MongoDB----modifier

$inc: Increases or decreases the specified key value and creates a key if the key does not exist.

$set: Specifies a value for a key that is created if the key does not exist.

$unset: Deletes the specified key.

$push: Adds an element to the end of the specified array, creates an array if the array does not exist, and can add the same elements repeatedly .

$addToSet: Adds an element to the specified array, creates an array if the array does not exist, and does not repeatedly add the same elements .

$pop: Deletes an element from the front or back end or either end of the specified array.

$pull: removes all matching elements from the specified array.

$ne: Not equal to