条件表达式COALESCE返回一组给定表达式的首个非空值。在遇见空值时，使用表达式COALESCE可有效替换默认值，特别是在计算过程或结果展示时，可用来避免空值。

<coalesce expression> ::= 
  "COALESCE" "(" <value expression> { "," <value expression> }... ")"

表达式COALESCE(V1, V2, V3)等同于以下CASE表达式：

CASE 
  WHEN NOT V1 IS NULL THEN V1
  WHEN NOT V2 IS NULL THEN V2
  ELSE V3
END

示例图集

以下示例根据该图集运行：

在空图集中运行以下语句创建示例图集：

INSERT (p1:Paper {_id:'P1', title:'Efficient Graph Search', score:6, author:'Alex', publisher:'PulsePress'}),
       (p2:Paper {_id:'P2', title:'Optimizing Queries', score:9, author:'Alex'}),
       (p3:Paper {_id:'P3', title:'Path Patterns', score:7, author:'Zack', publisher:'BrightLeaf'}),
       (p1)-[:Cites {weight:2}]->(p2),
       (p2)-[:Cites {weight:1}]->(p3)

示例

MATCH (n:Paper)
RETURN n.title, COALESCE(n.publisher, "N/A") AS publisher

结果：