MongoDB Query Documents

MongoDB query documents use thefind()、findOne()method.

The find() method displays all documents in an unstructured format.

Syntax

The syntax format for MongoDB querying data is as follows:

db.collection.find(query, projection)

• query: The query criteria used to find documents. Defaults to{}, which matches all documents.
• projection(Optional): Specifies the fields to include or exclude in the returned results.

Find all documents:

Find documents by criteria:

Find documents by criteria and return only the specified fields:

If you need to read the data in a readable format, you can use the pretty() method. The syntax format is as follows:

>db.col.find().pretty()

The pretty() method displays all documents in a formatted way.

2、findOne()

The findOne() method is used to find a single document in a collection. If multiple matching documents are found, it returns only the first one.

Syntax:

db.collection.findOne(query, projection)

• query: The query criteria used to find documents. Defaults to{}, which matches all documents.
• projection(Optional): Specifies the fields to include or exclude in the returned results.

Find a single document:

db.myCollection.findOne({ name: "Alice" });

db.myCollection.findOne(
    { name: "Alice" },
    { name: 1, age: 1, _id: 0 }
);

Advanced Query Methods

1. Using Comparison Operators

MongoDB supports various comparison operators, such as$gt、$lt、$gte、$lte、$eq、$neetc.

Find documents where age is greater than 25:

db.myCollection.find({ age: { $gt: 25 } });

2. Using Logical Operators

MongoDB supports various logical operators, such as$and、$or、$not、$noretc.

Find documents where age is greater than 25 and city is "New York":

3. Using Regular Expressions

Regular expressions can be used for pattern matching queries.

Find documents where the name starts with "A":

db.myCollection.find({ name: /^A/ });

4. Projection

Projection is used to control which fields are returned in the query results. You can use either inclusion mode or exclusion mode.

Return only the name and age fields:

5. Sorting

You can sort the query results.

Sort by age in descending order:

db.myCollection.find().sort({ age: -1 });

6. Limit and Skip

You can limit the query results and skip a specified number of documents.

Return the first 10 documents:

db.myCollection.find().limit(10);

Skip the first 5 documents and return the next 10 documents:

db.myCollection.find().skip(5).limit(10);

Example

Find documents where age is greater than 25 and city is "New York", return only the name and age fields, sort by age in descending order, and return the first 10 documents.

MongoDB vs RDBMS WHERE Clause Comparison

If you are familiar with conventional SQL data, you can better understand MongoDB's conditional query statements through the following table:

MongoDB AND Conditions

MongoDB's find() method can accept multiple keys, with each key separated by a comma, which is the AND condition in conventional SQL.

The syntax format is as follows:

>db.col.find({key1:value1, key2:value2}).pretty()

Example

The following example uses thebyandtitlekey to queryExample TutorialMediumMongoDB Tutorialdata

> db.col.find({"by":"Example", "title":"MongoDB 教程"}).pretty()
{
        "_id" : ObjectId("56063f17ade2f21f36b03133"),
        "title" : "MongoDB 教程",
        "description" : "MongoDB 是一个 Nosql 数据库",
        "by" : "Example",
        "url" : "http://www.example.com",
        "tags" : [
                "mongodb",
                "database",
                "NoSQL"
        ],
        "likes" : 100
}

The above example is similar to the WHERE statement:WHERE by='Example Tutorial' AND title='MongoDB Tutorial'

MongoDB OR Conditions

MongoDB OR condition statements use the keyword$or, the syntax format is as follows:

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

Example

In the following example, we demonstrate querying the keybywith the valueExample Tutorialor the keytitlewith the valueMongoDB Tutorialdocuments.

>db.col.find({$or:[{"by":"Example"},{"title": "MongoDB 教程"}]}).pretty()
{
        "_id" : ObjectId("56063f17ade2f21f36b03133"),
        "title" : "MongoDB 教程",
        "description" : "MongoDB 是一个 Nosql 数据库",
        "by" : "Example",
        "url" : "http://www.example.com",
        "tags" : [
                "mongodb",
                "database",
                "NoSQL"
        ],
        "likes" : 100
}
>

Combining AND and OR

The following example demonstrates the combined use of AND and OR, similar to the conventional SQL statement:'where likes>50 AND (by = 'Example Tutorial' OR title = 'MongoDB Tutorial')'

>db.col.find({"likes": {$gt:50}, $or: [{"by": "Example"},{"title": "MongoDB 教程"}]}).pretty()
{
        "_id" : ObjectId("56063f17ade2f21f36b03133"),
        "title" : "MongoDB 教程",
        "description" : "MongoDB 是一个 Nosql 数据库",
        "by" : "Example",
        "url" : "http://www.example.com",
        "tags" : [
                "mongodb",
                "database",
                "NoSQL"
        ],
        "likes" : 100
}