nosql-2.md

02 — Conditional Queries (Find Many)

Goal: Filter results based on status or numerical ranges. Key Idea: MongoDB uses Operators (words starting with $) instead of math symbols.

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The Core Difference

SQL	MongoDB Operator	Meaning
=	{ field: value }	Equals
>	$gt	Greater Than
>=	$gte	Greater Than or Equal
<	$lt	Less Than
<=	$lte	Less Than or Equal
!=	$ne	Not Equal
IN (...)	$in: [...]	Value is in array
NOT IN (...)	$nin: [...]	Value is not in array
AND	, (comma)	Both conditions must match
OR	$or: [...]	Either condition matches

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Task: Find all "active" users over age 21

SQL Way

SELECT *
FROM Users
WHERE status = 'active'
  AND age > 21;

MongoDB Way

db.users.find({
  status: "active",
  age: { $gt: 21 }
});

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🔍 Breaking Down the MongoDB Syntax

db.users.find({
  status: "active",   // Plain equals: status = 'active'
  age: { $gt: 21 }    // Operator: age > 21
  //     ↑
  //   $gt wraps inside an object { }
});

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More Examples

OR condition

-- SQL
SELECT * FROM Users WHERE status = 'active' OR status = 'pending';

// MongoDB
db.users.find({
  $or: [
    { status: "active" },
    { status: "pending" }
  ]
});

IN list

-- SQL
SELECT * FROM Users WHERE role IN ('admin', 'editor');

// MongoDB
db.users.find({
  role: { $in: ["admin", "editor"] }
});

Range (between)

-- SQL
SELECT * FROM Users WHERE age BETWEEN 18 AND 30;

// MongoDB
db.users.find({
  age: { $gte: 18, $lte: 30 }
});

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💡 Notes

• Multiple conditions inside {} are treated as AND by default.
• Operators always go inside an object: { $gt: 21 }, not $gt: 21 on its own.