SQL JOINs are one of the most confusing concepts for beginners. Reading about them in textbooks is one thing, but truly seeing how tables combine makes it click.
That’s why I built the SQL JOIN Visualizer: a free, browser-based tool where you can experiment with INNER, LEFT, RIGHT, and FULL JOINs using healthcare data (Patients + Medications). You can even upload your own CSVs.
Jump straight to the tool here: SQL JOIN Visualizer
Why JOINs Matter
When working with real data, you’ll often need to pull information from two or more tables. In healthcare, that might look like:
- Patients table → demographic info (ID, name, age).
- Medications table → prescriptions (patient_id, drug, frequency).
A JOIN lets you combine these into a single query result. Different JOIN types determine what gets included when there isn’t a match.
Types of SQL JOINs (with Examples)
INNER JOIN
Returns only rows where a patient_id exists in both tables.
Example: Only patients who actually have a medication record.
LEFT JOIN
Returns all rows from Patients, with matching Medications where available.
Example: Every patient, even those without medications, will show up.
RIGHT JOIN
Returns all rows from Medications, with matching Patients where possible.
Example: Medications prescribed even if the patient record isn’t in the Patients table.
FULL OUTER JOIN
Returns all rows from both tables, matching where possible.
Example: Every patient and every medication, combined.
Try It Yourself — Interactive SQL JOIN Visualizer
Here’s the live tool (no database required — it runs entirely in your browser): SQL JOIN Visualizer
⚡ Features:
- Switch between INNER / LEFT / RIGHT / FULL JOINs
- See SQL code + explanation instantly
- Upload your own CSVs to experiment
- Download results as CSV
Healthcare Data Example
In the demo dataset:
- Patient Mary Johnson (ID 2) appears with medications (Metformin, Amlodipine).
- Patient Alice Brown (ID 4) is included even without meds in a LEFT JOIN.
- Medications for patient_id 11 appear in a RIGHT JOIN, even though that patient isn’t in the Patients table.
This makes JOINs visual, not abstract.
Conclusion
Learning SQL JOINs doesn’t have to feel like memorizing diagrams. With the SQL JOIN Visualizer, you can play with real healthcare-style data, toggle JOINs, and see instantly what changes.
Try the SQL JOIN Visualizer now, and make JOINs finally “click.”