Part 61: Relational Algebra & The Theory of Joins

The relational database model is founded on mathematical set theory and relational algebra. A JOIN combines fields from two tables by using values common to each.

1. Visualizing Joins with Venn Diagrams

[ Table A ]                 [ Table B ]
    │                            │
    └───────────┬────────────────┘
                ▼
      [ JOIN OPERATION ]
   (Based on join predicate: A.id = B.a_id)

• INNER JOIN: Intersection (A ∩ B). Only rows matching on both sides.
• LEFT JOIN: All of A, plus matched rows from B. Unmatched B values are NULL.
• RIGHT JOIN: All of B, plus matched rows from A.
• FULL OUTER JOIN: Union (A ∪ B). All rows from both sides, matching where possible.
• CROSS JOIN: Cartesian Product (A × B). Every row of A combined with every row of B.

Key Takeaways

• Joins reconstruct normalized data into unified tabular results based on relational keys.

In Part 62, we will master INNER JOIN: Matching Rows Across Multiple Tables!