how many joins is too many

SQL Apprentice Question
Does anyone have any good sources of information that talk about how many
joins is too many? I've heard the number 4 or 5 thrown around but haven't
found any 'evidence' to back them up.

Celko Answers

>> I've heard the number 4 or 5 thrown around but haven't found any 'evidence' to back them up. <<


The "Rule of Five" first came from a psychologist named Brown. The
idea is that huymans have a hard time with more than five distinctions
in an input (i.e. if I ask you to rank a movie, food, etc. on a 1-5
scale, you will tend to produce the same results next week. But if I
give you a 1-10 scale, you will not repeat the same answers next week.
This shows up in matching colors, taste, tones, phrases, grid patterns
and a ton of other things.

Fewer than five distinctions is better; more than seven is nearly
impossible. just for fun, look at the combinations 1 items =1, 2
items = 2!, .but .5! = 120 possible ways to arrange the items.


This was quoted in a Yourdon book back in the early days of Software
Engineering.


Now, we have a wonderful mental tool called "Chunking"; in SQL it is
views, CTEs, and derived tables. We take several things and aggregate
them into a new "single unit" until you get to five or fewer chunks.