# Databases SQL

> SQL database querying, optimization, and data management for analytics

- Skill: `majiayu000/databases-sql` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/databases-sql`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/databases-sql/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Data & Analytics
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/databases-sql

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# Databases & SQL Skill

## Overview
Master SQL and database concepts essential for data analysts, from basic queries to advanced optimization and data warehousing.

## Core Topics

### SQL Fundamentals
- SELECT, FROM, WHERE, ORDER BY
- JOINs (INNER, LEFT, RIGHT, FULL)
- GROUP BY and aggregate functions
- Subqueries and CTEs

### Advanced SQL
- Window functions (ROW_NUMBER, RANK, LAG, LEAD)
- Recursive queries
- Query optimization and execution plans
- Index strategies

### Database Concepts
- Relational database design principles
- Normalization and denormalization
- Data warehousing concepts (star schema, snowflake)
- ETL processes

### Popular Platforms
- PostgreSQL
- MySQL
- SQL Server
- BigQuery, Redshift, Snowflake

## Learning Objectives
- Write efficient SQL queries for data extraction
- Understand database design and optimization
- Work with cloud data warehouses
- Implement ETL processes for analytics pipelines

## Error Handling

| Error Type | Cause | Recovery |
|------------|-------|----------|
| Syntax error | Invalid SQL | Validate query syntax before execution |
| Timeout | Long-running query | Add indexes, optimize query |
| Connection failed | Network/auth issue | Retry with exponential backoff |
| Permission denied | Access rights | Request appropriate permissions |
| Deadlock | Concurrent transactions | Retry transaction |

## Related Skills
- foundations (for data concepts)
- programming (for SQL with Python/R)
- advanced (for big data processing)