MySQL: Get column name or alias from query

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Question :

MySQL: Get column name or alias from query

I’m not asking for the SHOW COLUMNS command.

I want to create an application that works similarly to heidisql, where you can specify an SQL query and when executed, returns a result set with rows and columns representing your query result. The column names in the result set should match your selected columns as defined in your SQL query.

In my Python program (using MySQLdb) my query returns only the row and column results, but not the column names. In the following example the column names would be ext, totalsize, and filecount. The SQL would eventually be external from the program.

The only way I can figure to make this work, is to write my own SQL parser logic to extract the selected column names.

Is there an easy way to get the column names for the provided SQL?
Next I’ll need to know how many columns does the query return?

# Python

import MySQLdb

# connect to mysql

    db = MySQLdb.connect(host="myhost", user="myuser", passwd="mypass",db="mydb")
except MySQLdb.Error, e:
    print "Error %d: %s" % (e.args[0], e.args[1])
    sys.exit (1)

# query select from table

cursor = db.cursor ()   

cursor.execute ("""
     select ext,
        sum(size) as totalsize,
        count(*) as filecount
     from fileindex
    group by ext
    order by totalsize desc;

while (1):
    row = cursor.fetchone ()
    if row == None:
    print "%s %s %sn" % (row[0], row[1], row[2])

Asked By: panofish


Answer #1:

cursor.description will give you a tuple of tuples where [0] for each is the column header.

num_fields = len(cursor.description)
field_names = [i[0] for i in cursor.description]
Answered By: user625477

Answer #2:

This is the same as thefreeman but more in pythonic way using list and dictionary comprehension

columns = cursor.description 
result = [{columns[index][0]:column for index, column in enumerate(value)} for value in cursor.fetchall()]

Answered By: James

Answer #3:

Similar to @James answer, a more pythonic way can be:

fields = map(lambda x:x[0], cursor.description)
result = [dict(zip(fields,row))   for row in cursor.fetchall()]

You can get a single column with map over the result:

extensions = map(lambda x: x['ext'], result)

or filter results:

filter(lambda x: x['filesize'] > 1024 and x['filesize'] < 4096, result)

or accumulate values for filtered columns:

totalTxtSize = reduce(
        lambda x,y: x+y,
        filter(lambda x: x['ext'].lower() == 'txt', result)
Answered By: juandesant

Answer #4:

I think this should do what you need (builds on the answer above) . I am sure theres a more pythony way to write it, but you should get the general idea.

columns = cursor.description
result = []
for value in cursor.fetchall():
    tmp = {}
    for (index,column) in enumerate(value):
        tmp[columns[index][0]] = column
Answered By: thefreeman

Answer #5:

You could also use MySQLdb.cursors.DictCursor. This turns your result set into a python list of python dictionaries, although it uses a special cursor, thus technically less portable than the accepted answer. Not sure about speed. Here’s the edited original code that uses this.

#!/usr/bin/python -u

import MySQLdb
import MySQLdb.cursors

# connect to mysql

    db = MySQLdb.connect(host='myhost', user='myuser', passwd='mypass', db='mydb', cursorclass=MySQLdb.cursors.DictCursor)
except MySQLdb.Error, e:
    print 'Error %d: %s' % (e.args[0], e.args[1])

# query select from table

cursor = db.cursor()

sql = 'SELECT ext, SUM(size) AS totalsize, COUNT(*) AS filecount FROM fileindex GROUP BY ext ORDER BY totalsize DESC;'

all_rows = cursor.fetchall()

print len(all_rows) # How many rows are returned.
for row in all_rows: # While loops always make me shudder!
    print '%s %s %sn' % (row['ext'], row['totalsize'], row['filecount'])


Standard dictionary functions apply, for example, len(row[0]) to count the number of columns for the first row, list(row[0]) for a list of column names (for the first row), etc. Hope this helps!

Answered By: Eugene

Answer #6:

This is only an add-on to the accepted answer:

def get_results(db_cursor):
    desc = [d[0] for d in db_cursor.description]
    results = [dotdict(dict(zip(desc, res))) for res in db_cursor.fetchall()]
    return results

where dotdict is:

class dotdict(dict):
    __getattr__ = dict.get
    __setattr__ = dict.__setitem__
    __delattr__ = dict.__delitem__

This will allow you to access much easier the values by column names.
Suppose you have a user table with columns name and email:

cursor.execute('select * from users')
results = get_results(cursor)
for res in results:
Answered By: MikeL

Answer #7:

Looks like MySQLdb doesn’t actually provide a translation for that API call. The relevant C API call is mysql_fetch_fields, and there is no MySQLdb translation for that

Answered By: Daniel DiPaolo

Answer #8:



mysql connector version:

Answered By: Amin

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