Mark Needham

Thoughts on Software Development

Neo4j: Detecting rogue spaces in CSV headers with LOAD CSV

without comments

Last week I was helping someone load the data from a CSV file into Neo4j and we were having trouble filtering out rows which contained a null value in one of the columns.

This is what the data looked like:

load csv with headers from "file:///foo.csv" as row
RETURN row
╒══════════════════════════════════╕
│row                               │
╞══════════════════════════════════╡
│{key1: a,  key2: (null),  key3: c}│
├──────────────────────────────────┤
│{key1: d,  key2: e,  key3: f}     │
└──────────────────────────────────┘

We’d like to filter out any rows which have ‘key2’ as null, so let’s tweak our query to do that:

load csv with headers from "file:///foo.csv" as row
WITH row WHERE NOT row.key2 is null
RETURN row
(no rows)

Hmmm that’s odd, it’s got rid of both rows. We’d expect to see the 2nd row since that doesn’t have a null value.

At this point we might suspect that what we’re seeing on the screen isn’t actually what the data looks like. Let’s write the following query to check our header values:

load csv with headers from "file:///foo.csv" as row
WITH row LIMIT 1
UNWIND keys(row) AS key
RETURN key, SIZE(key)
╒═════╤═════════╕
│key  │SIZE(key)│
╞═════╪═════════╡
│key1 │4        │
├─────┼─────────┤
│ key2│5        │
├─────┼─────────┤
│ key3│5        │
└─────┴─────────┘

The second column tells us that there are some extra characters in the columns for ‘key2’ and ‘key3’ or rather ‘ key2’ and ‘ key3’. In this case they are spaces, but it could easily be another character:

load csv with headers from "file:///foo.csv" as row
WITH row LIMIT 1
UNWIND keys(row) AS key
RETURN key, replace(key, " ", "_SPACE_") AS spaces
╒═════╤═══════════╕
│key  │spaces     │
╞═════╪═══════════╡
│key1 │key1       │
├─────┼───────────┤
│ key2│_SPACE_key2│
├─────┼───────────┤
│ key3│_SPACE_key3│
└─────┴───────────┘

If we clean up our CSV file and try again everything works as expected:

load csv with headers from "file:///foo.csv" as row
WITH row LIMIT 1
UNWIND keys(row) AS key
RETURN key, SIZE(key)
╒════╤═════════╕
│key │SIZE(key)│
╞════╪═════════╡
│key1│4        │
├────┼─────────┤
│key2│4        │
├────┼─────────┤
│key3│4        │
└────┴─────────┘
load csv with headers from "file:///foo.csv" as row
WITH row WHERE NOT row.key2 is null
RETURN row
╒═══════════════════════════╕
│row                        │
╞═══════════════════════════╡
│{key1: d, key2: e, key3: f}│
└───────────────────────────┘
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Written by Mark Needham

October 19th, 2016 at 5:16 am

Posted in neo4j

Tagged with