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Support pattern expressions in WHERE clause via GLR parser (issue #1577)
Enable bare graph patterns as boolean expressions in WHERE clauses: MATCH (a:Person), (b:Person) WHERE (a)-[:KNOWS]->(b) -- now valid, equivalent to EXISTS(...) RETURN a.name, b.name Previously, this required wrapping in EXISTS(): WHERE EXISTS((a)-[:KNOWS]->(b)) The bare pattern syntax is standard openCypher and is used extensively in Neo4j. Its absence was the most frequently cited migration blocker. Implementation approach: - Switch the Cypher parser from LALR(1) to Bison GLR mode. GLR handles the inherent ambiguity between parenthesized expressions '(' expr ')' and graph path nodes '(' var_name label_opt props ')' by forking the parse stack and discarding the failing path. - Add anonymous_path as an expr_atom alternative with %dprec 1 (lower priority than expression path at %dprec 2). The action wraps the pattern in a cypher_sub_pattern + EXISTS SubLink, reusing the same transform_cypher_sub_pattern() machinery as explicit EXISTS(). - Extract make_exists_pattern_sublink() helper shared by both EXISTS(pattern) and bare pattern rules. - Fix YYLLOC_DEFAULT to use YYRHSLOC() for GLR compatibility. - %dprec annotations on expr_var/var_name_opt resolve the reduce/reduce conflict between expression variables and pattern node variables. Conflict budget: 7 shift/reduce (path extension vs arithmetic on -/<), 3 reduce/reduce (expr_var vs var_name_opt on )/}/=). All are expected and handled correctly by GLR forking + %dprec disambiguation. All 32 regression tests pass (31 existing + 1 new). New pattern_expression test covers: bare patterns, NOT patterns, labeled nodes, AND/OR combinations, left-directed patterns, anonymous nodes, multi-hop patterns, EXISTS() backward compatibility, and non-pattern expression regression checks. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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‎Makefile‎

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@@ -111,6 +111,7 @@ REGRESS = scan \
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name_validation \
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jsonb_operators \
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list_comprehension \
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pattern_expression \
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map_projection \
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direct_field_access \
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security

‎regress/expected/pattern_expression.out‎

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/*
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* Licensed to the Apache Software Foundation (ASF) under one
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* or more contributor license agreements. See the NOTICE file
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* distributed with this work for additional information
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* regarding copyright ownership. The ASF licenses this file
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* to you under the Apache License, Version 2.0 (the
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* "License"); you may not use this file except in compliance
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* with the License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing,
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* software distributed under the License is distributed on an
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* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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* KIND, either express or implied. See the License for the
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* specific language governing permissions and limitations
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* under the License.
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*/
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LOAD 'age';
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SET search_path TO ag_catalog;
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SELECT create_graph('pattern_expr');
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NOTICE: graph "pattern_expr" has been created
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create_graph
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--------------
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(1 row)
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--
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-- Setup test data
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--
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SELECT * FROM cypher('pattern_expr', $$
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CREATE (alice:Person {name: 'Alice'})-[:KNOWS]->(bob:Person {name: 'Bob'}),
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(alice)-[:WORKS_WITH]->(charlie:Person {name: 'Charlie'}),
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(dave:Person {name: 'Dave'})
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$$) AS (result agtype);
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result
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--------
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(0 rows)
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--
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-- Basic pattern expression in WHERE
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--
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-- Bare pattern: (a)-[:REL]->(b)
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a)-[:KNOWS]->(b)
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RETURN a.name, b.name
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ORDER BY a.name, b.name
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$$) AS (a agtype, b agtype);
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a | b
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---------+-------
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"Alice" | "Bob"
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(1 row)
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--
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-- NOT pattern expression
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--
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-- Find people who don't KNOW anyone
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person)
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WHERE NOT (a)-[:KNOWS]->(:Person)
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RETURN a.name
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ORDER BY a.name
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$$) AS (result agtype);
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result
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-----------
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"Bob"
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"Charlie"
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"Dave"
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(3 rows)
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--
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-- Pattern with labeled first node
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a:Person)-[:KNOWS]->(b)
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RETURN a.name, b.name
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ORDER BY a.name
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$$) AS (a agtype, b agtype);
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a | b
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---------+-------
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"Alice" | "Bob"
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(1 row)
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--
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-- Pattern combined with AND
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a)-[:KNOWS]->(b) AND a.name = 'Alice'
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RETURN a.name, b.name
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$$) AS (a agtype, b agtype);
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a | b
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---------+-------
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"Alice" | "Bob"
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(1 row)
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--
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-- Pattern combined with OR
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a)-[:KNOWS]->(b) OR (a)-[:WORKS_WITH]->(b)
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RETURN a.name, b.name
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ORDER BY a.name, b.name
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$$) AS (a agtype, b agtype);
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a | b
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---------+-----------
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"Alice" | "Bob"
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"Alice" | "Charlie"
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(2 rows)
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--
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-- Left-directed pattern
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a)<-[:KNOWS]-(b)
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RETURN a.name, b.name
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ORDER BY a.name
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$$) AS (a agtype, b agtype);
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a | b
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-------+---------
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"Bob" | "Alice"
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(1 row)
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--
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-- Pattern with anonymous nodes
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--
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-- Find anyone who has any outgoing KNOWS relationship
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person)
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WHERE (a)-[:KNOWS]->()
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RETURN a.name
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ORDER BY a.name
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$$) AS (result agtype);
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result
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---------
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"Alice"
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(1 row)
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--
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-- Multiple relationship pattern
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (c:Person)
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WHERE (a)-[:KNOWS]->()-[:WORKS_WITH]->(c)
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RETURN a.name, c.name
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ORDER BY a.name
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$$) AS (a agtype, c agtype);
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a | c
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---+---
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(0 rows)
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--
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-- Existing EXISTS() syntax still works (backward compatibility)
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE EXISTS((a)-[:KNOWS]->(b))
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RETURN a.name, b.name
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ORDER BY a.name
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$$) AS (a agtype, b agtype);
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a | b
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---------+-------
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"Alice" | "Bob"
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(1 row)
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--
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-- Pattern expression produces same results as EXISTS()
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person)
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WHERE (a)-[:KNOWS]->(:Person)
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RETURN a.name
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ORDER BY a.name
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$$) AS (result agtype);
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result
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---------
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"Alice"
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(1 row)
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person)
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WHERE EXISTS((a)-[:KNOWS]->(:Person))
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RETURN a.name
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ORDER BY a.name
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$$) AS (result agtype);
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result
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---------
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"Alice"
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(1 row)
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--
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-- Regular expressions still work (no regression)
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--
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SELECT * FROM cypher('pattern_expr', $$
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RETURN (1 + 2)
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$$) AS (result agtype);
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result
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--------
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3
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(1 row)
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (n:Person)
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WHERE n.name = 'Alice'
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RETURN (n.name)
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$$) AS (result agtype);
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result
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---------
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"Alice"
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(1 row)
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--
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-- Cleanup
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--
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SELECT * FROM drop_graph('pattern_expr', true);
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NOTICE: drop cascades to 5 other objects
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DETAIL: drop cascades to table pattern_expr._ag_label_vertex
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drop cascades to table pattern_expr._ag_label_edge
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drop cascades to table pattern_expr."Person"
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drop cascades to table pattern_expr."KNOWS"
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drop cascades to table pattern_expr."WORKS_WITH"
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NOTICE: graph "pattern_expr" has been dropped
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drop_graph
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------------
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(1 row)
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‎regress/sql/pattern_expression.sql‎

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/*
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* Licensed to the Apache Software Foundation (ASF) under one
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* or more contributor license agreements. See the NOTICE file
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* distributed with this work for additional information
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* regarding copyright ownership. The ASF licenses this file
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* to you under the Apache License, Version 2.0 (the
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* "License"); you may not use this file except in compliance
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* with the License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing,
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* software distributed under the License is distributed on an
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* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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* KIND, either express or implied. See the License for the
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* specific language governing permissions and limitations
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* under the License.
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*/
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LOAD 'age';
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SET search_path TO ag_catalog;
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SELECT create_graph('pattern_expr');
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--
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-- Setup test data
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--
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SELECT * FROM cypher('pattern_expr', $$
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CREATE (alice:Person {name: 'Alice'})-[:KNOWS]->(bob:Person {name: 'Bob'}),
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(alice)-[:WORKS_WITH]->(charlie:Person {name: 'Charlie'}),
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(dave:Person {name: 'Dave'})
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$$) AS (result agtype);
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--
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-- Basic pattern expression in WHERE
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--
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-- Bare pattern: (a)-[:REL]->(b)
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a)-[:KNOWS]->(b)
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RETURN a.name, b.name
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ORDER BY a.name, b.name
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$$) AS (a agtype, b agtype);
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--
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-- NOT pattern expression
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--
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-- Find people who don't KNOW anyone
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person)
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WHERE NOT (a)-[:KNOWS]->(:Person)
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RETURN a.name
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ORDER BY a.name
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$$) AS (result agtype);
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--
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-- Pattern with labeled first node
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a:Person)-[:KNOWS]->(b)
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RETURN a.name, b.name
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ORDER BY a.name
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$$) AS (a agtype, b agtype);
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--
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-- Pattern combined with AND
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a)-[:KNOWS]->(b) AND a.name = 'Alice'
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RETURN a.name, b.name
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$$) AS (a agtype, b agtype);
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--
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-- Pattern combined with OR
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a)-[:KNOWS]->(b) OR (a)-[:WORKS_WITH]->(b)
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RETURN a.name, b.name
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ORDER BY a.name, b.name
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$$) AS (a agtype, b agtype);
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--
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-- Left-directed pattern
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE (a)<-[:KNOWS]-(b)
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RETURN a.name, b.name
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ORDER BY a.name
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$$) AS (a agtype, b agtype);
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--
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-- Pattern with anonymous nodes
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--
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-- Find anyone who has any outgoing KNOWS relationship
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person)
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WHERE (a)-[:KNOWS]->()
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RETURN a.name
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ORDER BY a.name
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$$) AS (result agtype);
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--
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-- Multiple relationship pattern
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (c:Person)
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WHERE (a)-[:KNOWS]->()-[:WORKS_WITH]->(c)
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RETURN a.name, c.name
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ORDER BY a.name
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$$) AS (a agtype, c agtype);
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--
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-- Existing EXISTS() syntax still works (backward compatibility)
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person), (b:Person)
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WHERE EXISTS((a)-[:KNOWS]->(b))
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RETURN a.name, b.name
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ORDER BY a.name
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$$) AS (a agtype, b agtype);
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--
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-- Pattern expression produces same results as EXISTS()
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--
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person)
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WHERE (a)-[:KNOWS]->(:Person)
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RETURN a.name
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ORDER BY a.name
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$$) AS (result agtype);
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (a:Person)
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WHERE EXISTS((a)-[:KNOWS]->(:Person))
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RETURN a.name
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ORDER BY a.name
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$$) AS (result agtype);
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--
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-- Regular expressions still work (no regression)
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--
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SELECT * FROM cypher('pattern_expr', $$
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RETURN (1 + 2)
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$$) AS (result agtype);
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SELECT * FROM cypher('pattern_expr', $$
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MATCH (n:Person)
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WHERE n.name = 'Alice'
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RETURN (n.name)
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$$) AS (result agtype);
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--
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-- Cleanup
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--
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SELECT * FROM drop_graph('pattern_expr', true);

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