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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 |
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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. |
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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 | |
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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 |
121 | metadata keys. All metadata keys will be identifiable as such by being prefixed |
122 | with an underscore. All keys will be in lowercase. |
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123 | |
124 | =head2 Metadata keys |
125 | |
126 | These are the additional metadata keys that the AST provides for. |
127 | |
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128 | =head3 _query |
129 | |
130 | This denotes what kind of query this AST should be interpreted as. Different |
131 | Visitors may accept additional values for _query. For example, a MySQL Visitor |
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132 | may choose to accept 'replace' for REPLACE INTO. If a _query value is |
133 | unrecognized by the Visitor, the Visitor is expected to throw an error. |
df35a525 |
134 | |
135 | All Visitors are expected to handle the following values for _query: |
136 | |
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137 | =over 4 |
138 | |
df35a525 |
139 | =item * select |
140 | |
141 | This is a SELECT statement. |
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142 | |
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143 | =item * insert |
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144 | |
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145 | This is an INSERT statement. |
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146 | |
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147 | =item * update |
148 | |
149 | This is an UPDATE statement. |
150 | |
151 | =item * delete |
152 | |
153 | This is a DELETE statement. |
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154 | |
155 | =back |
156 | |
df35a525 |
157 | =head3 _version |
158 | |
159 | This denotes the version of the AST. Different versions will indicate different |
160 | capabilities provided. Visitors will choose to respect the _version as needed |
161 | and desired. |
162 | |
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163 | =head2 Structural units |
164 | |
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165 | All structural units will be hashes. These hashes will have, at minimum, the |
166 | following keys: |
167 | |
168 | =over 4 |
169 | |
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170 | =item * type |
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171 | |
172 | This indicates the structural unit that this hash is representing. While this |
173 | specification provides for standard structural units, different Visitors may |
174 | choose to accept additional units as desired. If a Visitor encounters a unit it |
175 | doesn't know how to handle, it is expected to throw an exception. |
176 | |
177 | =back |
178 | |
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179 | Structural units in the AST are supported by loaded components. L<SQL::Abstract> |
180 | provides for the following structural units by default: |
181 | |
182 | =head3 Identifier |
183 | |
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184 | This is a (potentially) fully canonicalized identifier for a elemnt in the |
185 | query. This element could be a schema, table, or column. The Visitor will |
186 | determine validity within the context of that SQL dialect. The AST is only |
187 | responsible for validating that the elements are non-empty Strings. |
188 | |
189 | The hash will be structured as follows: |
190 | |
191 | { |
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192 | type => 'Identifier', |
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193 | element1 => Scalar, |
194 | element2 => Scalar, |
195 | element3 => Scalar, |
df35a525 |
196 | } |
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197 | |
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198 | If element3 exists, then element2 must exist. element1 must always exist. If a |
199 | given element exists, then it must be defined and of non-zero length. |
200 | |
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201 | Visitors are expected to, by default, quote all identifiers according to the SQL |
202 | dialect's quoting scheme. |
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203 | |
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204 | =head3 Value |
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205 | |
7c66a0ab |
206 | A Value is a Perl scalar. Depending on the type, a Visitor may be able to make |
207 | certain decisions. |
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208 | |
209 | =over 4 |
210 | |
211 | =item * String |
212 | |
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213 | A String is a quoted series of characters. The Visitor is expected to ensure |
214 | that embedded quotes are properly handled per the SQL dialect's quoting scheme. |
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215 | |
216 | =item * Number |
217 | |
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218 | A Number is an unquoted number in some numeric format. |
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219 | |
ad0f8fa6 |
220 | =item * Null |
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221 | |
ad0f8fa6 |
222 | Null is SQL's NULL and corresponds to Perl's C<undef>. |
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223 | |
224 | =item * BindParameter |
225 | |
226 | This corresponds to a value that will be passed in. This value is normally |
227 | quoted in such a fashion so as to protect against SQL injection attacks. (q.v. |
228 | L<DBI/quote()> for an example.) |
229 | |
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230 | BindParameters are normally represented by a '?'. |
231 | |
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232 | =back |
233 | |
a3872878 |
234 | The hash will be structured as follows: |
235 | |
236 | { |
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237 | type => 'Value' |
7c66a0ab |
238 | subtype => [ 'String' | 'Number' | 'Null' | 'BindParameter' ] |
239 | value => Scalar |
a3872878 |
240 | } |
241 | |
242 | The provided subtypes are the ones that all Visitors are expected to support. |
243 | Visitors may choose to support additional subtypes. Visitors are expected to |
244 | throw an exception upon encountering an unknown subtype. |
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245 | |
804bd4ab |
246 | =head3 Operator |
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247 | |
804bd4ab |
248 | An Operator would be, in SQL dialect terms, a unary operator, a binary operator, |
249 | a trinary operator, or a function. Since different dialects may have a given |
250 | functionality as an operator or a function (such as CONCAT in MySQl vs. || in |
251 | Oracle for string concatenation), they will be represented in the AST as generic |
252 | operators. |
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253 | |
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254 | The hash will be structured as follows: |
255 | |
256 | { |
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257 | type => 'Operator', |
258 | op => String, |
259 | args => ExpressionList, |
7c66a0ab |
260 | } |
261 | |
804bd4ab |
262 | Operators have a cardinality, or expected number of arguments. Some operators, |
ad0f8fa6 |
263 | such as MAX(), have a cardinality of 1. Others, such as IF(), have a cardinality |
264 | of N, meaning they can have any number of arguments greater than 0. Others, such |
804bd4ab |
265 | as NOW(), have a cardinality of 0. Several operators with the same meaning may |
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266 | have a different cardinality in different SQL dialects as different engines may |
804bd4ab |
267 | allow different behaviors. As cardinality may differ between dialects, enforcing |
268 | cardinality is necessarily left to the Visitor. |
ad0f8fa6 |
269 | |
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270 | Operators also have restrictions on the types of arguments they will accept. The |
271 | first argument may or may not restricted in the same fashion as the other |
272 | arguments. As with cardinality, this restriction will need to be managed by the |
273 | Visitor. |
274 | |
275 | The operator name needs to take into account the possibility that the RDBMS may |
276 | allow UDFs (User-Defined Functions) that have the same name as an operator, such |
277 | as 'AND'. This will have to be managed by the Visitor. |
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278 | |
d6e108eb |
279 | =head3 Subquery |
280 | |
281 | A Subquery is another AST whose _query metadata parameter is set to "SELECT". |
282 | |
283 | Most places that a Subquery can be used would require a single value to be |
284 | returned (single column, single row), but that is not something that the AST can |
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285 | easily enforce. The single-column restriction may possibly be enforced, but the |
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286 | single-row restriction is much more difficult and, in most cases, probably |
287 | impossible. |
288 | |
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289 | Subqueries, when expressed in SQL, must be bounded by parentheses. |
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290 | |
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291 | =head3 Expression |
292 | |
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293 | An Expression can be any one of the following: |
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294 | |
295 | =over 4 |
296 | |
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297 | =item * Identifier |
298 | |
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299 | =item * Value |
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300 | |
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301 | =item * Operator |
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302 | |
303 | =item * Subquery |
304 | |
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305 | =back |
306 | |
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307 | An Expression is a meta-syntactic unit. An "Expression" unit will never appear |
308 | within the AST. It acts as a junction. |
309 | |
310 | =head3 ExpressionList |
311 | |
312 | An ExpressionList is a list of Expressions, generally separated by commas |
313 | (though other separators may be appropriate at times or for different SQL |
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314 | dialects). An null separator may also be used. |
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315 | |
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316 | The hash for an ExpressionList is as follows: |
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317 | |
318 | { |
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319 | type => 'ExpressionList', |
7c66a0ab |
320 | separator => ',', |
321 | elements => Array of Expressions, |
ad0f8fa6 |
322 | } |
323 | |
7c66a0ab |
324 | An ExpressionList is always rendered in SQL with parentheses around it. |
325 | |
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326 | =head2 SQL clauses |
327 | |
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328 | These are all the legal and acceptable clauses within the AST that would |
329 | correpsond to clauses in a SQL statement. Not all clauses are legal within a |
330 | given RDBMS engine's SQL dialect and some clauses may be required in one and |
331 | optional in another. Detecting and enforcing those engine-specific restrictions |
332 | is the responsibility of the Visitor object. |
333 | |
334 | The clauses are defined with a yacc-like syntax. The various parts are: |
335 | |
336 | =over 4 |
337 | |
338 | =item * := |
339 | |
340 | This means "defined" and is used to create a new term to be used below. |
341 | |
342 | =item * [] |
343 | |
344 | This means optional and indicates that the items within it are optional. |
345 | |
346 | =item * []* |
347 | |
348 | This means optional and repeating as many times as desired. |
349 | |
350 | =item * | |
351 | |
352 | This means alternation. It is a binary operator and indicates that either the |
353 | left or right hand sides may be used, but not both. |
354 | |
355 | =item * C<< <> >> |
356 | |
357 | This is a grouping construct. It means that all elements within this construct |
358 | are treated together for the purposes of optional, repeating, alternation, etc. |
359 | |
360 | =back |
361 | |
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362 | The expected clauses are (name and structure): |
363 | |
364 | =head3 select |
365 | |
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366 | This corresponds to the SELECT clause of a SELECT statement. |
367 | |
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368 | A select clause unit is an array of one or more SelectComponent units. |
81cd86f1 |
369 | |
7c66a0ab |
370 | The hash for a SelectComponent unit is composed as follows: |
81cd86f1 |
371 | |
7c66a0ab |
372 | { |
804bd4ab |
373 | type => 'SelectComponent', |
7c66a0ab |
374 | value => Expression, |
cca4daf5 |
375 | as => String, |
7c66a0ab |
376 | } |
377 | |
378 | The 'as' component is optional. Visitors may choose to make it required in |
379 | certain situations. |
d6e108eb |
380 | |
381 | =head3 tables |
382 | |
383 | This is a list of tables that this clause is affecting. It corresponds to the |
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384 | FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in |
385 | those respective statements. Depending on the _query metadata entry, the |
386 | appropriate clause name will be used. |
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387 | |
388 | The tables clause has several RDBMS-specific variations. The AST will support |
389 | all of them and it is up to the Visitor object constructing the actual SQL to |
390 | validate and/or use what is provided as appropriate. |
391 | |
cca4daf5 |
392 | A TableJoin is a junction of the following elements: |
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393 | |
cca4daf5 |
394 | =over 4 |
7c66a0ab |
395 | |
cca4daf5 |
396 | =item * TableIdentifier |
397 | |
398 | =item * Operator |
399 | |
400 | =back |
7c66a0ab |
401 | |
402 | The hash for a TableIdentifier will be composed as follows: |
d6e108eb |
403 | |
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404 | # TableIdentifier |
405 | { |
804bd4ab |
406 | type => 'TableIdentifier', |
cca4daf5 |
407 | value => Expression, |
408 | as => String, |
7c66a0ab |
409 | } |
410 | |
cca4daf5 |
411 | The value should be either an Identifier or a SubQuery. |
7c66a0ab |
412 | |
cca4daf5 |
413 | The hash for an Operator within a tables clause will be composed as follows: |
414 | |
415 | # Operator |
7c66a0ab |
416 | { |
cca4daf5 |
417 | type => 'Operator', |
418 | op => '< LEFT|RIGHT|FULL [ OUTER ] > | INNER | CROSS', |
419 | on => Expression, |
7c66a0ab |
420 | } |
d6e108eb |
421 | |
cca4daf5 |
422 | A USING clause is syntactic sugar for an ON clause and, as such, is not provided |
423 | for by the AST. A join of a comma is identical to a CROSS JOIN. The on clause is |
424 | optional. |
d6e108eb |
425 | |
426 | =head3 where |
427 | |
81cd86f1 |
428 | This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement. |
429 | |
430 | A where clause is composed as follows: |
431 | |
432 | WhereOperator := AND | OR |
433 | WhereExpression := Expression | Expression WhereOperator Expression |
434 | |
435 | WhereExpression |
436 | |
d6e108eb |
437 | =head3 set |
438 | |
81cd86f1 |
439 | This corresponds to the SET clause in an INSERT or UPDATE statement. |
440 | |
441 | A set clause is composed as follows: |
442 | |
443 | SetComponent := Identifier = Expression |
444 | |
445 | SetComponent [ , SetComponent ]* |
446 | |
447 | =head3 columns |
448 | |
449 | This corresponds to the optional list of columns in an INSERT statement. |
450 | |
7c66a0ab |
451 | A columns clause is an IdentifierList and the unit is composed as follows: |
81cd86f1 |
452 | |
7c66a0ab |
453 | columns => [ |
454 | Identifier, |
455 | [ Identifier, ]* |
456 | ], |
81cd86f1 |
457 | |
d6e108eb |
458 | =head3 values |
459 | |
81cd86f1 |
460 | This corresponds to the VALUES clause in an INSERT statement. |
461 | |
7c66a0ab |
462 | A values clause is an ExpressionList and the unit is composed as follows. |
81cd86f1 |
463 | |
7c66a0ab |
464 | values => [ |
465 | Expression, |
466 | [ Expression, ]* |
467 | ], |
81cd86f1 |
468 | |
469 | If there is a columns clause, the number of entries in the values clause must be |
470 | equal to the number of entries in the columns clause. |
471 | |
d6e108eb |
472 | =head3 orderby |
473 | |
81cd86f1 |
474 | This corresponds to the ORDER BY clause in a SELECT statement. |
475 | |
476 | An orderby clause is composed as follows: |
477 | |
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478 | OrderByComponent := XXX-TODO-XXX |
81cd86f1 |
479 | OrderByDirection := ASC | DESC |
480 | |
481 | OrderByComponent [ OrderByDirection ] |
482 | [ , OrderByComponent [ OrderByDirection ] ]* |
483 | |
d6e108eb |
484 | =head3 groupby |
485 | |
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486 | This corresponds to the GROUP BY clause in a SELECT statement. |
487 | |
488 | An groupby clause is composed as follows: |
489 | |
10000e9e |
490 | GroupByComponent := XXX-TODO-XXX |
81cd86f1 |
491 | |
492 | GroupByComponent [ , GroupByComponent ]* |
493 | |
d6e108eb |
494 | =head3 rows |
495 | |
81cd86f1 |
496 | This corresponds to the clause that is used in some RDBMS engines to limit the |
497 | number of rows returned by a query. In MySQL, this would be the LIMIT clause. |
498 | |
499 | A rows clause is composed as follows: |
500 | |
501 | Number [, Number ] |
502 | |
d6e108eb |
503 | =head3 for |
504 | |
81cd86f1 |
505 | This corresponds to the clause that is used in some RDBMS engines to indicate |
506 | what locks are to be taken by this SELECT statement. |
507 | |
508 | A for clause is composed as follows: |
509 | |
510 | UPDATE | DELETE |
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 | |
517 | A connectby clause is composed as follows: |
518 | |
519 | Identifier, WhereExpression |
520 | |
7c66a0ab |
521 | =head1 EXAMPLES |
522 | |
523 | The following are example SQL statements and a possible AST for each one. |
524 | |
525 | =over 4 |
526 | |
527 | =item * SELECT 1 |
528 | |
529 | { |
530 | _query => 'select', |
531 | _ast_version => 0.0001, |
532 | select => [ |
533 | { |
804bd4ab |
534 | type => 'SelectComponent', |
7c66a0ab |
535 | value => { |
804bd4ab |
536 | type => 'Value', |
7c66a0ab |
537 | subtype => 'number', |
538 | value => 1, |
539 | }, |
540 | }, |
541 | ], |
542 | } |
543 | |
544 | =item * SELECT NOW() AS time FROM dual AS duality |
545 | |
546 | { |
547 | _query => 'select', |
548 | _ast_version => 0.0001, |
549 | select => [ |
550 | { |
804bd4ab |
551 | type => 'SelectComponent', |
7c66a0ab |
552 | value => { |
804bd4ab |
553 | type => 'Function', |
7c66a0ab |
554 | function => 'NOW', |
555 | }, |
556 | as => { |
804bd4ab |
557 | type => 'Identifier', |
7c66a0ab |
558 | element1 => 'time', |
559 | }, |
560 | }, |
561 | ], |
cca4daf5 |
562 | tables => { |
563 | type => 'TableIdentifier', |
564 | value => { |
565 | type => 'Identifier', |
566 | element1 => 'dual', |
567 | }, |
568 | as => 'duality', |
569 | }, |
570 | } |
571 | |
572 | =item * SELECT 1 FROM foo LEFT OUTER JOIN bar ON ( foo.col1 = bar.col2 ) |
573 | |
574 | { |
575 | _query => 'select', |
576 | _ast_version => 0.0001, |
577 | select => [ |
7c66a0ab |
578 | { |
cca4daf5 |
579 | type => 'SelectComponent', |
7c66a0ab |
580 | value => { |
cca4daf5 |
581 | type => 'Value', |
582 | subtype => 'number', |
583 | value => 1, |
7c66a0ab |
584 | }, |
cca4daf5 |
585 | }, |
7c66a0ab |
586 | ], |
cca4daf5 |
587 | tables => { |
588 | type => 'Operator', |
589 | op => 'LEFT OUTER', |
590 | args => [ |
591 | { |
592 | type => 'TableIdentifier', |
593 | value => { |
594 | type => 'Identifier', |
595 | element1 => 'foo', |
596 | }, |
597 | }, |
598 | { |
599 | type => 'TableIdentifier', |
600 | value => { |
601 | type => 'Identifier', |
602 | element1 => 'bar', |
603 | }, |
604 | }, |
605 | ], |
606 | on => { |
607 | type => 'Operator', |
608 | op => '=', |
609 | args => [ |
610 | { |
611 | type => 'Identifier', |
612 | element1 => 'foo', |
613 | element2 => 'col1', |
614 | }, |
615 | { |
616 | type => 'Identifier', |
617 | element1 => 'bar', |
618 | element2 => 'col2', |
619 | }, |
620 | ], |
621 | }, |
622 | }, |
7c66a0ab |
623 | } |
624 | |
625 | =back |
626 | |
cca4daf5 |
627 | =head1 TODO |
628 | |
629 | =over 4 |
630 | |
631 | =item * sproc unit |
632 | |
633 | =back |
634 | |
d6e108eb |
635 | =head1 AUTHORS |
636 | |
81cd86f1 |
637 | robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >> |
d6e108eb |
638 | |
639 | =head1 LICENSE |
640 | |
641 | You may distribute this code under the same terms as Perl itself. |
642 | |
643 | =cut |