Snowflake Sql

---
name: snowflake-sql
description: "Snowflake-specific SQL patterns: QUALIFY for window filtering, LATERAL FLATTEN for arrays, semi-structured VARIANT data, ILIKE for case-insensitive matching, date functions, and time travel."
type: skill
---

# Snowflake SQL Skill

## 1. Window Function Filtering - Use QUALIFY

Instead of wrapping in a subquery, use QUALIFY:

```sql
-- Find the latest record per customer
SELECT customer_id, order_date, amount
FROM orders
QUALIFY ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY order_date DESC) = 1;

-- Top 5 sales ranks including boundary ties
SELECT product_id, total_sales
FROM sales_summary
QUALIFY DENSE_RANK() OVER (ORDER BY total_sales DESC) <= 5;
```

## 2. Case-Insensitive Matching - Use ILIKE

```sql
-- Case-insensitive LIKE
WHERE product_name ILIKE '%widget%'

-- Case-insensitive equality
WHERE UPPER(status) = 'ACTIVE'
-- or
WHERE status ILIKE 'active'
```

## 3. Arrays and Semi-Structured Data - LATERAL FLATTEN

```sql
-- Explode an ARRAY column
SELECT t.id, f.value AS item
FROM table t,
LATERAL FLATTEN(input => t.array_col) f;

-- Access VARIANT fields
SELECT col:field_name::STRING AS field_value
FROM table;

-- Parse JSON string
SELECT PARSE_JSON(json_col):key::STRING AS val
FROM table;
```

## 4. Date Functions

```sql
-- Add/subtract time
DATEADD(day, 7, order_date)          -- 7 days from order_date
DATEADD(month, -1, current_date())   -- 1 month ago

-- Difference between dates
DATEDIFF(day, start_date, end_date)  -- days between dates
DATEDIFF(month, start_date, end_date)

-- Truncate to period
DATE_TRUNC('month', event_ts)
DATE_TRUNC('year', event_ts)

-- Current timestamp
CURRENT_TIMESTAMP()
CURRENT_DATE()
```

## 5. String Functions

```sql
SPLIT_PART(col, '/', 1)              -- split by delimiter, get Nth part
REGEXP_SUBSTR(col, '[0-9]+')         -- first match of regex
TRIM(col)                            -- remove leading/trailing whitespace
LTRIM(col, '0')                      -- remove leading zeros
UPPER(col) / LOWER(col)
CONCAT(col1, '-', col2)              -- or col1 || '-' || col2
```

## 6. Null-Safe Equality

```sql
-- NULL-safe: TRUE when both are NULL or both equal
col1 IS NOT DISTINCT FROM col2

-- COALESCE for default values
COALESCE(col, 'unknown')
```

## 7. Time Travel (querying historical data)

```sql
-- Query table as it was 1 hour ago
SELECT * FROM my_table AT (OFFSET => -3600);

-- Query at a specific timestamp
SELECT * FROM my_table AT (TIMESTAMP => '2024-01-01'::TIMESTAMP);
```

## 8. Common Anti-Patterns to Avoid

- Do NOT use `= NULL` - use `IS NULL`
- Do NOT use `<>` for NULL comparison - use `IS NOT NULL`
- Prefer `QUALIFY` over subquery wrapping for window filters
- When accessing VARIANT fields, always cast: `col:field::STRING`

## 9. Dialect Patterns

- **Numeric precision**: Snowflake returns DECIMAL/NUMBER with configurable precision. Do NOT cast to FLOAT unless needed - precision loss changes exact numeric results.
- **IDENTIFIER case**: Snowflake upper-cases identifiers by default. Use double-quotes `"lower_case_col"` when column names are lowercase in source. Always check with `describe_table`.
- **LISTAGG**: Use `LISTAGG(col, ',') WITHIN GROUP (ORDER BY col)` for string aggregation (not GROUP_CONCAT).
- **TRY_CAST / TRY_TO_NUMBER**: Use for safe type conversion that returns NULL instead of error.
- **OBJECT_KEYS / ARRAY_SIZE**: Useful for introspecting semi-structured data before querying.