SQL Research Skill

Use this skill when researching SQL syntax and behavior for any database backend before implementing translations or features in dbplyr.

When to use this skill

• Before implementing any SQL translation for a database backend
• When you need to understand SQL syntax, behavior, or edge cases
• When documenting database-specific SQL features
• Before writing SQL-generating code in dbplyr

Critical principle

SQL correctness is paramount in dbplyr. You MUST complete research and documentation BEFORE implementing any SQL-related code.

Research workflow

1. Search for official documentation

Use WebSearch to find official documentation for "{dialect} {function/command}":

• Prioritize official database documentation and reputable sources
• Search for syntax, behavior, edge cases, and version-specific differences
• Look for:
• Function signatures and argument types
• Return types and behavior
• NULL handling
• Type coercion rules
• Limitations or restrictions
• Differences across database versions

2. Document your findings

Create research/{dialect}-{command}.md with the following structure:

markdown
# {Dialect} - {Function/Command}

## Summary
[1-2 sentence summary focused on R-to-SQL translation]

## Syntax
[Minimal syntax examples from official sources]

## Key behaviors
[Only behaviors that matter for dbplyr translation]

## Limitations
[Only restrictions that affect dbplyr usage]

## Sources
- [Source name](URL)
- [Source name](URL)

Documentation guidelines:

• Keep it minimal and focused on dbplyr use cases
• Include only what's relevant to translating R code to SQL
• ALL citations with URLs are REQUIRED (no exceptions)
• NO comparisons with other databases
• Use concrete examples from official sources
• Keep it as concise as possible

3. Verify your research

Cross-reference multiple sources when:

• Documentation seems incomplete or unclear
• Behavior differs across database versions
• Edge cases aren't well documented
• Official docs contradict community sources

Best practices:

• Check at least 2-3 authoritative sources
• Note any version-specific differences
• Document uncertainties or ambiguities
• When in doubt, test with actual database if possible

4. Proceed to implementation

Only after completing research and documentation should you:

• Implement SQL translations
• Write SQL-generating code
• Add tests for the functionality

Example research files

Minimal example

markdown
# PostgreSQL - POSITION

## Summary
Returns the starting position of a substring within a string (1-indexed).

## Syntax
POSITION(substring IN string)

## Key behaviors
- Returns integer position (1-indexed)
- Returns 0 if substring not found
- Case-sensitive by default
- NULL if any argument is NULL

## Sources
- [PostgreSQL String Functions](https://www.postgresql.org/docs/current/functions-string.html)

Complex example

markdown
# SQL Server - STRING_AGG

## Summary
Concatenates string values with a specified separator, optionally ordering results.

## Syntax
STRING_AGG(expression, separator) [WITHIN GROUP (ORDER BY order_expression)]

## Key behaviors
- Available in SQL Server 2017+ (compatibility level 110+)
- Returns NULL for empty groups
- Separator must be a literal or variable, not an expression
- WITHIN GROUP clause is optional but commonly used for deterministic ordering
- Maximum output length is 2GB

## Limitations
- Not available in SQL Server 2016 or earlier
- Cannot use with DISTINCT (use subquery instead)
- Separator cannot be a computed expression

## Sources
- [SQL Server STRING_AGG](https://docs.microsoft.com/en-us/sql/t-sql/functions/string-agg-transact-sql)
- [Compatibility requirements](https://docs.microsoft.com/en-us/sql/t-sql/functions/string-agg-transact-sql#compatibility-support)

Common research patterns

String functions

• Character encoding and collation
• 0-indexed vs 1-indexed positions
• NULL handling
• Regular expression support and syntax

Date/time functions

• Date/time types and precision
• Timezone handling
• Format strings and conventions
• Interval arithmetic

Aggregate functions

• NULL handling in aggregates
• Empty group behavior
• DISTINCT support
• Window function variants

Window functions

• OVER clause syntax
• Frame specifications (ROWS vs RANGE)
• Partitioning and ordering
• Function-specific restrictions

Checklist

Before completing SQL research:

• ] Searched official database documentation
• ] Identified syntax and key behaviors
• ] Documented edge cases and limitations
• ] Created research file in research/{dialect}-{function}.md
• ] Included ALL source URLs
• ] Kept documentation minimal and focused
• ] Cross-referenced multiple sources if needed
• ] Ready to proceed with implementation

Tips

• Start broad, then narrow: Search for the general command first, then dig into specifics
• Use official docs first: Official documentation is most authoritative
• Check version availability: Many SQL features are version-specific
• Note NULL behavior: NULL handling often differs across databases
• Document what matters: Focus on dbplyr translation needs, not general SQL education
• Keep it short: Research docs should be scannable reference material, not tutorials