MongoDB QueryData
From mi-linux
Main Page >> MongoDB >>MongoDB Workbook >> Querying Collections
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
