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👐 WHERE → .find()

Similar to SQL's WHERE clause, the .find() method in MongoDB retrieves documents from a collection that matches a specified query.

Syntax​

db.collection.find({ <query> })
  • <query>: Specifies conditions to filter documents

Example: Find all books from 2020​

db.books.find({ year: 2020 });

Equivalent SQL query​

SELECT * FROM books WHERE year = 2020;

Filtering with find()​

The find() method takes a document as its first argument. This document specifies the filter criteria. You can use a variety of expressions within the filter document:

  • Comparison operators: $eq (equals), $ne (not equals), $gt (greater than), $lt (less than), $gte (greater than or equals), $lte (less than or equals), $in (in an array), $nin (not in an array)
  • Logical operators: $and, $or, $not
  • Element operators: $exists (check for field existence), $type (check data type)
  • Evaluation operators: $regex (regular expression matching), $where (JavaScript code execution)
  • Geo-spatial operators: For location-based queries
  • Array operators: For querying arrays

Now, let's utilize a few MongoDB operators and create more sophisticated queries:

$or and $gt operators​

Suppose we want to get all of the books that were written in 2010 or that have more than 200 pages.

  SELECT * FROM books WHERE year = 2010 OR pages > 200;

Equivalent MongoDB query:

db.books.find({
  $or: [
    { year: 2010 }, // <--- 1st condition
    { pages: { $gt: 200 } }, // <--- 2nd condition
  ],
});

$and operator​

This time, instead of OR, let's query using AND:

  SELECT * FROM books WHERE year = 2010 AND pages > 200;

Equivalent MongoDB query:

db.books.find({
  $and: [
    { year: 2010 }, // <--- 1st condition
    { pages: { $gt: 200 } }, // <--- 2nd condition
  ],
});

Shorthand $and​

When we are querying on two different fields and want to utilize $and, we can do so by passing a document with all the conditions, like this:

db.books.find({ year: 2010, pages: { $gt: 200 } });

As you can see, we don't have to pass an array of conditions. MongoDB implicitly considers this as $and.

👐 Challenge​

Now, translate the following into a MongoDB query.

1. Find all books where totalInventory is exactly 5.​

Answer
  db.books.find({ totalInventory: 5 });

2. Find all books with more than 300 pages.​

Answer
  db.books.find({ pages: {$gt: 300} });

3. Find books in the Science genre that are more than 300 pages long.​

Answer
  db.books.find({ genres: "Science", pages: {$gt: 300} });