cypher官方文档example解析

EXAMPLE1

companies.csv
Id,Name,Location, Email, BusinessType
1, Neo4j, San Mateo,contact@neo4j.com,P
2, AAA, ,info@aaa.com ,
3, BBB, Chicago, ,G

//skip null values
LOAD CSV WITH HEADERS FROM 'file:///companies.csv' AS row
WITH row WHERE row.Id IS NOT NULL
MERGE (c:Company {companyId: row.Id}); 

// clear data
MATCH (n:Company) DELETE n;  

//set default for null values 为空值设定特定值
LOAD CSV WITH HEADERS FROM 'file:///companies.csv' AS row
MERGE (c:Company {companyId: row.Id, hqLocation: coalesce(row.Location, "Unknown")})

COALESCE( )函数
主流数据库系统都支持COALESCE()函数，这个函数主要用来进行空值处理，其参数格式如下：
COALESCE ( expression,value1,value2……,valuen)
COALESCE()函数的第一个参数expression为待检测的表达式，而其后的参数个数不定。
COALESCE()函数将会返回包括expression在内的所有参数中的第一个非空表达式。

// clear data
MATCH (n:Company) DELETE n;

//change empty strings to null values (not stored)
LOAD CSV WITH HEADERS FROM 'file:///companies.csv' AS row
MERGE (c:Company {companyId: row.Id})
SET c.emailAddress = CASE trim(row.Email) WHEN "" THEN null ELSE row.Email END

SQL 中的 TRIM 函数是用来移除掉一个字串中的字头或字尾。最常见的用途是移除字首或字尾的空白。
TRIM ( [ [位置] [要移除的字串] FROM ] 字串): [位置] 的可能值为 LEADING (起头), TRAILING (结尾), or BOTH (起头及结尾)。 这个函数将把 [要移除的字串] 从字串的起头、结尾，或是起头及结尾移除。如果我们没有列出 [要移除的字串] 是什么的话，那空白就会被移除。

EXAMPLE2

如果CSV中有一个字段是要拆分的项目列表，则可以使用Cypher split（）函数分隔单元格中的数组。

employees.csv
Id,Name, Skills, Email
1,Joe Smith,Cypher:Java:JavaScript,joe@neo4j.com
2,Mary Jones,Java,mary@neo4j.com
3,Trevor Scott,Java:JavaScript,trevor@neo4j.com

LOAD CSV WITH HEADERS FROM 'file:///employees.csv' AS row
MERGE (e:Employee {employeeId: row.Id, email: row.Email})
WITH e, row
UNWIND split(row.Skills, ':') AS skill
MERGE (s:Skill {name: skill})
MERGE (e)-[r:HAS_EXPERIENCE]->(s)

UNWIND expands a list into a sequence of rows.
（将一列转换为连续行）
UNWIND [1, 2, 3, NULL ] AS x
RETURN x, ‘val’ AS y

x	y
1	“val”
2	“val”
3	“val”
null	“val”