Oct 11, 2007
A Visual Explanation of SQL Joins
I thought Ligaya Turmelle's post on SQL joins was a great primer for novice developers. Since SQL joins appear to be set-based, the use of Venn diagrams to explain them seems, at first blush, to be a natural fit. However, like the commenters
to her post, I found that the Venn diagrams didn't quite match the
SQL join syntax
reality in my testing.
I love the concept, though, so let's see if we can make it work. Assume we have the following two tables. Table A is on the left, and
Table B is on the right. We'll populate them with four records each.
id name id name
-- ---- -- ----
1 Pirate 1 Rutabaga
2 Monkey 2 Pirate
3 Ninja 3 Darth Vader
4 Spaghetti 4 Ninja
Let's join these tables by the name field in a few different ways and see if we can get a conceptual match to those nifty Venn diagrams.
SELECT * FROM TableA
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