# Duckdb SQL

> Load when hitting DuckDB syntax errors or writing DuckDB-specific SQL. Covers gotchas that differ from PostgreSQL/MySQL.

- Skill: `signalpilot-labs/duckdb-sql-2` (Agent Skill)
- Install (CLI): `npx skillmds@latest add signalpilot-labs/duckdb-sql-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/signalpilot-labs/duckdb-sql-2/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Data & Analytics
- Author: signalpilot-labs (https://skillmd.com/u/signalpilot-labs)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/signalpilot-labs/duckdb-sql-2

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# DuckDB SQL - Key Differences from PostgreSQL/MySQL

## Gotchas

- **Integer division truncates**: `5/2 = 2`. Fix: `CAST(numerator AS DOUBLE) / denominator`
- **DATE_TRUNC returns TIMESTAMP**: Cast result if DATE needed: `CAST(DATE_TRUNC('month', col) AS DATE)`
- **INTERVAL syntax**: `INTERVAL '1' DAY` (quoted), NOT `INTERVAL 1 DAY`
- **No DATEADD/DATEDIFF**: In standalone queries, use `col + INTERVAL '1' DAY` and `DATE_DIFF('day', start, end)` - DuckDB has no `DATEADD`/`DATEDIFF` function.
- **SUM(NULL) = NULL**: Not 0. Use `COALESCE(SUM(col), 0)` if 0 is needed.
- **inf/NaN checks**: use `isinf(col)` and `isnan(col)`, NOT `IS_INF`/`IS_NAN` (those are BigQuery function names).
- **ROUND precision**: If the YML specifies a decimal type like `decimal(6,2)`,
  cast the FINAL output to match: `CAST(ROUND(AVG(col), 2) AS DECIMAL(6,2))`.
  Do NOT cast the input - cast the result after rounding.
- **Avoid CURRENT_DATE for date spines** - when generating or capping a calendar over historical data, use `(SELECT MAX(date_col) FROM source)` to anchor to the data's actual date range

## Date Parsing

- Non-ISO strings: `STRPTIME(col, '%d/%m/%Y')::DATE`
- `TRY_STRPTIME` returns NULL on failure (safe)
- Never `CAST(date_str AS DATE)` on non-ISO strings

## QUALIFY Clause

Filter window function results without a subquery:
```sql
SELECT *, ROW_NUMBER() OVER (PARTITION BY group ORDER BY col DESC) AS rn
FROM table
QUALIFY rn <= 10
```

## Date Spines

```sql
SELECT UNNEST(GENERATE_SERIES(min_date::DATE, max_date::DATE, INTERVAL '1' DAY)) AS date_day
```
Always use the primary fact table's max date as endpoint - call `get_date_boundaries` first.

## Type Casting

`CAST(x AS INTEGER)`, `CAST(x AS DOUBLE)`, `CAST(x AS VARCHAR)`, `CAST(x AS DATE)`
`TRY_CAST` returns NULL on failure.