ClickHouse Hello World

Overview

Create a MergeTree table, insert rows with JSONEachRow, and run your first analytical query -- all using the official @clickhouse/client. This is the smoke test that proves your connection works and teaches the four MergeTree concepts (ORDER BY, PARTITION BY, TTL, LowCardinality) reused in every real schema.

Prerequisites

@clickhouse/client installed and connected (see the clickhouse-install-auth skill for connection setup).
A reachable ClickHouse server (local Docker, ClickHouse Cloud, or self-hosted) with CLICKHOUSE_HOST / CLICKHOUSE_USER / CLICKHOUSE_PASSWORD set as environment variables.

Instructions

Step 1: Create a MergeTree Table

import { createClient } from '@clickhouse/client';

const client = createClient({
  url: process.env.CLICKHOUSE_HOST ?? 'http://localhost:8123',
  username: process.env.CLICKHOUSE_USER ?? 'default',
  password: process.env.CLICKHOUSE_PASSWORD ?? '',
});

await client.command({
  query: `
    CREATE TABLE IF NOT EXISTS events (
      event_id    UUID DEFAULT generateUUIDv4(),
      event_type  LowCardinality(String),
      user_id     UInt64,
      payload     String,
      created_at  DateTime DEFAULT now()
    )
    ENGINE = MergeTree()
    ORDER BY (event_type, created_at)
    PARTITION BY toYYYYMM(created_at)
    TTL created_at + INTERVAL 90 DAY
  `,
});
console.log('Table "events" created.');

Key concepts:

MergeTree() -- the foundational ClickHouse engine for analytics
ORDER BY -- defines the primary index (sort key); pick columns you filter/group on
PARTITION BY -- splits data into parts by month for efficient pruning
TTL -- automatic data expiration
LowCardinality(String) -- dictionary-encoded string, ideal for columns with < 10K distinct values

For the full engine menu (ReplacingMergeTree, SummingMergeTree, etc.) and the column-type table, see MergeTree engines & data types.

Step 2: Insert Data with JSONEachRow

await client.insert({
  table: 'events',
  values: [
    { event_type: 'page_view', user_id: 1001, payload: '{"url":"/home"}' },
    { event_type: 'click',     user_id: 1001, payload: '{"button":"signup"}' },
    { event_type: 'page_view', user_id: 1002, payload: '{"url":"/pricing"}' },
    { event_type: 'purchase',  user_id: 1002, payload: '{"amount":49.99}' },
    { event_type: 'page_view', user_id: 1003, payload: '{"url":"/docs"}' },
  ],
  format: 'JSONEachRow',
});
console.log('Inserted 5 events.');

Step 3: Query the Data

// Count events by type
const rs = await client.query({
  query: `
    SELECT
      event_type,
      count()          AS total,
      uniqExact(user_id) AS unique_users
    FROM events
    GROUP BY event_type
    ORDER BY total DESC
  `,
  format: 'JSONEachRow',
});

const rows = await rs.json<{
  event_type: string;
  total: string;        // ClickHouse returns numbers as strings in JSON
  unique_users: string;
}>();

for (const row of rows) {
  console.log(`${row.event_type}: ${row.total} events, ${row.unique_users} users`);
}

Step 4: Explore System Tables (optional)

Once data lands, inspect on-disk size and part counts via system.parts to confirm your partitioning is healthy. Full query and column reference: exploring system tables.

Output

Running the three core steps against a fresh table produces:

Step 1 -- Table "events" created. (idempotent via IF NOT EXISTS).
Step 2 -- Inserted 5 events.
Step 3 -- one aggregated row per event_type, sorted by count descending:

page_view: 3 events, 3 users
click: 1 events, 1 users
purchase: 1 events, 1 users

ClickHouse returns numeric aggregates as JSON strings, so cast total / unique_users before doing arithmetic in TypeScript.

Error Handling

Error	Cause	Solution
`Table already exists`	Re-running CREATE	Use `IF NOT EXISTS`
`Unknown column`	Typo in column name	Check `DESCRIBE TABLE events`
`Type mismatch`	Wrong data type in insert	Match types to schema
`Memory limit exceeded`	Query too broad	Add WHERE clauses, use LIMIT

Examples

Steps 1-3 form the canonical end-to-end example: create → insert → aggregate. Two extensions live in the reference files:

Different engine or column types -- swap MergeTree for ReplacingMergeTree (upserts) or add a Decimal(18,2) column: MergeTree engines & data types.
Verifying on-disk layout -- the system.parts size/part-count query: exploring system tables.

Resources

Next Steps

Proceed to the clickhouse-local-dev-loop skill for Docker-based local development and an iterative schema-editing workflow.