MongoDB (6): Querying documents

Tags: overview structure syntax ... pretty ODB strong embedded--

Find () method

To query data from a MongoDB collection, you need to use MongoDB's find() method.

Grammar

find()The basic syntax of the command is as follows:

>db.COLLECTION_NAME.find(document)

Shell

find()Method will display all documents in an unstructured manner.

Pretty () method

To display the results in a formatted manner, you can use the pretty() method.

Grammar

> db.mycol.find().pretty()

Example

>db.mycol.find().pretty(){   "_id": 100,   "title": "MongoDB Overview",    "description": "MongoDB is no sql database",   "by": "yiibai tutorials",   "url": "http://www.yiibai.com",   "tags": ["mongodb", "database", "NoSQL"],   "likes": "100"}>

In addition find() to the method, there is a findOne() method that returns only one document.

the equivalent Where clause for MongoDB and RDBMS

To query a document based on some criteria, you can use the following actions.

Operation	Grammar	Example	RDBMS equivalent Statement
Equal	{<key>:<value>}	db.mycol.find({"by":"yiibai"}).pretty()	where by = ‘yiibai‘
Less than	{<key>:{$lt:<value>}}	db.mycol.find({"likes":{$lt:50}}).pretty()	where likes < 50
Less than or equal	{<key>:{$lte:<value>}}	db.mycol.find({"likes":{$lte:50}}).pretty()	where likes <= 50
Greater than	{<key>:{$gt:<value>}}	db.mycol.find({"likes":{$gt:50}}).pretty()	where likes > 50
Greater than or equal	{<key>:{$gte:<value>}}	db.mycol.find({"likes":{$gte:50}}).pretty()	where likes >= 50
Not equal to	{<key>:{$ne:<value>}}	db.mycol.find({"likes":{$ne:50}}).pretty()	where likes != 50

Here we will show you all the actions in the table above-

and operator in MongoDB

Grammar

In a find() method, MongoDB treats it as a condition if it is passed by using ' ， separate ' multiple keys AND . The following is AND the basic syntax-

>db.mycol.find(   {      $and: [         {key1: value1}, {key2:value2}      ]   }).pretty()

Example

The following example shows all the yiibai tutorials tutorials written by "" and titled "MongoDB Overview."

> db.mycol.find({$and:[{"by":"yiibai tutorials"},{"title": "MongoDB Overview"}]}).pretty(){        "_id" : 100,        "title" : "MongoDB Overview",        "description" : "MongoDB is no sql database",        "by" : "yiibai tutorials",        "url" : "http://www.yiibai.com",        "tags" : [                "mongodb",                "database",                "NoSQL"        ],        "likes" : 100}>

For the example given above, the equivalent SQL where clause is-

SELECT * FROM mycol where by =‘yiibai tutorials‘ AND title =‘MongoDB Overview‘

Sql

You can find pass any number of key values in the clause.

the OR operator in MongoDB

Grammar

You need to use keywords when you want to query a document based on a OR condition $or . The following is the OR basic syntax for a condition-

>db.mycol.find(   {      $or: [         {key1: value1}, {key2:value2}      ]   }).pretty()

Example

The following example displays yiibai tutorials all tutorials written or titled "MongoDB Overview".

>db.mycol.find({$or:[{"by":"yiibai tutorials"},{"title": "MongoDB Overview"}]}).pretty(){   "_id": 100,   "title": "MongoDB Overview",    "description": "MongoDB is no sql database",   "by": "yiibai tutorials",   "url": "http://www.yiibai.com",   "tags": ["mongodb", "database", "NoSQL"],   "likes": "100"}>

using and and or conditions together

Example

The following example displays likes 10 all documents that are greater than and are titled " MongoDB Overview or" yiibai tutorials . The equivalent SQL WHERE clause is-

SELECT * FROM mycol where likes> 10 AND(by =‘yiibai tutorials‘ OR title =‘MongoDB Overview‘)

Sql

>db.mycol.find({"likes": {$gt:10}, $or: [{"by": "yiibai tutorials"},   {"title": "MongoDB Overview"}]}).pretty(){   "_id": 100,   "title": "MongoDB Overview",    "description": "MongoDB is no sql database",   "by": "yiibai tutorials",   "url": "http://www.yiibai.com",   "tags": ["mongodb", "database", "NoSQL"],   "likes": "100"}>

querying embedded/nested documents

Here's how to use db.collection.find() the: method for an example of a query operation on an embedded/nested document. The samples on this page use the inventory collection. To populate the inventory ( inventory ) collection to prepare some data, run the following command:

db.inventory.insertMany( [   { item: "journal", qty: 25, size: { h: 14, w: 21, uom: "cm" }, status: "A" },   { item: "notebook", qty: 50, size: { h: 8.5, w: 11, uom: "in" }, status: "A" },   { item: "paper", qty: 100, size: { h: 8.5, w: 11, uom: "in" }, status: "D" },   { item: "planner", qty: 75, size: { h: 22.85, w: 30, uom: "cm" }, status: "D" },   { item: "postcard", qty: 45, size: { h: 10, w: 15.25, uom: "cm" }, status: "A" }]);

Matching embedded/nested documents

To specify equality criteria on a field that is an embedded/nested document, use the query filter document {<field>：<value>} , which <value> is the document that you want to match.

For example, the following query size selects { h: 14, w: 21, uom: "cm" } All documents that have a field equal to:

db.inventory.find( { size: { h: 14, w: 21, uom: "cm" } } )

An equality match in an entire embedded document requires an exact match to the specified <value> document, including the order of the fields.
For example, the following query inventory does not match any of the documents in the Inventory () collection:

db.inventory.find(  { size: { w: 21, h: 14, uom: "cm" } }  )

Querying nested fields

To specify a query condition on a field in an embedded/nested document, use the dot symbol (" field.nestedField ").

Specify equals match on nested fields

The following example selects size all documents that are nested in fields equal to " uom in ":

db.inventory.find( { "size.uom": "in" } )

Use the query operator to specify a match

Query filter documents can be specified by using the query operator, such as the following form of conditions:

{ <field1>: { <operator1>: <value1> }, ... }

The following query uses the size h less-than operator () in fields that are embedded in fields $lt :

db.inventory.find( { "size.h": { $lt: 15 } } )

Specifying and Conditions

The following query selects h All documents that have a nested field that is less than 15 , and the nested field uom equals " in " and the status field equals " D ":

db.inventory.find( { "size.h": { $lt: 15 }, "size.uom": "in", status: "D" } )

MongoDB (6): Querying documents