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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 |
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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 |
7c66a0ab |
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. |
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141 | |
df35a525 |
142 | =item * insert |
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143 | |
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144 | This is an INSERT statement. |
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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 | |
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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', |
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192 | element1 => Scalar, |
193 | element2 => Scalar, |
194 | element3 => Scalar, |
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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 | |
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206 | =head3 Value |
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207 | |
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208 | A Value is a Perl scalar. Depending on the type, a Visitor may be able to make |
209 | certain decisions. |
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210 | |
211 | =over 4 |
212 | |
213 | =item * String |
214 | |
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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 | |
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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 |
81cd86f1 |
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, |
261 | args => ExpressionList, |
7c66a0ab |
262 | } |
263 | |
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264 | Operators have a cardinality, or expected number of arguments. Some operators, |
ad0f8fa6 |
265 | such as MAX(), have a cardinality of 1. Others, such as IF(), have a cardinality |
266 | of N, meaning they can have any number of arguments greater than 0. Others, such |
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267 | as NOW(), have a cardinality of 0. Several operators with the same meaning may |
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268 | have a different cardinality in different SQL dialects as different engines may |
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269 | allow different behaviors. As cardinality may differ between dialects, enforcing |
270 | cardinality is necessarily left to the Visitor. |
ad0f8fa6 |
271 | |
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272 | Operators also have restrictions on the types of arguments they will accept. The |
273 | first argument may or may not restricted in the same fashion as the other |
274 | arguments. As with cardinality, this restriction will need to be managed by the |
275 | Visitor. |
276 | |
277 | The operator name needs to take into account the possibility that the RDBMS may |
278 | allow UDFs (User-Defined Functions) that have the same name as an operator, such |
279 | as 'AND'. This will have to be managed by the Visitor. |
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280 | |
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281 | =head3 Subquery |
282 | |
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283 | A Subquery is another AST whose type metadata parameter is set to "SELECT". |
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284 | |
285 | Most places that a Subquery can be used would require a single value to be |
286 | returned (single column, single row), but that is not something that the AST can |
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287 | easily enforce. The single-column restriction may possibly be enforced, but the |
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288 | single-row restriction is much more difficult and, in most cases, probably |
289 | impossible. |
290 | |
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291 | Subqueries, when expressed in SQL, must be bounded by parentheses. |
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292 | |
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293 | =head3 Expression |
294 | |
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295 | An Expression can be any one of the following: |
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296 | |
297 | =over 4 |
298 | |
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299 | =item * Identifier |
300 | |
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301 | =item * Value |
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302 | |
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303 | =item * Operator |
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304 | |
305 | =item * Subquery |
306 | |
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307 | =back |
308 | |
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309 | An Expression is a meta-syntactic unit. An "Expression" unit will never appear |
310 | within the AST. It acts as a junction. |
311 | |
312 | =head3 ExpressionList |
313 | |
314 | An ExpressionList is a list of Expressions, generally separated by commas |
315 | (though other separators may be appropriate at times or for different SQL |
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316 | dialects). An null separator may also be used. |
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317 | |
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318 | The hash for an ExpressionList is as follows: |
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319 | |
320 | { |
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321 | type => 'ExpressionList', |
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322 | separator => ',', |
323 | elements => Array of Expressions, |
ad0f8fa6 |
324 | } |
325 | |
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326 | An ExpressionList is always rendered in SQL with parentheses around it. |
327 | |
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328 | =head3 Nesting |
329 | |
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330 | There is no specific operator or nodetype for nesting. Instead, nesting is |
331 | explicitly specified by node descent in the AST. |
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332 | |
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333 | =head2 SQL clauses |
334 | |
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335 | These are all the legal and acceptable clauses within the AST that would |
336 | correpsond to clauses in a SQL statement. Not all clauses are legal within a |
337 | given RDBMS engine's SQL dialect and some clauses may be required in one and |
338 | optional in another. Detecting and enforcing those engine-specific restrictions |
339 | is the responsibility of the Visitor object. |
340 | |
341 | The clauses are defined with a yacc-like syntax. The various parts are: |
342 | |
343 | =over 4 |
344 | |
345 | =item * := |
346 | |
347 | This means "defined" and is used to create a new term to be used below. |
348 | |
349 | =item * [] |
350 | |
351 | This means optional and indicates that the items within it are optional. |
352 | |
353 | =item * []* |
354 | |
355 | This means optional and repeating as many times as desired. |
356 | |
357 | =item * | |
358 | |
359 | This means alternation. It is a binary operator and indicates that either the |
360 | left or right hand sides may be used, but not both. |
361 | |
362 | =item * C<< <> >> |
363 | |
364 | This is a grouping construct. It means that all elements within this construct |
365 | are treated together for the purposes of optional, repeating, alternation, etc. |
366 | |
367 | =back |
368 | |
d6e108eb |
369 | The expected clauses are (name and structure): |
370 | |
371 | =head3 select |
372 | |
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373 | This corresponds to the SELECT clause of a SELECT statement. |
374 | |
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375 | A select clause unit is an array of one or more SelectComponent units. |
81cd86f1 |
376 | |
7c66a0ab |
377 | The hash for a SelectComponent unit is composed as follows: |
81cd86f1 |
378 | |
7c66a0ab |
379 | { |
804bd4ab |
380 | type => 'SelectComponent', |
7c66a0ab |
381 | value => Expression, |
cca4daf5 |
382 | as => String, |
7c66a0ab |
383 | } |
384 | |
385 | The 'as' component is optional. Visitors may choose to make it required in |
386 | certain situations. |
d6e108eb |
387 | |
388 | =head3 tables |
389 | |
390 | This is a list of tables that this clause is affecting. It corresponds to the |
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391 | FROM clause in a SELECT statement and the INSERT INTO/UPDATE/DELETE clauses in |
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392 | those respective statements. Depending on the type metadata entry, the |
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393 | appropriate clause name will be used. |
d6e108eb |
394 | |
395 | The tables clause has several RDBMS-specific variations. The AST will support |
396 | all of them and it is up to the Visitor object constructing the actual SQL to |
397 | validate and/or use what is provided as appropriate. |
398 | |
cca4daf5 |
399 | A TableJoin is a junction of the following elements: |
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400 | |
cca4daf5 |
401 | =over 4 |
7c66a0ab |
402 | |
cca4daf5 |
403 | =item * TableIdentifier |
404 | |
405 | =item * Operator |
406 | |
407 | =back |
7c66a0ab |
408 | |
409 | The hash for a TableIdentifier will be composed as follows: |
d6e108eb |
410 | |
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411 | # TableIdentifier |
412 | { |
804bd4ab |
413 | type => 'TableIdentifier', |
cca4daf5 |
414 | value => Expression, |
415 | as => String, |
7c66a0ab |
416 | } |
417 | |
cca4daf5 |
418 | The value should be either an Identifier or a SubQuery. |
7c66a0ab |
419 | |
cca4daf5 |
420 | The hash for an Operator within a tables clause will be composed as follows: |
421 | |
422 | # Operator |
7c66a0ab |
423 | { |
cca4daf5 |
424 | type => 'Operator', |
425 | op => '< LEFT|RIGHT|FULL [ OUTER ] > | INNER | CROSS', |
426 | on => Expression, |
7c66a0ab |
427 | } |
d6e108eb |
428 | |
cca4daf5 |
429 | A USING clause is syntactic sugar for an ON clause and, as such, is not provided |
da74c1c8 |
430 | for by the AST. A join of a comma is identical to a CROSS JOIN and, as such, is |
431 | not provided for by the AST. The on clause is optional. |
d6e108eb |
432 | |
433 | =head3 where |
434 | |
81cd86f1 |
435 | This corresponds to the WHERE clause in a SELECT, UPDATE, or DELETE statement. |
436 | |
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437 | A where clause is composed of an Expression. |
81cd86f1 |
438 | |
d6e108eb |
439 | =head3 set |
440 | |
81cd86f1 |
441 | This corresponds to the SET clause in an INSERT or UPDATE statement. |
442 | |
443 | A set clause is composed as follows: |
444 | |
445 | SetComponent := Identifier = Expression |
446 | |
447 | SetComponent [ , SetComponent ]* |
448 | |
449 | =head3 columns |
450 | |
451 | This corresponds to the optional list of columns in an INSERT statement. |
452 | |
7c66a0ab |
453 | A columns clause is an IdentifierList and the unit is composed as follows: |
81cd86f1 |
454 | |
7c66a0ab |
455 | columns => [ |
456 | Identifier, |
457 | [ Identifier, ]* |
458 | ], |
81cd86f1 |
459 | |
d6e108eb |
460 | =head3 values |
461 | |
81cd86f1 |
462 | This corresponds to the VALUES clause in an INSERT statement. |
463 | |
da74c1c8 |
464 | A values clause is an ExpressionList and the unit is composed as follows: |
81cd86f1 |
465 | |
7c66a0ab |
466 | values => [ |
467 | Expression, |
468 | [ Expression, ]* |
469 | ], |
81cd86f1 |
470 | |
471 | If there is a columns clause, the number of entries in the values clause must be |
472 | equal to the number of entries in the columns clause. |
473 | |
d6e108eb |
474 | =head3 orderby |
475 | |
81cd86f1 |
476 | This corresponds to the ORDER BY clause in a SELECT statement. |
477 | |
da74c1c8 |
478 | A orderby clause unit is an array of one or more OrderbyComponent units. |
81cd86f1 |
479 | |
da74c1c8 |
480 | The hash for a OrderbyComponent unit is composed as follows: |
81cd86f1 |
481 | |
da74c1c8 |
482 | { |
483 | type => 'OrderbyComponent', |
484 | value => < Identifier | Number > |
485 | dir => '< ASC | DESC >', |
486 | } |
487 | |
488 | The dir element, if omitted, will be defaulted to ASC by the AST. The number |
489 | corresponds to a column in the select clause. |
81cd86f1 |
490 | |
d6e108eb |
491 | =head3 groupby |
492 | |
81cd86f1 |
493 | This corresponds to the GROUP BY clause in a SELECT statement. |
494 | |
da74c1c8 |
495 | A groupby clause unit is an array of one or more GroupbyComponent units. |
81cd86f1 |
496 | |
da74c1c8 |
497 | The hash for a GroupbyComponent unit is composed as follows: |
498 | |
499 | { |
500 | type => 'GroupbyComponent', |
501 | value => < Identifier | Number > |
502 | } |
81cd86f1 |
503 | |
da74c1c8 |
504 | The number corresponds to a column in the select clause. |
81cd86f1 |
505 | |
d6e108eb |
506 | =head3 rows |
507 | |
81cd86f1 |
508 | This corresponds to the clause that is used in some RDBMS engines to limit the |
509 | number of rows returned by a query. In MySQL, this would be the LIMIT clause. |
510 | |
e4a310cb |
511 | The hash for a rows clause is composed as follows: |
81cd86f1 |
512 | |
e4a310cb |
513 | { |
514 | type => 'Rows', |
515 | start => Number, |
516 | count => Number, |
517 | } |
518 | |
519 | The start attribute, if ommitted, will default to 0. The count attribute is |
520 | optional. |
81cd86f1 |
521 | |
d6e108eb |
522 | =head3 for |
523 | |
81cd86f1 |
524 | This corresponds to the clause that is used in some RDBMS engines to indicate |
525 | what locks are to be taken by this SELECT statement. |
526 | |
e4a310cb |
527 | The hash for a for clause is composed as follows: |
81cd86f1 |
528 | |
e4a310cb |
529 | { |
530 | value => '< UPDATE | DELETE >', |
531 | } |
81cd86f1 |
532 | |
533 | =head3 connectby |
534 | |
535 | This corresponds to the clause that is used in some RDBMS engines to provide for |
536 | an adjacency-list query. |
537 | |
22033e85 |
538 | The hash for a for clause is composed as follows: |
539 | |
540 | { |
541 | start_with => ExpressionList, |
542 | connect_by => { |
543 | option => '< PRIOR | NOCYCLE >' |
544 | cond => ExpressionList, |
545 | }, |
546 | order_siblings => orderby-clause, |
547 | } |
81cd86f1 |
548 | |
22033e85 |
549 | Both the start_with and order_siblings clauses are optional. |
81cd86f1 |
550 | |
cca4daf5 |
551 | =head1 TODO |
552 | |
553 | =over 4 |
554 | |
555 | =item * sproc unit |
556 | |
557 | =back |
558 | |
d6e108eb |
559 | =head1 AUTHORS |
560 | |
81cd86f1 |
561 | robkinyon: Rob Kinyon C<< <rkinyon@cpan.org> >> |
d6e108eb |
562 | |
563 | =head1 LICENSE |
564 | |
565 | You may distribute this code under the same terms as Perl itself. |
566 | |
567 | =cut |