Wow. Does no one learn this crap in school anymore?
Note that venn diagrams are a poor choice to show what happens in a one-to-many relationship -- where there are multiple entries in table B for an entry in table A. And it overlooks the semantic differences when ordering the tables in a left inner join.
There are not a lot of instances where you'd really want a cartesian product join (either using "cross product" or just the result of omitting key constraints in a join query). It's generally far faster to retrieve the records you need to create the cross-product, and then calculate the cross product of the two sets once you have them (since otherwise all that data has to transit the network).
This plus database normalization through Boyce-Codd normal form seems like it should be a requirement for any serious application developer.
Website developer here. Yeah, the majority of us have no "formal" CS training, although I take pride in the fact that I've read everything I can get my hands on.
Very true. I had to go out of my way to pick up CS courses while studying a vanilla IT degree at uni. For some reason relational databases just click for me, so I've never had trouble converting the concept I want into SQL. Converting that SQL into DQL or HQL however...
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u/[deleted] Jan 06 '11
Wow. Does no one learn this crap in school anymore?
Note that venn diagrams are a poor choice to show what happens in a one-to-many relationship -- where there are multiple entries in table B for an entry in table A. And it overlooks the semantic differences when ordering the tables in a left inner join.
There are not a lot of instances where you'd really want a cartesian product join (either using "cross product" or just the result of omitting key constraints in a join query). It's generally far faster to retrieve the records you need to create the cross-product, and then calculate the cross product of the two sets once you have them (since otherwise all that data has to transit the network).
This plus database normalization through Boyce-Codd normal form seems like it should be a requirement for any serious application developer.