Difference between revisions of "MongoDB QueryData"

Revision as of 10:35, 11 November 2017

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:

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()

To find just one document - department 10:

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

@@ Line 40: / Line 40: @@
   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()