MongoDB Operators & Vocabulary: SQL Intent to Documents

Mongo speaks documents, so what you’re translating is the operation’s intent, not SQL syntax.

{ field: {$op: value} }

• Equal $eq: the default, no operator needed. {field: <cond>}
• Others: { age: {$gt: 30} }
• { age: { $gt: 20, $lt: 40 } } // 20 < age < 40

Common operators

• Comparison
  • $gt / $gte / $lt / $lte
  • $eq / $ne
  • $in / $nin: matches any / matches none in a list, like SQL’s IN
    • { type: { $in: ["a","b","c"] } }
• String
  • $regex: like SQL’s LIKE
    • { name: { $regex: "abc", $options: "i" } } (i = case-insensitive)
• Existence
  • $exists: whether this field is present at all
    • In Mongo, Absent ≠ null: username: null means “the field is there, explicitly with no value”; Absent means “the document doesn’t have this key at all”
• Logical — $or / $and / $nor take an array
  • { $or: [ { age: { $lt: 18 } }, { vip: true } ] }

SQL → MongoDB

SQL	MongoDB
SELECT col	{ $project: {...} } (or the find projection parameter)
GROUP BY	{ $group: { _id: "$field", ... } }
ORDER BY	{ $sort: {...} }
IN (...)	{ field: { $in: [a, b, c] } }
IS NULL	{ field: null } or { field: { $exists: false } }
LIMIT	{ $limit: n }

Vocabulary: relational → MongoDB

Relational (Postgres)	MongoDB	Notes
Table	Collection	a group of documents
Row	Document	a JSON/BSON object
Column	Field	but no fixed schema — fields can differ document to document
Schema	(not enforced)	schema is flexible, varies by document
Primary key	_id	auto-added, unique

---

References

• MongoDB Query Operators — https://www.mongodb.com/docs/manual/reference/operator/query/

Related: back to the Cross-Engine DB Query overview; see how LIKE / ILIKE substring search maps to $regex, and how it all ties to the portable SQL core.