LOAD CSV Cypher clause
The LOAD CSV clause enables you to load and use data from a CSV file of your
choosing in a row-based manner, within a query. We support the Excel CSV
dialect, as it's the most commonly used one. For the syntax of the clause,
please check the Cypher manual
The clause reads row by row from a CSV file, and binds the contents of the parsed row to the variable you specified.
It's important to note that the parser parses the values as strings. It's up to
the user to convert the parsed row values to the appropriate type. This can be
done using the built-in conversion functions such as ToInteger, ToFloat,
ToBoolean etc. Consult the documentation on the available conversion
functions.
Header options
Depending on how you set the HEADER option (WITH or NO), a row will be
parsed as either a map or a list.
If the WITH HEADER option is set, the very first line in the file will be
parsed as the header, and any remaining rows will be parsed as regular rows. The
value bound to the row variable will be a map of the form:
{ ( "header_field" : "row_value" )? ( , "header_field" : "row_value" )* }
If the NO HEADER option is set, then each row is parsed as a list of values.
The contents of the row can be accessed using the list index syntax. Note that
in this mode, there are no restrictions on the number of values a row contains.
This isn't recommended, as the user must manually handle the varying number of
values in a row.
Notes about the clause
The clause can't stand on its own, meaning there has to be at least one more clause in the query, in addition to it. In other words, the following query will throw an exception:
LOAD CSV FROM "file.csv" WITH HEADER AS row ;
On the other hand, the following query is valid:
MERGE (n:A) LOAD CSV FROM "file.csv" WITH HEADER AS row ;
Note that the row variable doesn't have to be used in any subsequent clause.
Also, it's important to note how the LOAD CSV result exhaustion works. Let's
assume the following query:
MATCH (n)
LOAD CSV FROM "file.csv" WITH HEADER as row
SET n.p = row ;
Let's say the MATCH (n) clause finds five nodes, whereas the "file.csv" has
only 2 rows. Then only the first two nodes returned by the MATCH (n) will have
their properties set, using the two rows from the file. Similarly, if the MATCH
(n) clause finds two nodes, whereas the "file.csv" has five rows, the two nodes
returned by MATCH (n) will have their properties set with values from the
first two rows of the file. In general, the clause that exhausts its results
sooner will dictate how many times the "loop" will be executed.
Finally, note that the LOAD CSV clause can be used at most once per query.
Trying to use multiple LOAD CSV clauses within a single query will throw an
exception. In other words, queries such as
LOAD CSV FROM "x.csv" WITH HEADER as x
LOAD CSV FROM "y.csv" WITH HEADER as y
CREATE (n:A { p1 : x, p2 : y });
are invalid.