DuckDB

DuckDB

Overview

DuckDB is a high-performance, in-process analytical database management system (often called "SQLite for analytics"). Execute complex SQL queries directly on CSV, Parquet, JSON files, and Python DataFrames (pandas, Polars) without importing data or running a separate database server.

When to Use This Skill

Activate when the user:

• Wants to run SQL queries on data files (CSV, Parquet, JSON)
• Needs to perform complex analytical queries (aggregations, joins, window functions)
• Asks to query pandas or Polars DataFrames using SQL
• Wants to explore or analyze data without loading it into memory
• Needs fast analytical performance on medium to large datasets
• Mentions DuckDB explicitly or wants OLAP-style analytics

Installation

Check if DuckDB is installed:

python3 -c "import duckdb; print(duckdb.__version__)"

If not installed:

pip3 install duckdb

For Polars integration:

pip3 install duckdb 'polars[pyarrow]'

Core Capabilities

1. Querying Data Files Directly

DuckDB can query files without loading them into memory:

import duckdb

# Query CSV file
result = duckdb.sql("SELECT * FROM 'data.csv' WHERE age > 25")
print(result.df())  # Convert to pandas DataFrame

# Query Parquet file
result = duckdb.sql("""
    SELECT category, SUM(amount) as total
    FROM 'sales.parquet'
    GROUP BY category
    ORDER BY total DESC
""")

# Query JSON file
result = duckdb.sql("SELECT * FROM 'users.json' LIMIT 10")

# Query multiple files with wildcards
result = duckdb.sql("SELECT * FROM 'data/​*.parquet'")

2. Working with Pandas DataFrames

DuckDB can directly query pandas DataFrames:

import duckdb
import pandas as pd

# Create or load a DataFrame
df = pd.read_csv('data.csv')

# Query the DataFrame using SQL
result = duckdb.sql("""
    SELECT
        category,
        AVG(price) as avg_price,
        COUNT(*) as count
    FROM df
    WHERE price > 100
    GROUP BY category
    HAVING count > 5
""")

# Convert result to pandas DataFrame
result_df = result.df()
print(result_df)

3. Working with Polars DataFrames

DuckDB integrates seamlessly with Polars using Apache Arrow:

import duckdb
import polars as pl

# Create or load a Polars DataFrame
df = pl.read_csv('data.csv')

# Query Polars DataFrame with DuckDB
result = duckdb.sql("""
    SELECT
        date_trunc('month', date) as month,
        SUM(revenue) as monthly_revenue
    FROM df
    GROUP BY month
    ORDER BY month
""")

# Convert result to Polars DataFrame
result_df = result.pl()

# For lazy evaluation, use lazy=True
lazy_result = result.pl(lazy=True)

4. Creating Persistent Databases

Create database files for persistent storage:

import duckdb

# Connect to a persistent database (creates file if doesn't exist)
con = duckdb.connect('my_database.duckdb')

# Create table and insert data
con.execute("""
    CREATE TABLE users AS
    SELECT * FROM 'users.csv'
""")

# Query the database
result = con.execute("SELECT * FROM users WHERE age > 30").fetchdf()

# Close connection
con.close()

5. Complex Analytical Queries

DuckDB excels at analytical queries:

import duckdb

# Window functions
result = duckdb.sql("""
    SELECT
        name,
        department,
        salary,
        AVG(salary) OVER (PARTITION BY department) as dept_avg,
        RANK() OVER (PARTITION BY department ORDER BY salary DESC) as dept_rank
    FROM 'employees.csv'
""")

# CTEs and subqueries
result = duckdb.sql("""
    WITH monthly_sales AS (
        SELECT
            date_trunc('month', sale_date) as month,
            product_id,
            SUM(amount) as total_sales
        FROM 'sales.parquet'
        GROUP BY month, product_id
    )
    SELECT
        m.month,
        p.product_name,
        m.total_sales,
        LAG(m.total_sales) OVER (
            PARTITION BY m.product_id
            ORDER BY m.month
        ) as prev_month_sales
    FROM monthly_sales m
    JOIN 'products.csv' p ON m.product_id = p.id
    ORDER BY m.month DESC, m.total_sales DESC
""")

6. Joins Across Different Data Sources

Join data from multiple files and DataFrames:

import duckdb
import pandas as pd

# Load DataFrame
customers_df = pd.read_csv('customers.csv')

# Join DataFrame with Parquet file
result = duckdb.sql("""
    SELECT
        c.customer_name,
        c.email,
        o.order_date,
        o.total_amount
    FROM customers_df c
    JOIN 'orders.parquet' o ON c.customer_id = o.customer_id
    WHERE o.order_date >= '2024-01-01'
    ORDER BY o.order_date DESC
""")

Common Patterns

Pattern 1: Quick Data Exploration

import duckdb

# Get table schema
duckdb.sql("DESCRIBE SELECT * FROM 'data.parquet'").show()

# Quick statistics
duckdb.sql("""
    SELECT
        COUNT(*) as rows,
        COUNT(DISTINCT user_id) as unique_users,
        MIN(created_at) as earliest_date,
        MAX(created_at) as latest_date
    FROM 'data.csv'
""").show()

# Sample data
duckdb.sql("SELECT * FROM 'large_file.parquet' USING SAMPLE 1000").show()

Pattern 2: Data Transformation Pipeline

import duckdb

# ETL pipeline using DuckDB
con = duckdb.connect('analytics.duckdb')

# Extract and transform
con.execute("""
    CREATE TABLE clean_sales AS
    SELECT
        date_trunc('day', timestamp) as sale_date,
        UPPER(TRIM(product_name)) as product_name,
        quantity,
        price,
        quantity * price as total_amount,
        CASE
            WHEN quantity > 10 THEN 'bulk'
            ELSE 'retail'
        END as sale_type
    FROM 'raw_sales.csv'
    WHERE price > 0 AND quantity > 0
""")

# Create aggregated view
con.execute("""
    CREATE VIEW daily_summary AS
    SELECT
        sale_date,
        sale_type,
        COUNT(*) as num_sales,
        SUM(total_amount) as revenue
    FROM clean_sales
    GROUP BY sale_date, sale_type
""")

result = con.execute("SELECT * FROM daily_summary ORDER BY sale_date DESC").fetchdf()
con.close()

Pattern 3: Combining DuckDB + Polars for Optimal Performance

import duckdb
import polars as pl

# Read multiple parquet files with Polars
df = pl.read_parquet('data/​*.parquet')

# Use DuckDB for complex SQL analytics
result = duckdb.sql("""
    SELECT
        customer_segment,
        product_category,
        COUNT(DISTINCT customer_id) as customers,
        SUM(revenue) as total_revenue,
        AVG(revenue) as avg_revenue,
        PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY revenue) as median_revenue
    FROM df
    WHERE order_date >= CURRENT_DATE - INTERVAL '30 days'
    GROUP BY customer_segment, product_category
    HAVING total_revenue > 10000
    ORDER BY total_revenue DESC
""").pl()  # Return as Polars DataFrame

# Continue processing with Polars
final_result = result.with_columns([
    (pl.col('total_revenue') / pl.col('customers')).alias('revenue_per_customer')
])

Pattern 4: Export Query Results

import duckdb

# Export to CSV
duckdb.sql("""
    COPY (
        SELECT * FROM 'input.parquet' WHERE status = 'active'
    ) TO 'output.csv' (HEADER, DELIMITER ',')
""")

# Export to Parquet
duckdb.sql("""
    COPY (
        SELECT date, category, SUM(amount) as total
        FROM 'sales.csv'
        GROUP BY date, category
    ) TO 'summary.parquet' (FORMAT PARQUET)
""")

# Export to JSON
duckdb.sql("""
    COPY (SELECT * FROM users WHERE age > 21)
    TO 'filtered_users.json' (FORMAT JSON)
""")

Performance Tips

1. Use Parquet for large datasets: Parquet is columnar and compressed, ideal for analytical queries
2. Filter early: Push filters down to file reads when possible
3. Partition large files: Use DuckDB's automatic partitioning for large datasets
4. Use projections: Only select columns you need
5. Leverage indexes: For persistent databases, create indexes on frequently queried columns

# Good: Filter and project early
duckdb.sql("SELECT name, age FROM 'users.parquet' WHERE age > 25")

# Less efficient: Select all then filter
duckdb.sql("SELECT * FROM 'users.parquet'").df()[lambda x: x['age'] > 25]

Integration with Polars

DuckDB and Polars work together seamlessly via Apache Arrow:

import duckdb
import polars as pl

# Polars for data loading and transformation
df = (
    pl.scan_parquet('data/​*.parquet')
    .filter(pl.col('date') >= '2024-01-01')
    .collect()
)

# DuckDB for complex SQL analytics
result = duckdb.sql("""
    SELECT
        user_id,
        COUNT(*) as sessions,
        SUM(duration) as total_duration,
        AVG(duration) as avg_duration,
        MAX(duration) as max_duration
    FROM df
    GROUP BY user_id
    HAVING sessions > 5
""").pl()

# Back to Polars for final processing
top_users = result.top_k(10, by='total_duration')

See the polars skill for more Polars-specific operations and the references/​integration.md file for detailed integration examples.

Error Handling

Common issues and solutions:

import duckdb

try:
    result = duckdb.sql("SELECT * FROM 'data.csv'")
except duckdb.Error as e:
    print(f"DuckDB error: {e}")
except FileNotFoundError:
    print("File not found")
except Exception as e:
    print(f"Unexpected error: {e}")

Resources

• references/​integration.md: Detailed examples of DuckDB + Polars integration patterns
• Official docs: https://duckdb.org/docs/
• Python API: https://duckdb.org/docs/api/python/overview