Learn Mongo DB

Colection == table; document == row/tuble;  field == column
-> Connect DB
### - Show DB :            show dbs - dont show empty DB.
### - use or create DB :   use <DB-name>
### - Check DB using :     db
### - insert collection :  db.<coll-name>.insert({"name" :"xyz"})
                           db.createCollection("name-Coll", {option}) - option : capped, autoIndexId, size, max.
### - show Collection :    show collections
### - Drop DB (default "test" DB) :          db.dropDatabase()
### - Drop Collection :    db.<coll-name>.drop()
### - insert document :    db.<coll-name>.insert(obj) - obj: a obj or arr of obj.
### - query Document :     db.<coll-name>.find(obj-condition^) - return all result
                           db.<coll-name>.find(obj-condition^).pretty() - return format result
                           db.<coll-name>.findOne(obj-condition^) - return one result
                 => condition: AND: $and: [obj1, obj2] ; OR: $or: [obj1, obj2] ; NOR: $nor: [obj1, obj2] ; NOT: $not: [obj1]
    AND + OR : db.mycol.find({"likes": {$gt:10}, $or: [{"by": "admin"},{"title": "MongoDB"}]}).pretty()
                 Equality :   	    {key:value}; 
                 Less Than :  	    {key:{$lt:value}}; 
                 Less Than Equals :	{key:{$lte:value}}
                 Greater Than :   	{key:{$gt:value}}; 
                 Greater Than Equals :  {key:{$gte:value}}; 
                 Not Equals :         	{key:{$ne:value}}
### - update Document :    db.<coll-name>.update({obj-find}, {$set:{obj-set}}) - Add value to document
                           db.<coll-name>.save({"_id": Object(...), key:value, key:value} - Repalce value to document
### - Remove Document :    db.<coll-name>.remove() - Remove all document of collection
                           db.<coll-name>.remove({obj-remove}) - remove all doccument by condition
                           db.<coll-name>.remove({obj-remove}, 1//true) - remove 1 document first by condition
### - Profection :         db.<coll-name>.find({},{key: 0//1}) - Select field necessary (0: hiden, 1: show)
### - Limit record :       db.<coll-name>.find({}, {key: 0//1}).limit(num) - limit num record to be displayed
### - skip record :        db.<coll-name>.find({}, {key: 0//1}).skip(num) - skip num record to be displayed
### - sort record :        db.<coll-name>.find({}, {key: 0//1}).sort(key: 1*//-1) - sort record with -1: descending
### - Index :              db.<coll-name>.createIndex({key: 1, key: -1}) - create Index for field
                           db.<coll-name>.dropIndex({key: 1//-1}) - drop 1 index
                           db.<coll-name>.dropIndexs({key: 1, key: -1} - drop many Index.
                           db.<coll-name>.getIndexs() - get all Index.
### - Aggregation :        db.<coll-name>aggregate([{pipeline : {_id : "$by_user", num_tutorial : {expressions : 1}}}])
                      pipeline: $project − $match − $group − $sort − $skip − $limit - $unwind
                      expressions: $sum	- $avg - $min	- $max - $push - $addToSet - $first - $last

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