// Find all documents
db.students.find()

// Find one document
db.students.findOne({ name: "Ali Hassan" })

// Find by _id
db.students.findOne({ _id: new ObjectId("64a1f2c3e4b0a1b2c3d4e5f6") })

// Find with exact match
db.students.find({ grade: "10th" })

// Find with multiple exact matches — implicit $and
db.students.find({ grade: "10th", enrolled: true })

// Count matching documents
db.students.countDocuments({ grade: "10th" })

// Include specific fields only
db.students.find({ grade: "10th" }, { name: 1, grade: 1 })

// Exclude specific fields
db.students.find({ grade: "10th" }, { address: 0, enrollmentDate: 0 })

// Exclude _id
db.students.find({ grade: "10th" }, { name: 1, grade: 1, _id: 0 })

// Sort ascending
db.students.find().sort({ age: 1 })

// Sort descending
db.students.find().sort({ age: -1 })

// Sort by multiple fields
db.students.find().sort({ grade: 1, name: 1 })

// Limit results
db.students.find().limit(5)

// Skip results
db.students.find().skip(10).limit(5)

// Sort + limit + skip together
db.students.find({ enrolled: true }).sort({ name: 1 }).skip(5).limit(5)

// Equal
db.students.find({ age: { $eq: 16 } })

// Not equal
db.students.find({ age: { $ne: 16 } })

// Greater than
db.students.find({ age: { $gt: 15 } })

// Greater than or equal
db.students.find({ age: { $gte: 15 } })

// Less than
db.students.find({ age: { $lt: 18 } })

// Less than or equal
db.students.find({ age: { $lte: 18 } })

// Range — between 15 and 17 inclusive
db.students.find({ age: { $gte: 15, $lte: 17 } })

// In a list of values
db.students.find({ grade: { $in: ["9th", "10th", "11th"] } })

// Not in a list of values
db.students.find({ grade: { $nin: ["9th", "12th"] } })

// Date range
db.students.find({
  enrollmentDate: {
    $gte: new Date("2024-01-01"),
    $lte: new Date("2024-12-31")
  }
})

// AND — explicit
db.students.find({
  $and: [{ grade: "10th" }, { enrolled: true }]
})

// AND — implicit shorthand
db.students.find({ grade: "10th", enrolled: true })

// OR
db.students.find({
  $or: [{ grade: "9th" }, { grade: "11th" }]
})

// NOT
db.students.find({ age: { $not: { $gt: 16 } } })

// NOR
db.students.find({
  $nor: [{ grade: "9th" }, { enrolled: false }]
})

// AND combined with OR
db.students.find({
  enrolled: true,
  $or: [{ grade: "10th" }, { age: { $gt: 16 } }]
})

// Query nested field with dot notation
db.students.find({ "address.city": "Lahore" })

// Query deeply nested field
db.students.find({ "address.country": "Pakistan" })

// Array contains a value
db.students.find({ subjects: "Math" })

// Array contains all specified values
db.students.find({ subjects: { $all: ["Math", "Physics"] } })

// Array has exact size
db.students.find({ subjects: { $size: 3 } })

// Array has at least N elements — index trick
db.students.find({ "subjects.2": { $exists: true } })

// Array of objects — elemMatch
db.students.find({
  grades: {
    $elemMatch: {
      subject: "Math",
      score: { $gt: 90 }
    }
  }
})

// Array of objects — failed any subject
db.students.find({
  grades: {
    $elemMatch: { score: { $lt: 50 } }
  }
})

// Field exists
db.students.find({ phone: { $exists: true } })

// Field does not exist
db.students.find({ phone: { $exists: false } })

// Field is a specific type
db.students.find({ age: { $type: "double" } })
db.students.find({ enrollmentDate: { $type: "date" } })
db.students.find({ subjects: { $type: "array" } })

// Field exists AND is correct type
db.students.find({
  graduationDate: { $exists: true, $type: "date" }
})

// Regex — starts with
db.students.find({ name: { $regex: /^Ali/i } })

// Regex — contains
db.students.find({ name: { $regex: /ahmed/i } })

// Regex — ends with
db.teachers.find({ email: { $regex: /@school\.com$/i } })

// Compare two fields in same document
db.courses.find({
  $expr: { $gt: ["$totalStudents", "$capacity"] }
})

// Text search — requires text index
db.courses.createIndex({ title: "text" })
db.courses.find({ $text: { $search: "science" } })

// Text search — exact phrase
db.courses.find({ $text: { $search: "\"computer science\"" } })

// Text search — exclude word
db.courses.find({ $text: { $search: "science -computer" } })

// Text search — sort by relevance
db.courses.find(
  { $text: { $search: "math" } },
  { score: { $meta: "textScore" } }
).sort({ score: { $meta: "textScore" } })

// All enrolled students sorted by name
db.students.find({ enrolled: true }).sort({ name: 1 })

// Students in 10th grade, showing name and grade only
db.students.find(
  { grade: "10th" },
  { name: 1, grade: 1, _id: 0 }
)

// Students from Lahore who are enrolled
db.students.find({ "address.city": "Lahore", enrolled: true })

// Students aged between 15 and 17 in 10th or 11th grade
db.students.find({
  age: { $gte: 15, $lte: 17 },
  grade: { $in: ["10th", "11th"] }
})

// Students taking both Math and Physics
db.students.find({ subjects: { $all: ["Math", "Physics"] } })

// Students who scored above 90 in any subject
db.students.find({
  grades: { $elemMatch: { score: { $gt: 90 } } }
})

// Students who scored above 90 specifically in Math
db.students.find({
  grades: {
    $elemMatch: { subject: "Math", score: { $gt: 90 } }
  }
})

// Active teachers who joined before 2018
db.teachers.find({
  active: true,
  joinedDate: { $lt: new Date("2018-01-01") }
})

// Courses with between 20 and 30 students
db.courses.find({ totalStudents: { $gte: 20, $lte: 30 } })

// Courses where enrollment exceeds capacity
db.courses.find({
  $expr: { $gt: ["$totalStudents", "$capacity"] }
})

// Page 2 of enrolled students — 10 per page
db.students
  .find({ enrolled: true })
  .sort({ name: 1 })
  .skip(10)
  .limit(10)