Learn MQL with Real Code Examples

// Find users aged over 25

db.users.find({ age: { $gt: 25 } })

// Find users, only return name and email

db.users.find({}, { name: 1, email: 1 })

// Aggregation: group by status and count

db.orders.aggregate([
    { $group: { _id: "$status", total: { $sum: 1 } } }
])

Examples of finding documents, projecting fields, and using aggregation in MongoDB.

-- Select all users
SELECT * FROM users;

-- Select specific columns
SELECT name, email FROM users;

-- Select with filter
SELECT * FROM users WHERE age > 25;

-- Order results
SELECT * FROM users ORDER BY created_at DESC;

Simple SQL operations: SELECT, WHERE, ORDER BY.

-- Inner Join
SELECT u.name, o.id
FROM users u
INNER JOIN orders o ON u.id = o.user_id;

-- Left Join
SELECT u.name, o.id
FROM users u
LEFT JOIN orders o ON u.id = o.user_id;

-- Right Join
SELECT u.name, o.id
FROM users u
RIGHT JOIN orders o ON u.id = o.user_id;

Examples of INNER JOIN, LEFT JOIN, and RIGHT JOIN in PostgreSQL.

# Set and Get
SET user:1 "Alice"
GET user:1

# Hash operations
HSET user:2 name "Bob" age 30
HGETALL user:2

# List operations
LPUSH tasks "task1" "task2"
LRANGE tasks 0 -1

Basic Redis key-value operations.

-- Count users
SELECT COUNT(*) FROM users;

-- Average age
SELECT AVG(age) FROM users;

-- Total orders
SELECT SUM(amount) FROM orders;

Aggregations like COUNT, AVG, SUM in MySQL.

-- Create table
CREATE TABLE users (
    id INTEGER PRIMARY KEY,
    name TEXT,
    email TEXT
);

-- Insert row
INSERT INTO users (name, email) VALUES ('Alice', 'alice@mail.com');

-- Query
SELECT * FROM users;

Creating tables and inserting rows in SQLite.

-- Create keyspace
CREATE KEYSPACE shop WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 1};

-- Use keyspace
USE shop;

-- Create table
CREATE TABLE users (id UUID PRIMARY KEY, name TEXT, age INT);

-- Insert
INSERT INTO users (id, name, age) VALUES (uuid(), 'Alice', 29);

-- Select
SELECT * FROM users;

Basic CQL queries for Cassandra.

// Create nodes
CREATE (a:Person {name: "Alice"}), (b:Person {name: "Bob"});

// Create relationship
MATCH (a:Person {name: "Alice"}), (b:Person {name: "Bob"})
CREATE (a)-[:FRIEND]->(b);

// Query
MATCH (p:Person)-[:FRIEND]->(f)
RETURN p.name, f.name;

Basic graph queries using Cypher in Neo4j.

{
    "query": {
        "bool": {
        "must": [
        { "match": {title: "database" } },
        { "range": { "year": { "gte": 2020 } } }
        ]
        }
    }
}

JSON-based query DSL examples in Elasticsearch.

-- Create measurement
INSERT temperature,location=room1 value=23.5

-- Select with time filter
SELECT value FROM temperature WHERE time > now() - 1h;

-- Aggregate
SELECT MEAN(value) FROM temperature WHERE time > now() - 24h GROUP BY time(1h);

InfluxQL examples for querying time-series data.