// Find documents where the 'phone' field EXISTS (any value, even null)
db.users.find({ phone: { $exists: true } })

// Find documents where the 'phone' field does NOT EXIST
db.users.find({ phone: { $exists: false } })

// Find documents where the optional 'middleName' field is present
db.people.find({ middleName: { $exists: true } })

// Combine $exists with other operators
// Find documents where 'deletedAt' is present and is a date
db.posts.find({
  deletedAt: { $exists: true, $type: 'date' }
})

// { field: null } matches BOTH documents with null value AND documents missing the field
db.users.find({ phone: null })
// Matches: { name: 'Alice', phone: null }      <- null value
// Matches: { name: 'Bob' }                     <- field is absent
// Does NOT match: { name: 'Carol', phone: '555-1234' }

// { field: { $eq: null } } — same behavior as above
db.users.find({ phone: { $eq: null } })

// To match ONLY documents where field EXISTS but is null:
db.users.find({ phone: { $exists: true, $eq: null } })
// Matches: { name: 'Alice', phone: null }      <- null value (yes)
// Does NOT match: { name: 'Bob' }              <- field is absent (no)

// To match ONLY documents where field is MISSING:
db.users.find({ phone: { $exists: false } })

// Find documents where 'name' is stored as a string
db.users.find({ name: { $type: 'string' } })

// Find documents where 'age' is stored as a number (int or double)
db.users.find({ age: { $type: 'number' } })
// 'number' is an alias that matches double, int, long, and decimal

// Find documents where 'tags' is stored as an array
db.posts.find({ tags: { $type: 'array' } })

// Using numeric type codes (less readable but equivalent)
db.users.find({ name: { $type: 2 } })   // 2 = string
db.users.find({ age: { $type: 16 } })   // 16 = 32-bit int

// Find documents where 'tags' is stored as an array
// Useful when the same field may be a string in some docs and an array in others
db.posts.find({ tags: { $type: 'array' } })

// Find documents where 'tags' is a plain string (not an array)
db.posts.find({ tags: { $type: 'string' } })

// Useful during migrations where schema was inconsistent:
// Some old docs: { tags: 'mongodb' }     <- string
// Some new docs: { tags: ['mongodb'] }   <- array

// Match documents where 'age' is stored as any numeric type
db.users.find({
  age: { $type: ['int', 'double', 'long'] }
})

// Match documents where a field is either a string or null
db.products.find({
  description: { $type: ['string', 'null'] }
})

// Numeric type codes also work in arrays
db.users.find({
  age: { $type: [16, 18, 1] }  // int32, int64, double
})

// Find documents where 'price' was accidentally stored as a string
// (should be a number)
db.products.find({ price: { $type: 'string' } })

// Find documents where 'createdAt' was stored as a string instead of Date
db.users.find({ createdAt: { $type: 'string' } })

// Find documents where 'isActive' is stored as a string instead of boolean
db.users.find({ isActive: { $type: 'string' } })

// Count how many documents have the wrong type (for impact assessment)
db.products.countDocuments({ price: { $type: 'string' } })

// Fix them: convert string prices to numbers
db.products.find({ price: { $type: 'string' } }).forEach(doc => {
  db.products.updateOne(
    { _id: doc._id },
    { $set: { price: parseFloat(doc.price) } }
  )
})