SQL Query Builder

When to Use

• When a business question needs to be answered with data
• When building reporting queries or data pipeline transformations
• When optimizing slow queries or refactoring complex SQL
• When translating stakeholder requirements into analytical SQL

Procedure

Step 1 — Clarify the Question

Translate the business question into a precise data question:

• Business: "How are our sales trending?"
• Data: "Monthly revenue by product category for the last 12 months, with month-over-month growth rate"

Document:

• Expected output columns and format
• Filters and date ranges
• Granularity (daily, weekly, monthly)
• Sort order

Step 2 — Identify Tables & Joins

Map the data model:

Table	Role	Key Columns	Join
orders	Fact	order_id, customer_id, order_date, total	Primary
products	Dimension	product_id, category, name	orders.product_id = products.id
customers	Dimension	customer_id, region, segment	orders.customer_id = customers.id

Join type selection:

• INNER when both sides must exist
• LEFT when the left side may have no match (preserve all orders even without customer data)
• Never use RIGHT JOIN — rewrite as LEFT JOIN for clarity

Step 3 — Build CTE Pipeline

Structure complex queries as a pipeline of CTEs:

WITH
-- Step 1: Filter and clean base data
base AS (
    SELECT ...
    FROM orders
    WHERE order_date >= DATE_TRUNC('month', CURRENT_DATE - INTERVAL '12 months')
),

-- Step 2: Aggregate to desired granularity
monthly AS (
    SELECT
        DATE_TRUNC('month', order_date) AS month,
        category,
        SUM(total) AS revenue,
        COUNT(DISTINCT customer_id) AS unique_customers
    FROM base
    JOIN products USING (product_id)
    GROUP BY 1, 2
),

-- Step 3: Add calculations (window functions)
with_growth AS (
    SELECT *,
        LAG(revenue) OVER (PARTITION BY category ORDER BY month) AS prev_revenue,
        ROUND(100.0 * (revenue - LAG(revenue) OVER (PARTITION BY category ORDER BY month))
              / NULLIF(LAG(revenue) OVER (PARTITION BY category ORDER BY month), 0), 1) AS mom_growth_pct
    FROM monthly
)

-- Final output
SELECT month, category, revenue, unique_customers, mom_growth_pct
FROM with_growth
ORDER BY category, month;

Step 4 — Common Patterns Reference

Pattern	When to Use	SQL
Running total	Cumulative metrics	SUM(x) OVER (ORDER BY date ROWS UNBOUNDED PRECEDING)
Rank within group	Top N per category	ROW_NUMBER() OVER (PARTITION BY cat ORDER BY val DESC)
Period comparison	YoY, MoM	LAG(val, 12) OVER (ORDER BY month)
Deduplication	Remove exact dupes	ROW_NUMBER() OVER (PARTITION BY key ORDER BY updated DESC)
Cohort analysis	Retention / engagement	First-action date as cohort, then activity by period
Pivot	Rows to columns	CASE WHEN + SUM or database-specific PIVOT

Step 5 — Performance Notes

Add comments for production queries:

• Estimated row counts at each CTE stage
• Index usage hints
• Known limitations or edge cases
• Data freshness assumptions

Inputs

Input	Required	Format
Business question	Yes	Plain language
Table schema	Yes	Table names, columns, types
Database dialect	Recommended	PostgreSQL / MySQL / BigQuery / Snowflake
Sample data	Recommended	To validate output
Performance requirements	Optional	Max execution time

Output

## Query — [Business Question]

### Question
"What is the monthly revenue by category for the last 12 months with growth rates?"

### Query
[SQL with CTEs, comments, and formatting]

### Output Schema

| Column | Type | Description |
|--------|------|-------------|
| month | DATE | First day of month |
| category | VARCHAR | Product category |
| revenue | DECIMAL | Total revenue |
| unique_customers | INT | Distinct customers |
| mom_growth_pct | DECIMAL | Month-over-month growth % |

### Performance Notes
- Base table ~2M rows, filtered to ~200K
- Index on orders(order_date) recommended
- Runs in ~3s on PostgreSQL 15

Definition of Done

• Business question clearly translated to data question
• Tables and joins documented
• Query uses CTE pipeline (no nested subqueries)
• Window functions used where appropriate
• Output columns described
• Performance notes included

Quality Criteria

• Data sources and assumptions are explicitly stated
• Calculations are reproducible from provided inputs
• Visualizations or tables have clear labels, units, and time ranges
• Caveats and confidence levels are documented for estimates

Verification (4C)

Check	Question
Correctness	Are formulas, aggregations, and statistical methods applied correctly?
Completeness	Does the analysis cover all requested metrics and time ranges?
Context-fit	Are the chosen metrics relevant to the business question being answered?
Consequence	If this data were used for a decision today, what blind spots remain?

Edge Cases

• Missing or incomplete data — Document gaps and their potential impact on conclusions. Provide ranges instead of point estimates.
• Outliers skewing results — Report with and without outliers. Document the decision to include or exclude.
• Changing data definitions mid-period — Split analysis at the change boundary and note the schema difference.

Changelog

• v1.1.0 — Added Tier 2 verification checklist and examples for query safety.
• v1.0.0 — Initial release