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({"fieldName": "value"})

The fieldName must be in quotes, the value needs quotes if it is a string or date value.

Find all documents

For example, show all the data so far in the deptCollection:

db.deptCollection.find()

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

db.deptCollection.find().pretty()

Find One document

To find just one document - department 10:

db.deptCollection.find({"deptno":10}).pretty()

Finding an employee means using the array name too:

db.deptCollection.find({"employees.empno":7902}).pretty()

However, this does mean you get back all the employees in the department they were found in!

Since version 2.2 MongoDB's new $elemMatch can avoid this:

db.deptCollection.find({"deptno":20},  
  { _id: 0, employees: {$elemMatch: {empno: 7902}}}).pretty()

$elemMatch limits the contents of the employees array to contain only the first element matching the $elemMatch condition.

This is akin to a SQL query:

 SELECT * FROM Emp WHERE deptno=20 AND empno = 7902

_id is a unique value automatically generated by MongoDB (like a Primary Key, except it is unique for the whole database).

Using _id:0 suppresses the value, however to see it:

db.deptCollection.find({"deptno":20},  
  { employees: {$elemMatch: {empno: 7902}}}).pretty()

More about _ids in the next section.

Find with Query Criteria

The query criteria can be as complex as that found in SQL.

To find all employees earning more than 2000 in department 10:

db.deptCollection.find({"deptno":10},   
 { employees: {$elemMatch: {sal: { $gt: 2000}}}}).pretty()

Next Step

Updating the collection