Arduino extended database library data type. Is char str[] in Arduino the equivalent of VARCHAR in SQL?
If you mean, are these equivalent:

foo VARCHAR(100);


and:

char foo [100];


No, they are not equivalent, really. In the case of SQL you can put 100 characters into foo. In the case of C++ you would (generally speaking) put up to 99 characters into foo, with the final character being 0x00 which indicates the length of the string.

You could conceivably put 100 characters into foo but then you would have to store the size of foo in another variable.