Why does VARCHAR need length specification?

Why do we always need to specify VARCHAR(length) instead of just VARCHAR? It is dynamic anyway.

UPD: I'm puzzled specifically by the fact that it is mandatory (e.g. in MySQL).

The "length" of the VARCHAR is not the length of the contents, it is the maximum length of the contents.

The max length of a VARCHAR is not dynamic, it is fixed and therefore has to be specified.

If you don't want to define a maximum size for it then use VARCHAR(MAX).

First off, it does not needed it in all databases. Look at SQL Server, where it is optional.

Regardless, it defines a maximum size for the content of the field. Not a bad thing in itself, and it conveys meaning (for example - phone numbers, where you do not want international numbers in the field).

There's possible performance impact: in MySQL, temporary tables and MEMORY tables store a VARCHAR column as a fixed-length column, padded out to its maximum length.

If you design VARCHAR columns much larger than the greatest size you need, you will consume more memory than you have to. This affects cache efficiency, sorting speed, etc.

So you give the max length which is under your string come. like if you max length of character 10 so don't give his length 100 or more.

You can see it as a constraint on your data. It ensures that you don't store data that violates your constraint. It is conceptionally similar to e.g. a check constraint on a integer column that ensure that only positive values are entered.