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In the last blog, we talked about the basic operation of MongoDB, including insert, query, remove and update. Now in this blog, we will further hava a good understanding of  querying operation. 

At first, let's create 1000 recors for testing. As the following picture depicts.

Condition Expression 

1. <, <=, >, <=  is correspondent to ‘$lt’,'$Ite', '$gt' and '$gte'

2. $all

This operation is analogous to the $in operation. But the difference is that $all operation requires every elements in the array to be included in the return result. 

3. $exists

$exists operation check whether one field exists.