qpython to select columns from table - KX Learning Hub

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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 9 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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