query-writingClaude Skill

Query Writing Skill

Workflow for Simple Queries

For straightforward questions about a single table:

1. Identify the table - Which table has the data?
2. Get the schema - Use sql_db_schema to see columns
3. Write the query - SELECT relevant columns with WHERE/LIMIT/ORDER BY
4. Execute - Run with sql_db_query
5. Format answer - Present results clearly

Workflow for Complex Queries

For questions requiring multiple tables:

1. Plan Your Approach

Use write_todos to break down the task:

• Identify all tables needed
• Map relationships (foreign keys)
• Plan JOIN structure
• Determine aggregations

2. Examine Schemas

Use sql_db_schema for EACH table to find join columns and needed fields.

3. Construct Query

• SELECT - Columns and aggregates
• FROM/JOIN - Connect tables on FK = PK
• WHERE - Filters before aggregation
• GROUP BY - All non-aggregate columns
• ORDER BY - Sort meaningfully
• LIMIT - Default 5 rows

4. Validate and Execute

Check all JOINs have conditions, GROUP BY is correct, then run query.

Example: Revenue by Country

SELECT
    c.Country,
    ROUND(SUM(i.Total), 2) as TotalRevenue
FROM Invoice i
INNER JOIN Customer c ON i.CustomerId = c.CustomerId
GROUP BY c.Country
ORDER BY TotalRevenue DESC
LIMIT 5;

Error Recovery

If a query fails or returns unexpected results:

1. Empty results — Verify column names and WHERE conditions against the schema; check for case sensitivity or NULL values
2. Syntax error — Re-examine JOINs, GROUP BY completeness, and alias references
3. Timeout — Add stricter WHERE filters or LIMIT to reduce result set, then refine

Quality Guidelines

• Query only relevant columns (not SELECT *)
• Always apply LIMIT (5 default)
• Use table aliases for clarity
• For complex queries: use write_todos to plan
• Never use DML statements (INSERT, UPDATE, DELETE, DROP)