MongoDB QueryData

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Querying a collection

The find() function can be used to query the documents.

The format is:

 db.collectionName.find(optional_query_criteria)

Where the query_criteria follows a pattern:

 db.collectionName.find({keyField: "value"})

Note:

• the criteria is enclosed in curly brackets: {}
• the value needs quotes if it is a string or date value
• all names and values are case sensitive
• quotes are optional for the fieldName, so long as they do not contain spaces

Find all documents

For example, show all the data so far in the dept collection:

db.dept.find()

And the emp collection:

db.emp.find()

The data comes back messy. The pretty() function can be used to improve the layout:

db.emp.find().pretty()

Note the difference in the object ids returned.

Find with query criteria

If working with a large collection, you will not want all the documents returned.

Find all the employees in department 10:

db.emp.find({deptno:10})

Find One document

To find just one document requires the use of the equivalent of a primary key field. This can be a field that the user takes responsibility to keep unique, such as the deptno:

db.dept.find({deptno:10})

Or the object id can be used, which is will be unique:

db.dept.find({_id:10})

Object ids will be discussed in a later section.

Next Step

MongoDB Aggregation Pipeline