duckdb-query

name	duckdb-query
description	Queries local and remote files (Parquet, CSV, JSON) using DuckDB as SQL engine. Use for data analysis, file inspection, S3 queries, or answering questions about tabular data.

DuckDB Query Engine

Use DuckDB CLI to query local and remote data files without loading them into a database.

Prerequisites

Ensure DuckDB is installed: duckdb --version. If not, install via brew install duckdb (macOS) or see https://duckdb.org/docs/installation.

Workflow

1. For S3 Files (AWS)

Always configure credentials first before querying S3 paths:

-- Use AWS credential chain (reads from env, ~/.aws/credentials, IAM role, etc.)
CREATE OR REPLACE SECRET s3_secret (
    TYPE s3,
    PROVIDER credential_chain
);

To verify credentials work:

SELECT * FROM 's3://bucket/path/file.parquet' LIMIT 1;

If you need a specific profile:

CREATE OR REPLACE SECRET s3_secret (
    TYPE s3,
    PROVIDER credential_chain,
    PROFILE 'my-profile'
);

2. Query Files Directly

# Quick query (in-memory)
duckdb -c "SELECT * FROM 'data.parquet' LIMIT 10"
duckdb -c "SELECT * FROM 'data.csv' LIMIT 10"
duckdb -c "SELECT * FROM 'data.json' LIMIT 10"
duckdb -c "SELECT * FROM 'data.csv.gz' LIMIT 10"

# S3 files
duckdb -c "CREATE SECRET (TYPE s3, PROVIDER credential_chain); SELECT * FROM 's3://bucket/file.parquet' LIMIT 10"

3. Get Table Metadata

-- Column names and types
DESCRIBE SELECT * FROM 'file.parquet';

-- Or using pragma
PRAGMA table_info('file.parquet');

-- Parquet metadata
SELECT * FROM parquet_metadata('file.parquet');
SELECT * FROM parquet_schema('file.parquet');

4. Persist Data Locally (Large Files)

For large files or repeated queries, create a persistent database:

duckdb mydata.db

-- Create table from remote file
CREATE TABLE sales AS SELECT * FROM 's3://bucket/sales.parquet';

-- Create temporary table (session only)
CREATE TEMP TABLE temp_data AS SELECT * FROM 'large_file.csv';

DuckDB-Specific Syntax

EXCLUDE - Remove Columns from SELECT *

-- All columns except 'id' and 'created_at'
SELECT * EXCLUDE (id, created_at) FROM 'data.parquet';

-- With table alias
SELECT t.* EXCLUDE (internal_field) FROM 'data.parquet' t;

REPLACE - Transform Columns in SELECT *

-- Replace 'price' with rounded value
SELECT * REPLACE (round(price, 2) AS price) FROM 'sales.parquet';

-- Multiple replacements
SELECT * REPLACE (upper(name) AS name, price * 1.1 AS price) FROM 'products.csv';

COLUMNS - Dynamic Column Selection

-- Select columns matching pattern
SELECT COLUMNS('.*_id') FROM 'data.parquet';

-- Apply function to matching columns
SELECT COLUMNS('amount.*')::DECIMAL(10,2) FROM 'transactions.csv';

-- All numeric columns
SELECT MIN(COLUMNS(* EXCLUDE (name, date))) FROM 'data.parquet';

-- Using regex
SELECT COLUMNS('(first|last)_name') FROM 'users.csv';

COLUMNS with Lambda

-- Select columns where name contains 'price'
SELECT COLUMNS(c -> c LIKE '%price%') FROM 'products.parquet';

-- Select only numeric columns
SELECT COLUMNS(c -> typeof(c) IN ('INTEGER', 'DOUBLE')) FROM 'data.parquet';

Pattern Matching in Column Selection

-- LIKE pattern
SELECT * LIKE '%amount%' FROM 'transactions.parquet';

-- GLOB pattern
SELECT * GLOB '*_date' FROM 'events.csv';

Reading Multiple Files

-- Glob patterns
SELECT * FROM 'data/*.parquet';
SELECT * FROM 'logs/2024-*.csv';

-- Multiple specific files
SELECT * FROM read_parquet(['file1.parquet', 'file2.parquet']);

-- With filename column
SELECT *, filename FROM 'data/*.parquet';

-- Hive-partitioned data
SELECT * FROM 'data/*/*.parquet' WHERE year = 2024;

Output Formats

Default: Use -jsonlines for compact LLM-friendly output (one JSON object per row, no wasted tokens on table formatting):

# PREFERRED: JSONLines - compact, one object per line (best for LLM agents)
duckdb -jsonlines -c "SELECT * FROM 'data.parquet' LIMIT 5"

# JSON array - when you need a valid JSON array
duckdb -json -c "SELECT * FROM 'data.parquet' LIMIT 5"

# CSV - compact, good for exports
duckdb -csv -c "SELECT * FROM 'data.parquet' LIMIT 5"

Avoid for LLM queries: -markdown, -box, -duckbox, -table (verbose, wastes context tokens)

Only use table formats when explicitly showing results to users in documentation.

Common Patterns

Quick Data Exploration

-- Row count
SELECT COUNT(*) FROM 'data.parquet';

-- Sample rows
SELECT * FROM 'data.parquet' USING SAMPLE 10;

-- Distinct values in a column
SELECT DISTINCT category FROM 'products.csv';

-- Value distribution
SELECT column_name, COUNT(*) FROM 'data.parquet' GROUP BY 1 ORDER BY 2 DESC LIMIT 10;

Schema Discovery

-- Infer and show schema
DESCRIBE SELECT * FROM 'unknown_file.csv';

-- Check for nulls
SELECT COUNT(*) - COUNT(column_name) AS null_count FROM 'data.parquet';

Reference

• DuckDB Documentation
• S3 API Support
• Star Expression (EXCLUDE/COLUMNS)