option_match

//与match区别：如果没有匹配到，返回NULL
CREATE (olive:Person {name:'Olive Stone'}),
(charlie:Person {name:'Charlie Sheen'}),
(michael:Person {name:'Michael Douglas'}),
(martin:Person {name:'Martin Sheen'}),
(rob:Person {name:'Rob Reiner'}),
(wallstreet:Movie {name:'WallStreet',title:"Wall Street"}),
(president:Movie {name:'TheAmericanPresident',title:"The American President"}),
(olive)-[:DIRECTED]->(wallstreet),
(charlie)-[:ACTED_IN]->(wallstreet),
(michael)-[:ACTED_IN]->(wallstreet),
(martin)-[:ACTED_IN]->(wallstreet),

(michael)-[:ACTED_IN]->(president),
(rob)-[:DIRECTED]->(president),
(martin)-[:ACTED_IN]->(president),
(charlie)-[:FATHER]->(martin)
1.可选元素
//返回null，节点没有外向的关系
match(a:Movie{title:"Wall Street"}) optional match(a)-->(x) return x
2.可选元素的属性
// 如果可选的元素返回null，那么该元素的属性也返回null
match(a:Movie{title:"Wall Street"}) optional match(a)-->(x) return x,x.name
3.可选关系类型
//返回null
match(a:Movie{title:"Wall Street"}) optional match(a)-[r:ACTED_IN]->() return r