What is Query Selectors/Query Operators?

In MongoDB, query selectors (also called query operators) are special expressions used to filter and retrieve documents from a collection based on specific conditions. They allow you to perform complex searches, comparisons, and logical operations on your data.

Comparison Operators:

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$eq: Matches values that are equal to a specified value.
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$gt: Matches values that are greater than a specified value.
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$gte: Matches values that are greater than or equal to a specified value.
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$in: Matches any of the values specified in an array.
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$lt: Matches values that are less than a specified value.
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$lte: Matches values that are less than or equal to a specified value.
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$ne: Matches all values that are not equal to a specified value.
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$nin: Matches none of the values specified in an array.

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Logical Operators:

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$and: Joins query clauses with a logical AND returns all documents that match the conditions of both clauses.
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$not: Inverts the effect of a query predicate and returns documents that do not match the query predicate.
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$nor: Joins query clauses with a logical NOR returns all documents that fail to match both clauses.
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$or: Joins query clauses with a logical OR returns all documents that match the conditions of either clause.

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AND as well as OR Conditions

Array Operators:

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$all: Matches arrays that contain all elements specified in the query.
2
$elemMatch: Selects documents if element in the array field matches all the specified $elemMatch conditions.
3
$size: Selects documents if the array field is a specified size.

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Evaluation Operators: Evaluation operators in MongoDB allow you to evaluate the data in a document based on more complex logic, such as regular expressions, schema validation, or even custom JavaScript functions.

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$regex: Provides regular expression capabilities for pattern matching strings in queries.
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$mod: Performs a modulo operation on the value of a field and selects documents with a specified result.
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$text: Performs a text search on the content of the fields indexed with a text index. This is much more powerful than $regex for large blocks of text.
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$expr: Allows the use of aggregation expressions within the query language. This is powerful because it lets you compare two different fields within the same document.
5
$jsonSchema: Validates documents against a given JSON Schema. While usually used for collection validation, it can be used in a query.
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$where: Passes a string containing a JavaScript function or a full JavaScript function to the query system.

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Bitwise Operators: allow you to perform logic on the individual bits of integer values. For a senior developer, these are highly efficient tools for managing bitmasks, such as user permissions, feature flags, or status codes where multiple states are packed into a single number to save space.

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$bitsAllSet: Matches documents where all the bit positions specified in the query are set (1) in the document's field. Checking if a user has both Read and Write permissions.
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$bitsAnySet: Matches documents where at least one of the specified bit positions is set (1). Checking if a user has either Admin OR Superuser status.
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$bitsAllClear: Matches documents where all the bit positions specified are clear (0). Finding accounts that have no restrictions or blocks set.
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$bitsAnyClear: Matches documents where at least one of the specified bit positions is clear (0). Finding items that are missing at least one required validation flag.