Question

Should I choose the smallest datatype possible, or if I am storing the value 1 for example, it doesn’t matter what is the col datatype and the value will occupy the same memory size?

The question is also, cuz I will always have to convert it and play around in the application.

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UPDATE

I think that varchar(1) and varchar(50) is the same memory size if value is “a”, I thought it’s the same with int and tinyint, according to the answers I understand it’s not, is it?

Answer 1

Always choose the smallest data type possible. SQL can’t guess what you want the maximum value to be, but it can optimize storage and performance once you tell it the data type.

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To answer your update:

varchar does take up only as much space as you use and so you’re right when you say that the character “a” will take up 1 byte (in latin encoding) no matter how large a varchar field you choose. That is not the case with any other type of field in SQL.

However, you will likely be sacrificing efficiency for space if you make everything a varchar field. If everything is a fixed-size field then SQL can do a simple constant-time multiplication to find your value (like an array). If you have varchar fields in there, then the only way to find out where you data is stored it to go through all the previous fields (like a linked list).

If you’re beginning SQL then I advise just to stay away from varchar fields unless you expect to have fields that sometimes have very small amounts of text and sometimes very large amounts of text (like blog posts). It takes experience to know when to use variable length fields to the best effect and even I don’t know most of the time.