John D Cook (unofficial)

2019-12-31 13:15:36

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

Related posts

• Munging CSV files with cut, sort, and
  awk
• Exporting Excel files to
  CSV
• Wrangling wide files with
  cut

{width="1"
height="1"}

http://feedproxy.google.com/~r/TheEndeavour/~3/ENI-x5u_m3M/
#johndcook #Computing

Doing a database join with CSV files

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

Michael Goasdoue

2019-10-17 02:02:00

Men in shorts operating computers

Via Australian Kitsch‏ @OzKitsch on Twitter
#computing #retrocomputing #shorts

Michael Goasdoue

2019-10-12 03:04:53

Computer historians crack passwords of Unix's early pioneers

Early versions of the free/open Unix variant BSD came with password files that included hashed passwords for such Unix luminaries as Dennis Ritchie, Stephen R. Bourne, Eric Schmidt, Brian W. Kernighan and Stuart Feldman.
Leah Neukirchen recovered an BSD version 3 source tree and posted about it on the Unix Heritage Society mailing list, revealing that she was able to crack many of the weak passwords used by the equally weak hashing algorithm from those bygone days

Via Boing Boing
#computing #retrocomputing #UNIX

jagtar singh lakhyan

2019-06-11 09:01:36

As the bookies became aware of this success, they prevented the researchers from betting further. #computing #cs #coding #code

An online betting strategy that really works—if you can use it

A team of researchers found a way to make money legally from online bookies. But then their troubles began.

^{www.technologyreview.com}

Michael Goasdoue

2019-05-22 05:48:54

Arpanet logical map, march 1977

Via Wikimedia Commons
#computing #retrocomputing #ARPANET

Neil Carvin

2019-05-20 01:50:05

South Korea's government will switch to Linux over cost concerns

Some governments might have fallen out of love with Linux, but South Korea appears ready to start a torrid affair. The country's Ministry of the Interior and Safety has outlined plans to switch government computers from Windows to Linux due to both lower costs and a reduced dependency on a single operating system. The Ministry will trial Linux on its PCs and roll it out more broadly if there aren't any major compatibility or security issues.

#software #computing #technology #business #microsoft #linux #southkorea #opensource

South Korea's government will switch to Linux over cost concerns

South Korea wants to switch its government PCs to Linux after concerns that Windows will be too costly to maintain.

^{www.engadget.com}

Michael Goasdoue

2019-05-10 06:17:22

On-line Party Time

Commodore 64 ad, 1986
#retrocomputing #computing

Emmanuel Florac

2019-04-06 12:19:29

Introduction - Computer Graphics from scratch - Gabriel Gambetta

#book #computing #programming #3D

Introduction to 3D computer graphics.

Introduction - Computer Graphics from scratch - Gabriel Gambetta

^{www.gabrielgambetta.com}