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Title: Query Operators in MongoDB

MongoDB provides a set of operators to define queries more precisely. These operators are essential for filtering, matching, and evaluating documents based on certain conditions. Query operators are typically used within queries to specify the criteria that documents must meet.

1. Comparison Operators: $eq, $ne, $gt, $gte, $lt, $lte, $in

Comparison operators are used to compare field values in a document against specified values. These operators allow for more complex filtering in queries, enabling precise control over the documents retrieved from a collection.

  • $eq (Equal): Matches documents where the field is equal to the specified value.
  • $ne (Not Equal): Matches documents where the field is not equal to the specified value.
  • $gt (Greater Than): Matches documents where the field is greater than the specified value.
  • $gte (Greater Than or Equal): Matches documents where the field is greater than or equal to the specified value.
  • $lt (Less Than): Matches documents where the field is less than the specified value.
  • $lte (Less Than or Equal): Matches documents where the field is less than or equal to the specified value.
  • $in: Matches any of the values specified in an array.

Examples:

  • $eq (Equal):

    db.users.find({ age: { $eq: 30 } });
// Finds users where the "age" field is exactly 30.

  • $ne (Not Equal):

    db.users.find({ age: { $ne: 30 } });
// Finds users where the "age" field is not 30.

  • $gt (Greater Than):

    db.users.find({ age: { $gt: 25 } });
// Finds users where the "age" field is greater than 25.

  • $gte (Greater Than or Equal):

    db.users.find({ age: { $gte: 25 } });
// Finds users where the "age" field is greater than or equal to 25.

  • $lt (Less Than):

    db.users.find({ age: { $lt: 30 } });
// Finds users where the "age" field is less than 30.

  • $lte (Less Than or Equal):

    db.users.find({ age: { $lte: 30 } });
// Finds users where the "age" field is less than or equal to 30.

  • $in (Matches Any in Array):

    db.users.find({ age: { $in: [25, 30, 35] } });
// Finds users where the "age" field is 25, 30, or 35.

2. Logical Operators: $and, $or, $nor, $not

Logical operators are used to combine multiple query conditions. They allow for complex queries that need to satisfy multiple conditions.

  • $and: Joins multiple queries together and returns documents that match all conditions.
  • $or: Joins multiple queries together and returns documents that match at least one condition.
  • $nor: Returns documents that do not match any of the conditions.
  • $not: Inverts the effect of another query operator.

Examples:

  • $and (Logical AND):

    db.users.find({ $and: [{ age: { $gt: 25 } }, { isEmployed: true }] });
// Finds users where the age is greater than 25 AND they are employed.

  • $or (Logical OR):

    db.users.find({ $or: [{ age: { $lt: 25 } }, { isEmployed: false }] });
// Finds users where the age is less than 25 OR they are not employed.

  • $nor (Logical NOR):

    db.users.find({ $nor: [{ age: { $lt: 25 } }, { isEmployed: true }] });
// Finds users where the age is not less than 25 AND they are not employed.

  • $not (Logical NOT):

    db.users.find({ age: { $not: { $gt: 30 } } });
// Finds users where the age is not greater than 30 (i.e., age is less than or equal to 30).

3. Evaluation Operators: $regex, $text, $where

Evaluation operators provide advanced ways to match and evaluate documents based on specific criteria, such as regular expressions, full-text search, or custom JavaScript functions.

  • $regex: Matches documents where the field matches a regular expression.
  • $text: Performs a full-text search on a collection. This requires a text index.
  • $where: Allows the use of JavaScript expressions to match documents.

Examples:

  • $regex (Regular Expression):

    db.users.find({ name: { $regex: /^A/ } });
// Finds users where the "name" field starts with the letter "A".

  • $text (Full-Text Search):

    db.articles.createIndex({ content: "text" });
db.articles.find({ $text: { $search: "MongoDB" } });
// Finds articles where the "content" field contains the word "MongoDB".

  • $where (Custom JavaScript Expression):

    db.users.find({ $where: "this.age > 30 && this.isEmployed == true" });
// Finds users where the age is greater than 30 and they are employed.

Conclusion

These query operators provide powerful ways to interact with and filter data in MongoDB. Mastering them is essential for building efficient and complex queries, which is a critical skill for developers working with MongoDB, especially in high-performance environments.