mysql - The logic of field type in relationship -


i have tables user preferences currency, language , ...

currency structure , sample records :

id  |   title    |  slug ----+------------+-------- 1   | dollar  | usd 2   | ca dollar  | cad 3   | euro       | eur 4   | swiss franc| chf

language structure , sample records :

id  |   title     |    code   |   flag ----+-------------+-----------+---------- 1   | english     | en        | img/flags/en.png 2   | french      | fr        | img/flags/fr.png

i want create relation between these tables , users tables, want know field better foreign key ? example in currency table currency.id or currency.slug ?

if see sql speed should go currency.id because finding integer value lots many rows (in thousands) would faster finding varchar.

but here see currency won't more 500 can use both won't affect speed.

you can use both in particular example: currency.id or currency.slug


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