code making rule?

Dear postgresers

I'm making a tissue sample database. I have to assign a unique code to
each entry. Just integer code is the simplest one but it will  not show
any information of its objects.
Is there any rule or tips or habits making
a brilliant code?
Or just some examples will be welcomed.

Sincerly,

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Science is the game we play with God to find out what his rules are.
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Doo Suk Yang
Research Scientist                              Voice: 82-42-866-2222
LG Chemical Ltd. Research Park
Biotech Research Institute I                    FAX:   82-42-861-2566
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A fool hath no delight in understanding, but that his heart may discover itself.
(Proverbs 18:2)
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