Week 31 - SQL Chips and Chunks Newsletter

1. SQL Chip: Conditional Logic with CASE 

The CASE statement is SQL's way of implementing "if/then/else" logic. It allows you to evaluate conditions and return different values based on which condition is met. Think of it as classifying your data on the fly. This is incredibly useful for turning raw data into meaningful categories, like flagging systems as "High Risk" or "Low Risk" based on a score, or categorizing operational efficiency levels.

2. SQL Chunk: Simple Risk Classification

Let's use CASE to classify systems based on a hypothetical vulnerability_score.

SELECT
    system_id,
    vulnerability_score,
    CASE
        WHEN vulnerability_score >= 80 THEN 'High Risk'
        WHEN vulnerability_score >= 50 AND vulnerability_score < 80 THEN 'Medium Risk'
        ELSE 'Low Risk'
    END AS risk_level,
    last_audit_date
FROM
    systems_inventory
WHERE
    is_active = TRUE
ORDER BY
    vulnerability_score DESC
LIMIT 100;

3. SQL Challenge: Aggregating by Conditional Grouping

A more advanced use of CASE is combining it with aggregate functions like COUNT() and the GROUP BY clause. This allows you to count the number of items (like operational findings) that fall into your custom-defined risk categories.

SELECT
    CASE
        WHEN finding_severity = 'Critical' THEN 'Needs Immediate Action'
        WHEN finding_severity = 'Major' THEN 'Scheduled Review'
        ELSE 'Minor Observation'
    END AS action_priority,
    COUNT(finding_id) AS total_findings,
    AVG(time_to_resolve_days) AS avg_resolution_time
FROM
    audit_findings
WHERE
    finding_date >= '2025-01-01'
GROUP BY
    action_priority
HAVING
    COUNT(finding_id) > 5
ORDER BY
    total_findings DESC;

4. SQL Mistake: Misplacing the GROUP BY Clause ❌

A common mistake for beginners is incorrectly ordering the GROUP BY clause. In standard SQL query execution order, the GROUP BY clause must appear after the FROM, WHERE, and JOIN clauses, but before HAVING, SELECT column aliases (in some SQL dialects), ORDER BY, and LIMIT. Putting GROUP BY before WHERE will cause a syntax error.

5. SQL in Practice: Healthcare Compliance & Data Integrity

In a healthcare scenario, maintaining data integrity and compliance (like HIPAA) is paramount. You could use the CASE statement to generate a compliance report that flags patient records based on the last time their access permissions were reviewed, ensuring adherence to regulatory standards.

SELECT
    patient_record_id,
    last_access_review_date,
    CASE
        -- Assuming internal policy requires review every 180 days (approx. 6 months)
        WHEN DATEDIFF(CURRENT_DATE(), last_access_review_date) > 180 THEN 'Non-Compliant: Review Overdue'
        WHEN DATEDIFF(CURRENT_DATE(), last_access_review_date) BETWEEN 90 AND 180 THEN 'Approaching Review Deadline'
        ELSE 'Compliant'
    END AS compliance_status,
    system_owner
FROM
    patient_data_access_logs
WHERE
    is_active_record = TRUE
ORDER BY
    compliance_status DESC, last_access_review_date ASC;

6. SQL Resource: Advanced CASE Statement Patterns

A great resource to deepen your understanding is a guide focused on the more advanced, often-overlooked uses of CASE.

• Resource Name: SQL CASE Statement Advanced Techniques

• How it Helps: This guide breaks down patterns like search CASE statements (the type we used), using CASE for conditional aggregation (like creating pivot tables), and important performance considerations. Mastering these patterns is a key step toward becoming an SQL expert in data analysis and reporting.