Cypher on SQL stores: a node given by coalesce() or CASE isn't a node, so MATCH (x)-->(d) from it, or labels(x), is refused (port of drivine4j#440)
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Start in drivine/cypher/sql/sql_cypher_compiler.py, reading the coalesce handling around line 4830, pattern variable handling around lines 1657 and 2441, and labels() around line 4802. Use TCK Match7 #22 and the two proposed queries from the issue to guide the behavior and tests. Done means node-valued coalesce and CASE expressions work as pattern inputs and with labels(), while a null node used as a pattern matches nothing.
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描述
Port of liberation-data/drivine4j#440.
What happens. A node given by coalesce() or CASE is no longer a node in compiled Cypher. A pattern that starts from it, as in TCK Match7 #22's WITH coalesce(b, c) AS x MATCH (x)-->(d) RETURN d, is refused with "x is not a node, and is used as one in a pattern.", and labels(x) with "labels() takes a node.". Neo4j runs both. x.num works.
Where. coalesce always gives JSON, whatever its arguments (drivine/cypher/sql/sql_cypher_compiler.py:4830). A pattern accepts only a node value for a variable in scope (drivine/cypher/sql/sql_cypher_compiler.py:1657, and :2441 for writes). labels() takes only a node (drivine/cypher/sql/sql_cypher_compiler.py:4802). Read at drivine-python origin/main (7e2998e).
The fix. drivine4j has no fix yet. It proposes that a coalesce or CASE of nodes (or null literals) give a node value whose nid, labels, props and stamp are each the COALESCE or CASE of the arguments' columns, so a pattern from a null node matches nothing.
Tests to port. TCK Match7 #22, and from drivine4j#440: MATCH (a:Single) OPTIONAL MATCH (a)-->(c:NonExistent) WITH coalesce(c, a) AS x MATCH (x)-->(y) RETURN y.num and WITH CASE WHEN true THEN b ELSE a END AS x MATCH (x)-->(d) RETURN labels(d).
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