In SQL server 2005 this query
select len(cast('the quick brown fox jumped over the lazy dog' as varchar))
returns 30 as length while the supplied string has more characters. This seems to be the default. Why 30, and not 32 or any other power of 2?
[EDIT] I am aware that I should always specifiy the length when casting to varchar but this was a quick let's-check-something query. Questions remains, why 30?
Why don't you specify the varchar length? ie:
SELECT CAST('the quick brown fox jumped over the lazy dog' AS VARCHAR(45))
As far as why 30, that's the default length in SQL Server for that type.
From char and varchar (Transact-SQL):
When n is not specified in a data definition or variable declaration statement, the default length is 1. When n is not specified when using the CAST and CONVERT functions, the default length is 30.
My theory is that default 30 character length originated from the U.S. Postal Service specs for name and address lines:
I don't know why they chose 30, but it was the same in Sybase SQL Server, which Microsoft's SQL Server was developed from. It seems to be a peculiarity of those RDBMSs as it's not in the SQL standards, and other servers differ in their behaviour.