that have the same name as an operator, such |
281 | as 'AND'. This will have to be managed by the Visitor. |
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282 | |
d6e108eb |
283 | =head3 Subquery |
284 | |
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285 | A Subquery is another AST whose type metadata parameter is set to "SELECT". |
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286 | |
287 | Most places that a Subquery can be used would require a single value to be |
288 | returned (single column, single row), but that is not something that the AST can |
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289 | easily enforce. The single-column restriction may possibly be enforced, but the |
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290 | single-row restriction is much more difficult and, in most cases, probably |
291 | impossible. |
292 | |
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293 | Subqueries, when expressed in SQL, must be bounded by parentheses. |
81cd86f1 |
294 | |
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295 | =head3 Expression |
296 | |
7c66a0ab |
297 | An Expression can be any one of the following: |
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298 | |
299 | =over 4 |
300 | |
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301 | =item * Identifier |
302 | |
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303 | =item * Value |
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304 | |
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305 | =item * Operator |
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306 | |
307 | =item * Subquery |
308 | |
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309 | =back |
310 | |
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311 | An Expression is a meta-syntactic unit. An "Expression" unit will never appear |
312 | within the AST. It acts as a junction. |
313 | |
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314 | =head3 Nesting |
315 | |
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316 | There is no specific operator or nodetype for nesting. Instead, nesting is |
317 | explicitly specified by node descent in the AST. |
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318 | |
d6e108eb |
319 | =head2 SQL clauses |
320 | |
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321 | These are all the legal and acceptable clauses within the AST that would |
322 | correpsond to clauses in a SQL statement. Not all clauses are legal within a |
323 | given RDBMS engine's SQL dialect and some clauses may be required in one and |
324 | optional in another. Detecting and enforcing those engine-specific restrictions |
325 | is the responsibility of the Visitor object. |
326 | |
327 | The clauses are defined with a yacc-like syntax. The various parts are: |
328 | |
329 | =over 4 |
330 | |
331 | =item * := |
332 | |
333 | This means "defined" and is used to create a new term to be used below. |
334 | |
335 | =item * [] |
336 | |
337 | This means optional and indicates that the items within it are optional. |
338 | |
339 | =item * []* |
340 | |
341 | This means optional and repeating as many times as desired. |
342 | |
343 | =item * | |
344 | |
345 | This means alternation. It is a binary operator and indicates that either the |
346 | left or right hand sides may be used, but not both. |
347 | |
348 | =item * C<< <> >> |
349 | |
350 | This is a grouping construct. It means that all elements within this construct |
351 | are treated together for the purposes of optional, repeating, alternation, etc. |
352 | |
353 | =back |
354 | |
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355 | The expected clauses are (name and structure): |
356 | |
357 | =head3 select |
358 | |
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359 | This corresponds to the SELECT clause of a SELECT statement. |
360 | |
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361 | A select clause unit is an array of one or more SelectComponent units. |
81cd86f1 |
362 | |
7c66a0ab |
363 | The hash for a SelectComponent unit is composed as follows: |
81cd86f1 |
364 | |
7c66a0ab |
365 | { |
804bd4ab |
366 | type => 'SelectComponent', |
7c66a0ab |
367 | value => Expression, |
cca4daf5 |
368 | as => String, |
7c66a0ab |
369 | } |
370 | |
371 | The 'as' component is optional. Visitors may choose to make it required in |
372 | certain situations. |
d6e108eb |
373 | |
374 | =head3 tables |
375 | |
376 | This is a list of tables that this clause is affecting. It corresponds to the |
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377 | FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in |
37f2cc3f |
378 | those respective statements. Depending on the type metadata entry, the |
81cd86f1 |
379 | appropriate clause name will be used. |
d6e108eb |
380 | |
381 | The tables clause has several RDBMS-specific variations. The AST will support |
382 | all of them and it is up to the Visitor object constructing the actual SQL to |
383 | validate and/or use what is provided as appropriate. |
384 | |
cca4daf5 |
385 | A TableJoin is a junction of the following elements: |
7c66a0ab |
386 | |
cca4daf5 |
387 | =over 4 |
7c66a0ab |
388 | |
cca4daf5 |
389 | =item * TableIdentifier |
390 | |
391 | =item * Operator |
392 | |
393 | =back |
7c66a0ab |
394 | |
395 | The hash for a TableIdentifier will be composed as follows: |
d6e108eb |
396 | |
7c66a0ab |
397 | # TableIdentifier |
398 | { |
804bd4ab |
399 | type => 'TableIdentifier', |
cca4daf5 |
400 | value => Expression, |
401 | as => String, |
7c66a0ab |
402 | } |
403 | |
cca4daf5 |
404 | The value should be either an Identifier or a SubQuery. |
7c66a0ab |
405 | |
cca4daf5 |
406 | The hash for an Operator within a tables clause will be composed as follows: |
407 | |
408 | # Operator |
7c66a0ab |
409 | { |
cca4daf5 |
410 | type => 'Operator', |
411 | op => '< LEFT|RIGHT|FULL [ OUTER ] > | INNER | CROSS', |
412 | on => Expression, |
7c66a0ab |
413 | } |
d6e108eb |
414 | |
cca4daf5 |
415 | A USING clause is syntactic sugar for an ON clause and, as such, is not provided |
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416 | for by the AST. A join of a comma is identical to a CROSS JOIN and, as such, is |
417 | not provided for by the AST. The on clause is optional. |
d6e108eb |
418 | |
419 | =head3 where |
420 | |
81cd86f1 |
421 | This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement. |
422 | |
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423 | A where clause is composed of an Expression. |
81cd86f1 |
424 | |
d6e108eb |
425 | =head3 set |
426 | |
81cd86f1 |
427 | This corresponds to the SET clause in an INSERT or UPDATE statement. |
428 | |
753e226d |
429 | A set clause unit is an array of one or more SetComponent units. |
81cd86f1 |
430 | |
753e226d |
431 | The hash for SetComponent unit is composed as follows: |
81cd86f1 |
432 | |
753e226d |
433 | { |
434 | type => 'SetComponent', |
435 | col => Identifier, |
436 | value => Expression, |
437 | } |
81cd86f1 |
438 | |
439 | =head3 columns |
440 | |
441 | This corresponds to the optional list of columns in an INSERT statement. |
442 | |
338df86b |
443 | A columns clause unit is an array of one or more Identifier units. |
81cd86f1 |
444 | |
d6e108eb |
445 | =head3 values |
446 | |
81cd86f1 |
447 | This corresponds to the VALUES clause in an INSERT statement. |
448 | |
338df86b |
449 | A values clause unit is an array of one or more Expression units. |
81cd86f1 |
450 | |
451 | If there is a columns clause, the number of entries in the values clause must be |
452 | equal to the number of entries in the columns clause. |
453 | |
d6e108eb |
454 | =head3 orderby |
455 | |
81cd86f1 |
456 | This corresponds to the ORDER BY clause in a SELECT statement. |
457 | |
da74c1c8 |
458 | A orderby clause unit is an array of one or more OrderbyComponent units. |
81cd86f1 |
459 | |
da74c1c8 |
460 | The hash for a OrderbyComponent unit is composed as follows: |
81cd86f1 |
461 | |
da74c1c8 |
462 | { |
463 | type => 'OrderbyComponent', |
464 | value => < Identifier | Number > |
465 | dir => '< ASC | DESC >', |
466 | } |
467 | |
468 | The dir element, if omitted, will be defaulted to ASC by the AST. The number |
469 | corresponds to a column in the select clause. |
81cd86f1 |
470 | |
d6e108eb |
471 | =head3 groupby |
472 | |
81cd86f1 |
473 | This corresponds to the GROUP BY clause in a SELECT statement. |
474 | |
da74c1c8 |
475 | A groupby clause unit is an array of one or more GroupbyComponent units. |
81cd86f1 |
476 | |
da74c1c8 |
477 | The hash for a GroupbyComponent unit is composed as follows: |
478 | |
479 | { |
480 | type => 'GroupbyComponent', |
481 | value => < Identifier | Number > |
482 | } |
81cd86f1 |
483 | |
da74c1c8 |
484 | The number corresponds to a column in the select clause. |
81cd86f1 |
485 | |
d6e108eb |
486 | =head3 rows |
487 | |
81cd86f1 |
488 | This corresponds to the clause that is used in some RDBMS engines to limit the |
489 | number of rows returned by a query. In MySQL, this would be the LIMIT clause. |
490 | |
e4a310cb |
491 | The hash for a rows clause is composed as follows: |
81cd86f1 |
492 | |
e4a310cb |
493 | { |
e4a310cb |
494 | start => Number, |
495 | count => Number, |
496 | } |
497 | |
498 | The start attribute, if ommitted, will default to 0. The count attribute is |
499 | optional. |
81cd86f1 |
500 | |
d6e108eb |
501 | =head3 for |
502 | |
81cd86f1 |
503 | This corresponds to the clause that is used in some RDBMS engines to indicate |
504 | what locks are to be taken by this SELECT statement. |
505 | |
e4a310cb |
506 | The hash for a for clause is composed as follows: |
81cd86f1 |
507 | |
e4a310cb |
508 | { |
509 | value => '< UPDATE | DELETE >', |
510 | } |
81cd86f1 |
511 | |
512 | =head3 connectby |
513 | |
514 | This corresponds to the clause that is used in some RDBMS engines to provide for |
515 | an adjacency-list query. |
516 | |
22033e85 |
517 | The hash for a for clause is composed as follows: |
518 | |
519 | { |
f32d60b9 |
520 | start_with => [ |
521 | Expression, |
522 | ], |
22033e85 |
523 | connect_by => { |
524 | option => '< PRIOR | NOCYCLE >' |
f32d60b9 |
525 | cond => [ |
526 | Expression, |
527 | ], |
22033e85 |
528 | }, |
529 | order_siblings => orderby-clause, |
530 | } |
81cd86f1 |
531 | |
22033e85 |
532 | Both the start_with and order_siblings clauses are optional. |
81cd86f1 |
533 | |
cca4daf5 |
534 | =head1 TODO |
535 | |
536 | =over 4 |
537 | |
538 | =item * sproc unit |
539 | |
540 | =back |
541 | |
d6e108eb |
542 | =head1 AUTHORS |
543 | |
81cd86f1 |
544 | robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >> |
d6e108eb |
545 | |
546 | =head1 LICENSE |
547 | |
548 | You may distribute this code under the same terms as Perl itself. |
549 | |
550 | =cut |