Visualizing SQL Joins with Venn Diagrams: Inner, Outer & Cross Join Explained

SQL join operations can be visualized with Venn diagrams, though the analogy is not perfect.

Sample tables

id name   id name
-- ----   -- ----
1 Pirate   1 Rutabaga
2 Monkey   2 Pirate
3 Ninja    3 Darth Vader
4 Spaghetti 4 Ninja

Inner Join

Returns rows with matching values in both tables.

SELECT * FROM TableA
INNER JOIN TableB
ON TableA.name = TableB.name

Result:

id name   id name
-- ----   -- ----
1 Pirate   2 Pirate
3 Ninja    4 Ninja

Full Outer Join

Returns all rows from both tables, with NULL where there is no match.

SELECT * FROM TableA
FULL OUTER JOIN TableB
ON TableA.name = TableB.name

Result includes matched rows and rows with NULLs for non‑matching sides.

Left Outer Join

Returns all rows from the left table and matching rows from the right table; non‑matching right side values are NULL.

SELECT * FROM TableA
LEFT OUTER JOIN TableB
ON TableA.name = TableB.name

To find rows present only in TableA, add a WHERE clause filtering NULLs on the right side.

WHERE TableB.id IS null

Cross Join (Cartesian Product)

A cross join pairs every row of the first table with every row of the second, producing a Cartesian product that can quickly become large.

SELECT * FROM TableA
CROSS JOIN TableB

This generates 4 × 4 = 16 rows for the example tables, illustrating why cross joins on large tables are dangerous.