Skill: SQL Analysis

Use this skill when writing, reviewing, or modifying SQL queries.

Rules

• Use explicit column names — avoid SELECT * except for quick exploration.
• Use CTEs (Common Table Expressions) for readability and modularity.
• Add date filters for large fact tables to limit scan scope.
• Check join keys and join cardinality before writing joins.
• Validate row counts before and after joins to detect fanout or data loss.
• Avoid implicit cross joins.
• Never run DROP, TRUNCATE, DELETE, or UPDATE unless explicitly requested by the user.
• If destructive SQL is requested, propose a dry-run, backup, or transaction strategy first.

Query Structure

WITH base AS (
    SELECT
        column_a,
        column_b,
        event_date
    FROM schema.table_name
    WHERE event_date BETWEEN '2024-01-01' AND '2024-01-31'
),
aggregated AS (
    SELECT
        column_a,
        COUNT(*) AS row_count,
        SUM(column_b) AS total_b
    FROM base
    GROUP BY column_a
)
SELECT
    column_a,
    row_count,
    total_b
FROM aggregated
ORDER BY row_count DESC;

Review Checklist

Before finalizing a query, verify:

• Are grains (unit of analysis per row) clear?
• Are date ranges explicit and appropriate?
• Are NULLs handled (filtered, coalesced, or documented)?
• Are duplicates considered (distinct, dedup logic)?
• Is join cardinality validated (1:1, 1:N, M:N)?
• Are business definitions documented in comments or CTEs?
• Are row counts checked before and after key transformations?
• Is there any risk of implicit cross join?
• Are destructive operations absent or explicitly approved?