2010/9/17 MRAB <pyt...@mrabarnett.plus.com>:
> On 16/09/2010 23:11, Vlastimil Brom wrote:
>>
>>...
>> I put together some code, which works as expected, but I suspect
>> somehow, that there must be better ways of doing it.
>>
>> Two things I am not quite clear about are using the placeholders for
>> the data identifiers and "chaining" the SELECT parameters.
>>
>> I Couldn't find a way to use "?" placeholder for table or column
>> names, hence I ended up using string interpolation for them and
>> placeholders for the data values, like.
>> curs.execute('SELECT * FROM "%s" WHERE "%s"==?' % (text_name,
>> index_col), (text_index,))
>> is there a better way or is it not supposed to supply these
>> identifiers programatically?
>>
> You would normally expect the structure of the database to be fixed and
> only the contents to vary.
>
>> For getting the matching text indices given the tags, tag_values
>> combination I ended up with a clumsy query:
>>
>> combined_query_list = ['SELECT "%s" FROM "%s" WHERE "%s"==?' %
>> (index_col, text_name, tag) for tag in tags]
>> sql_query = " INTERSECT ".join(combined_query_list)
>> curs.execute(sql_query, tag_values)
>>
>> which produces e.g.:
>> SELECT "ind" FROM "n" WHERE "KC"==? INTERSECT SELECT "ind" FROM "n"
>> WHERE "VN"==?
>>
>> or alternatively:
>>
>> select_begin = 'SELECT "%s" FROM "%s" WHERE ' % (index_col, text_name)
>> where_subquery = " AND ".join('"%s"==?' % (tag,) for tag in tags)
>> sql_query = select_begin + where_subquery
>>
>> with the resulting query string like:
>> SELECT "ind" FROM "n" WHERE "KC"==? AND "VN"==? ('12', '1')
>>
>> (BTW, are these queries equivalent, as the outputs suggest, or are
>> there some distinctions to be aware of?)
>>
>> Anyway, I can't really believe, this would be the expected way ...
>>
> If you're selecting rows of a table then using 'AND' would seem the
> obvious way.
>
>
Thanks for the answer,
Well, that may be a part of the problem, the database structure is
going to be fixed once I'll have the text sources complete, but I was
trying to keep it more general, also allowing the identifiers to be
passed programmatically (based on the tagged text in question).
yes, I am just selecting rows - based on the combination of the column
values (which, I guess, might be an usual database approach(?).
However, I was unsure, whether it is usual to construct the query
string this way - partly using string interpolation or sequence
joining.
Or should there normally be no need for construct like the above and I
am doing something wrong in a more general sense?
Thanks again,
Vlastimil Brom
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