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Cardinality;PRIMARY Index;Indexes

Posted on 2006-07-19
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Can someone please explain what these terms mean/do?

Cardinality;PRIMARY Index;Indexes

I've seen them in phpmyadmin and are wondering what they are used for.
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Question by:mnb93
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Guy Hengel [angelIII / a3] earned 500 total points
ID: 17136887
Cardinality:
  means how many different values you have for 1 column. for a primary key or unique constraints , it must be 1
 

primary key:
  is a constraint enforcing you have each value only once in the table. trying to update or insert duplicate values will be rejected

indexes:
  like in books, a index is to find information quicker.
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by:mnb93
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if I set something to an index that does what? (one hand typing atm ,soory.)
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by:Guy Hengel [angelIII / a3]
Guy Hengel [angelIII / a3] earned 500 total points
ID: 17137097
an index is a additional structure besides the table, kept up-to-date automatically, that helps WHERE clause to be fast.

like Persons table : PersonID, PersonName

if you don't put a index on PersonID, getting a single row can take more and more time, the larger the table.
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