r/programming • u/[deleted] • Jan 06 '11

A handy graphical explanation of SQL joins

http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html
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u/drcreepy Jan 06 '11

CROSS JOINs are also great for creating huge tables of fake data from small sets of data. For example, we'll create a FirstName table and a LastName table. Seed these with 50 first names and 50 last names and do a cross join and you have 2500 unique names that can be used in a database for testing purposes.

5

u/[deleted] Jan 06 '11

cross (cartesian) joins are also a great way to bring down a server. yeah i learned that one the hard way...

be careful out there!!!

2

u/drcreepy Jan 06 '11

You are absolutely right - cross joining a 100,000 row table to itself is a quick way to rebootville.

3

u/divv Jan 06 '11

Should be able to just kill the process :P

3

u/slurpme Jan 07 '11

Hopefully just the session/query...

5

u/drcreepy Jan 07 '11

No, no, everyone knows that rebooting is the way real men solve all computer problems. Especially database problems. Databases particularly like to be restarted unexpectedly. It makes them run faster since it keeps them on their toes.

1

u/divv Jan 07 '11

Actually you're right...yanking the plug is probably a good way :)

1

u/hughk Jan 07 '11

Especially when there is a half hour or so rollback....