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 results should look like:

{ "_id" : 10, "deptno" : 10, "dname" : "ACCOUNTING", "loc" : "NEW YORK" }
{ "_id" : 20, "deptno" : 20, "dname" : "RESEARCH", "loc" : "DALLAS" }
{ "_id" : 40, "deptno" : 40, "dname" : "OPERATIONS", "loc" : "BOSTON" }
{ "_id" : 50, "deptno" : 40, "dname" : "OPERATIONS V2", "loc" : "BOSTON" }
{ "_id" : 30, "deptno" : 30, "dname" : "SALES", "loc" : "CHICAGO" }

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 will be unique:

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

In the emp collection, the _ids are system generated and generally along the lines of: '5a0727e99ba81dee9b1cc6a3', so less easy to use!

List all the records in emp:

db.emp.find().pretty()

and pick an _id from the collection and then try and find one record.

For example (note, your object id will be different):

db.emp.find( {_id : ObjectId("5a0727e99ba81dee9b1cc6a3")}).pretty()

The function ObjectId() must be used to convert the value into an object id.

Next Step

MongoDB Aggregation Pipeline