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()

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

db.dept.find().pretty()

Find One document

To find just one document - department 10:

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

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()

Same again for department 20 and the managers:

db.deptCollection.find({deptno:20},  { employees: {$elemMatch: {sal: { $gt: 2000}, job: "MANAGER"}}}).pretty()

employees is an array, so $elemMatch only returns the first matching value. What if we try this instead:

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

Things to note:

• This time employees.sal must be enclosed in matching single or double quotes.
• Comment on what the above query returns.
• If you examine the data carefully:
  • either find() means if one element of an array is found to be true, then all the elements are returned, even if they do not satisfy the query criteria
  • or including $elemMatch means only the first element matching the query criteria is returned.
• Is this good practice?

Find departments with no managers:

db.deptCollection.find({ "employees.job" : { $ne: "MANAGER"}}).pretty()

Next Step

MongoDB Aggregation Pipeline