How to delete columns from an existing table in SQLite.

SQLite has limited ALTER TABLE support that you can use to add a column to the end of a table or to change the name of a table. If you want to make more complex changes in the structure of a table, you will have to recreate the table. You can save existing data to a temporary table, drop the old table, create the new table, then copy the data back in from the temporary table.
For example, suppose you have a table named "t1" with columns names "a", "b", and "c" and that you want to delete column "c" from this table. The following steps illustrate how this could be done:

BEGIN TRANSACTION;
CREATE TEMPORARY TABLE t1_backup(a,b);
INSERT INTO t1_backup SELECT a,b FROM t1;
DROP TABLE t1;
CREATE TABLE t1(a,b);
INSERT INTO t1 SELECT a,b FROM t1_backup;
DROP TABLE t1_backup;
COMMIT;

how to: python mongodb map-reduce

it's simple example how-to use map-reduce in mongo with python script
use:

• python3
• pymongo3 library

import pymongo

from datetime import datetime, timedelta

from  pymongo.code import Code



MAP_FUNCTION = Code("function () { emit(this.player, { count:1, score:this.score} ); }")

REDUCE_FUNCTION = Code("function (key, values) {var result = {count:0, score:0}; values.forEach(function (value) {result.count += value.count; result.score += value.score;}); return result; }")



dbhost=""

dbport=27017



mongodb = pymongo.Connection(dbhost, dbport)

db = mongodb.

age=10 

query={'last_visited': {'$gte': datetime.utcnow()-timedelta(age)}}

result = db..map_reduce(map=MAP_FUNCTION, reduce=REDUCE_FUNCTION, out="output", query=query)


for item in result.find():

    player = item["_id"]

    score = item["value"]['score']

    print(player, score)