# SQL Analysis

> Write SQL queries to extract, filter, and analyze data for business insights and requirements validation

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

---


# SQL Analysis Skill

## Purpose
Enable BAs to validate data requirements, understand current data states, and perform independent data analysis without relying solely on developers.

## When to Use
- Validating migration data.
- Understanding current data structures (As-Is).
- Troubleshooting user issues ("Why can't I see this order?").
- Generating ad-hoc reports for stakeholders.

## Core SQL Concepts for BAs

### 1. The SELECT Statement
```sql
SELECT column1, column2   -- What do you want?
FROM table_name           -- From where?
WHERE condition           -- Filter logic
ORDER BY column1 DESC;    -- Sorting
```

### 2. Filtering (WHERE)
- `WHERE status = 'Active'` (Exact match)
- `WHERE quantity > 10` (Comparison)
- `WHERE name LIKE 'Smith%'` (Partial match)
- `WHERE status IN ('New', 'Open')` (List)
- `WHERE end_date IS NULL` (Finding missing data)

### 3. Aggregation (GROUP BY)
Summarizing data.
```sql
SELECT status, COUNT(*) as total_orders
FROM orders
GROUP BY status;
```

### 4. Joining Tables (JOIN)
Connecting related data.
- **INNER JOIN**: Only matching records (A ∩ B). "Show me customers who have orders."
- **LEFT JOIN**: All from Left, matches from Right (A + A∩B). "Show me ALL customers, and their orders if any." (Good for finding customers with NO orders).

## Common Analysis Queries

### Data Quality Check
"Are there any duplicate customers by email?"
```sql
SELECT email, COUNT(*)
FROM customers
GROUP BY email
HAVING COUNT(*) > 1;
```

"Are there orders without a valid user?"
```sql
SELECT o.order_id
FROM orders o
LEFT JOIN users u ON o.user_id = u.id
WHERE u.id IS NULL;
```

### Process Analysis
"How long does it take to ship an order?"
```sql
SELECT 
    AVG(DATEDIFF(day, order_date, shipped_date)) as avg_days_to_ship,
    MAX(DATEDIFF(day, order_date, shipped_date)) as max_days
FROM orders
WHERE shipped_date IS NOT NULL;
```

### Integration Spec Validation
"What are the distinct values for 'Payment Method' so we can map them?"
```sql
SELECT DISTINCT payment_method
FROM transactions;
```

## Best Practices
- **Read-Only Access**: Ensure you only have SELECT permissions (ro-user).
- **Limit Results**: Always use `LIMIT 10` (or `TOP 10`) when exploring new tables.
- **No Production Performance Impact**: Avoid running heavy queries (e.g., Joining 5 huge tables) during peak hours.
- **Understand NULLs**: `COUNT(column)` ignores NULLs; `COUNT(*)` counts everything.

## Tools
- **DBeaver**: Universal DB Tool.
- **Metabase / Redash**: Visual SQL Builders.
- **Lark Base**: Can import CSV results for sharing.