MongoDB Read

Read operations or queries, fetch data in the database. In MongoDB, queries select document from a single collection. The queries are like other database queries that specify the condition to fetch the data from the database.

Query:

In MongoDB find() method accepts both query criteria and projection to fetch the records.

Example:

db.employees.find(
{
_id: 1
}
, {
firstName: true
, lastName: true
, phone: true
, email: true
});

Below query is the equivalent SQL query.

SELECT _id, firstName, lastName, phone, email
FROM employees
WHERE _id = 1

Sorting:

The order of the results by a query is not not defined. If you need the result set in a order you need to specify the order. To apply sorting to the result set use sort() method. It accepts a field and sort order. Specify in the sort parameter the field or fields to sort by and a value of 1 or -1 to specify an ascending or descending sort respectively.

Example:

db.employees.find().sort({modAt: -1}) // Descending order
db.employees.find().sort({modAt: 1}) // Ascending order

SQL comparison

SELECT * FROM Employees ORDER BY ModAt DESC
SELECT * FROM Employees ORDER BY ModAt ASC

Limit:

Use of limit() method on a cursor to specify the maximum no of records. It's equivalent in SQL server is TOP

Example:

db.employees.find().limit(5)

SQL comparison

SELECT TOP 5 * FROM Employees


Skip:

We can use the skip method to control where MongoDB starts fetching records. It is mainly used for paged results like pagination.

Example:

var pageNumber = 2, pageLimit = 5, skipNumber = (pageNumber  1) * pageLimit;
db.employees.find().skip( skipNumber).limit(pageLimit);

The above query returns the records for second page. It will skip first 5 records and will fetch second 5 records.

Conclusion:

In this tutorial, I have explained the basics of read operations on MongoDB. I will add more in my next articles.

Reference:

http://docs.mongodb.org/

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 Srikant Biswal

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