Neo4j的暗號查詢空收集使整個結果空

Question

我一直面臨着奇怪的問題，我試圖讓從Neo4j的圖形，我有built.Here數據是我的查詢

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，當結果COLLECT（{REST：r2.res_id}）爲空，整個結果變爲空。此外，查詢無法識別第一個匹配的行，並返回undefined rows。請告訴我。謝謝！

Answer 1

如果模式不匹配任何路徑，結果確實是空的。

你必須一分爲2 MATCH，使第二個OPTIONAL，或在您的實際情況下，停止同u1節點一遍又一遍匹配：

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 

我不知道的第一個OPTIONAL MATCH在你的情況下（你只提到第二個collect作爲一個攔截器），但如果你想兩個模式的聚合，其中每個可以是空的，在這裏你去。