1. MongoDB Read Operations

2. Agenda
Query Interface
Query Behavior
Query Statements

3. Read operations Queries select documents from a single collection.
db.collection.find()
Find() accepts the query criteria (what to return)
It returns a cursor to the document set
Cursor points to the first 20 docs in the result set, is iterable.
Limits can be applied.
Projections return only wanted fields
Limit will limit the number of documents returned

4. Projections
Queries will automatically return all fields in all matching docs by default.
Limit the number of fields returned by each document with a projection
Projections are the second argument in the find() method.
Can specify a list of fields to return
Can also specify a list of fields to exclude
You cannot mix inclusive and exclusive projections together. Except for excluding the _id field in an inclusive projection.

5. Projections

6. Projection Examples db.restaurants.find({"cuisine": "Bakery"},
{"name": 1, "address": 1
}).pretty()
{"address": 0, "grades": 0
{"name": 1, "address": 1, "_id": 0

7. Read Operations MongoDB

8. Read Operations Mysql

9. Query Selectors (Comparisons)
Name
Description
$eq
Matches values that are equal to a specified value.
$gt
Matches values that are greater than a specified value.
$gte
Matches values that are greater than or equal to a specified value.
$lt
Matches values that are less than a specified value.
$lte
Matches values that are less than or equal to a specified value.
$ne
Matches all values that are not equal to a specified value.
$in
Matches any of the values specified in an array.
$nin
Matches none of the values specified in an array.

10. Query Selectors (Comparisons)
Name
Description
$eq
db.restaurants.find({"address.building": {$eq: "195"}}).pretty()
$gt
db.restaurants.find({"address.zipcode": {$gt: "10020"}}).pretty()
$gte
db.restaurants.find({"address.zipcode": {$gte: "10020"}}).pretty()
$lt
db.restaurants.find({"address.zipcode": {$lt: "10045"}}).pretty()
$lte
db.restaurants.find({"address.zipcode": {$lte: "10020"}}).pretty()
$ne
db.restaurants.find({"borough": {$ne: "Manhattan"}}).pretty()
$in
db.restaurants.find({"address.zipcode": {$in: ["10020", "10021", "10022"]}}).pretty()
$nin
db.restaurants.find({"address.zipcode": {$nin: ["10020", "10021", "10022"]}}).pretty()

11. Query Selectors (Logical)
Name
Description
$or
Joins query clauses with a logical OR returns all documents that match the conditions of either clause.
$and
Joins query clauses with a logical AND returns all documents that match the conditions of both clauses.
$not
Inverts the effect of a query expression and returns documents that do not match the query expression.

12. Query Selectors (Logical)
Name
Description
$or
db.restaurants.find({ $or: [{"cuisine": "Italian"}, {"address.zipcode": "10075"}]}).pretty()
$and
db.restaurants.find({ $and: [{"cuisine": "Italian"}, {"address.zipcode": "10075"}]}).pretty()
$not
db.restaurants.find({"cuisine": {$not: {$eq: "Italian"}}}).pretty()

13. Query Behavior
All queries in MongoDB refer to a single collection (Ex. restaurants)
Modify the results via: Limits, Skips, Sort Orders
The order of the documents can be random unless you specify a sort() order.
Updates also use the same syntax as Reads to select documents to update.

14. How Query Statements Work