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Doing a database join with CSV files


It’s easy to manipulate CSV files with basic command line tools until
you need to do a join. When your data is spread over two different
files, like two tables in a normalized database, joining the files is
more difficult unless the two files have the same keys in the same
order. Fortunately, the xsv utility
is just the tool for the job. Among other useful features, xsv
supports database-like joins.

Suppose you want to look at weights broken down by sex, but weights are
in one file and sex is in another. The weight file alone doesn’t tell
you whether the weights belong to men or women.

Suppose a file weight.csv has the following rows:
ID,weight 
    123,200 
    789,155 
    999,160

and a file person.csv has the following:
ID,sex 
    123,M 
    456,F 
    789,F

Note that the two files have different ID values: 123 and 789 are in
both files, 999 is only in weight.csv and 456 is only in person.csv.
We want to join the two tables together, analogous to the JOIN command
in SQL.

The command
xsv join ID person.csv ID weight.csv

does just this, producing
ID,sex,ID,weight 
    123,M,123,200 
    789,F,789,155

by joining the two tables on their ID columns.

The command includes ID twice, once for the field called ID in
person.csv and once for the field called ID in weight.csv. The
fields could have different names. For example, if the first column of
person.csv were renamed Key, then the command
xsv join Key person.csv ID weight.csv

would produce
Key,sex,ID,weight 
    123,M,123,200 
    789,F,789,155

We’re not interested in the ID columns per se; we only want to use
them to join the two files. We could suppress them in the output by
asking xsv to select the second and fourth columns of the output
join Key person.csv ID weight.csv | xsv select 2,4

which would return
sex,weight 
    M,200 
    F,155

We can do other kinds of joins by passing a modifier to join. For
example, if we do a left join, we will include all rows in the left
file, person.csv, even if there isn’t a match in the right file,
weight.csv. The weight will be missing for such records, and so
$ xsv join --left Key person.csv ID weight.csv

produces
Key,sex,ID,weight 
    123,M,123,200 
    456,F,, 
    789,F,789,155

Right joins are analogous, including every record from the second
file, and so
xsv join --right Key person.csv ID weight.csv

produces
Key,sex,ID,weight 
    123,M,123,200 
    789,F,789,155 
    ,,999,160

You can also do a full join, with
xsv join --full Key person.csv ID weight.csv

producing
Key,sex,ID,weight 
    123,M,123,200 
    456,F,, 
    789,F,789,155 
    ,,999,160

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Doing a database join with CSV files

John D. Cook: Doing a SQL join with CSV files with xsv