Neo4j的暗号查询空收集使整个结果空

Question

我一直面临着奇怪的问题，我试图让从Neo4j的图形，我有built.Here数据是我的查询

MATCH (u1:User {user_id: 4})-[:FOLLOWS]->(u2:User)-[]->(r1:Rest{city_id: 1}) 
WITH COLLECT ({ REST: r1.res_id}) as rows 
MATCH (u1:User {user_id: 4})-[rel]->(r2:Rest{city_id: 1}) 
WHERE NOT (u1:User {user_id: 4})-[rel : BEEN_THERE | ADD_REVIEW]->(r2:Rest{city_id: 1}) 
WITH rows + COLLECT ({ REST: r2.res_id}) AS allrows 
UNWIND allrows as row 
RETURN row.REST as RESTAURANT_ID, count(row.REST) as COUNT 
ORDER BY COUNT desc 
LIMIT 15; 

然而，当结果COLLECT（{REST：r2.res_id}）为空，整个结果变为空。此外，查询无法识别第一个匹配的行，并返回undefined rows。请告诉我。谢谢！

Answer 1

如果模式不匹配任何路径，结果确实是空的。

你必须一分为2 MATCH，使第二个OPTIONAL，或在您的实际情况下，停止同u1节点一遍又一遍匹配：

MATCH (u1:User {user_id: 4}) 
OPTIONAL MATCH (u1)-[:FOLLOWS]->(:User)-->(r1:Rest {city_id: 1}) 
WITH u1, collect({ REST: r1.res_id }) AS rows 
OPTIONAL MATCH (u1)-->(r2:Rest {city_id: 1}) 
WHERE NOT (u1)-[:BEEN_THERE | ADD_REVIEW]->(r2) 
WITH rows + collect({ REST: r2.res_id }) AS allrows 
UNWIND allrows as row 
RETURN row.REST AS RESTAURANT_ID, count(row.REST) AS COUNT 
ORDER BY COUNT desc 
LIMIT 15 

我不知道的第一个OPTIONAL MATCH在你的情况下（你只提到第二个collect作为一个拦截器），但如果你想两个模式的聚合，其中每个可以是空的，在这里你去。