• kdb+

  qpython to select columns from table

  Posted by powerpeanuts on July 12, 2022 at 12:00 am

  This is about qpython code. Let’s say we have a table named “t” with 2 columns “name” and “iq” in kdb:

   

  from qpython import qconnection, qcollection 
  q = qconnection.QConnection(host='localhost', port=5000): q.open() 
  q("t:flip `name`iq!(`Dent`Beeblebrox`Prefect;98 42 126)")

   

   

  How can we pass the column name and table name through qpython? Thanks.

  pseudo code:

   

  q({select x from y}, "iq", "t")

   

  powerpeanuts replied 8 months, 1 week ago 2 Members · 3 Replies
• 3 Replies

• 

  rocuinneagain

  Member

  July 12, 2022 at 12:00 am

  Some ways:

  q("{((),x)#y}", numpy.string_("iq"), numpy.string_("t")) 
  q("{value"select ",string[x],"from ",string y}", "iq", "t") 
  q("select %s from %s" % ("iq","t")) 
  q("{?[y;();0b;{x!x}(),x]}", numpy.string_("iq"), numpy.string_("t"))

   

   

  1. Using # https://code.kx.com/q/ref/take/
  2. Using value https://code.kx.com/q/ref/value/
  3. Python string formatting
  4. Functional select https://code.kx.com/q/basics/funsql/

  Functional selects are the most powerful way https://code.kx.com/q/wp/parse-trees/

• 

  powerpeanuts

  Member

  July 12, 2022 at 12:00 am

  Thanks @rocuinneagain. I was trying the functional select and have to convert the strings to <class ‘numpy.bytes_’> first (I am using python 3), i.e.

   

  q("{?[y;();0b;{x!x}(),x]}", np.bytes_("iq"), np.bytes_("t"))

   

   

• 

  rocuinneagain

  Member

  July 12, 2022 at 12:00 am

  See docs for more on how the library handles difference between symbols and strings:

  https://qpython.readthedocs.io/en/latest/type-conversion.html#string-and-symbols

  (Answer above updated to use numpy.string_ to pass symbols as needed)

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