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Example 1: find all the keys in the set and the number of times each key corresponds

MongoDB doesn't have a schema, so you don't know how many keys there are in each document. Usually the best way to find all the keys in a collection is to make use of MapReduce. All right, let's look at the examples.

1. First, generate several test data records through mongo- > shell.

2. Write map function

3. Write reduce function

4. Call MapReduce to see the result

5. View the results

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