Neo4j: LOAD CSV - The Sneaky Null Character

Mark Needham

Oct. 31, 14 · Interview

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I spent some time earlier in the week trying to import a CSV file extracted from Hadoop into Neo4j using Cypher’s LOAD CSV command and initially struggled due to some rogue characters.

The CSV file looked like this:

$ cat foo.csv
foo,bar,baz
1,2,3

I wrote the following LOAD CSV query to extract some of the fields and compare others:

load csv with headers from "file:/Users/markneedham/Downloads/foo.csv" AS line
RETURN line.foo, line.bar, line.bar = "2"

==> +--------------------------------------+
==> | line.foo | line.bar | line.bar = "2" |
==> +--------------------------------------+
==> | <null>   | "2"     | false          |
==> +--------------------------------------+
==> 1 row

I had expect to see a “1” in the first column and a ‘true’ in the third column, neither of which happened.

I initially didn’t have a text editor with hexcode mode available so I tried checking the length of the entry in the ‘bar’ field:

load csv with headers from "file:/Users/markneedham/Downloads/foo.csv" AS line
RETURN line.foo, line.bar, line.bar = "2", length(line.bar)

==> +---------------------------------------------------------+
==> | line.foo | line.bar | line.bar = "2" | length(line.bar) |
==> +---------------------------------------------------------+
==> | <null>   | "2"     | false          | 2                |
==> +---------------------------------------------------------+
==> 1 row

The length of that value is 2 when we’d expect it to be 1 given it’s a single character.

I tried trimming the field to see if that made any difference…

load csv with headers from "file:/Users/markneedham/Downloads/foo.csv" AS line
RETURN line.foo, trim(line.bar), trim(line.bar) = "2", length(line.bar)

==> +---------------------------------------------------------------------+
==> | line.foo | trim(line.bar) | trim(line.bar) = "2" | length(line.bar) |
==> +---------------------------------------------------------------------+
==> | <null>   | "2"            | true                 | 2                |
==> +---------------------------------------------------------------------+
==> 1 row

…and it did! I thought there was probably a trailing whitespace character after the “2” which trim had removed and that ‘foo’ column in the header row had the same issue.

I was able to see that this was the case by extracting the JSON dump of the query via the Neo4j browser:

{  
   "table":{  
      "_response":{  
         "columns":[  
            "line"
         ],
         "data":[  
            {  
               "row":[  
                  {  
                     "foo\u0000":"1\u0000",
                     "bar":"2\u0000",
                     "baz":"3"
                  }
               ],
               "graph":{  
                  "nodes":[  
 
                  ],
                  "relationships":[  
 
                  ]
               }
            }
         ],
      ...
}

It turns out there were null characters scattered around the file so I needed to pre process the file to get rid of them:

$ tr < foo.csv -d '\000' > bar.csv

Now if we process bar.csv it’s a much smoother process:

load csv with headers from "file:/Users/markneedham/Downloads/bar.csv" AS line
RETURN line.foo, line.bar, line.bar = "2", length(line.bar)

==> +---------------------------------------------------------+
==> | line.foo | line.bar | line.bar = "2" | length(line.bar) |
==> +---------------------------------------------------------+
==> | "1"      | "2"      | true           | 1                |
==> +---------------------------------------------------------+
==> 1 row

Note to self: don’t expect data to be clean, inspect it first!

CSV Neo4j

Published at DZone with permission of Mark Needham, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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