SQL Server default values: why with one or two parentheses?

It's just how SQL stores them internally.

They aren't needed except for when IN is expanded to (.. OR ...) for some reason.

You should try a check constraint with ANDs and ORs. Lordy lord.

This answer and gbn's answer suffice. However, FWIW, I noticed a pattern, in my databases at least, for when SQL Server stores the default values in single vs. double parentheses.

• Double parentheses for numbers (all integers in my case).
• Single parentheses for non-numbers (i.e. strings and functions).
• All definitions stored in at least a single set of parentheses.
• No definitions stored in more than two sets of parentheses.

While this may not affect functionality, etc., as a programmer, it feels good to know there is a pattern.

Below is a query to check defaults in a database against these rules (it may not be perfect, but it's a start).

-- Find number of distinct defaults. This should match the number of records in following query 
-- to ensure all are accounted for.
select count(*) as NumberOfDistinctConstraints 
from (select distinct [definition] from sys.default_constraints) t
;

-- Find defaults that follow and don't follow the rules.
;with c1 as (
    select [definition] as DefaultValue, 3 as NumberOfParentheses
    from sys.default_constraints dc
    where [definition] like '(((%'

    union

    select [definition], 2
    from sys.default_constraints dc
    where [definition] like '(([^(]%'

    union

    select [definition], 1
    from sys.default_constraints dc
    where [definition] like '([^(]%' --and ([definition] like '(N''%' or [definition] like '(''%')

    union

    select [definition], 0
    from sys.default_constraints dc
    where [definition] like '[^(]%'
)
, c2 as (
    select 
        DefaultValue
        , NumberOfParentheses
        , case
            when 
                NumberOfParentheses >= 3 -- None exists.
                or NumberOfParentheses = 0 -- None exists.
                or (
                    -- Any double parentheses not followed by a digit or negative sign.
                    NumberOfParentheses = 2 
                    and substring(DefaultValue, 3, 1) not like ('[0-9]') 
                    and substring(DefaultValue, 3, 1) not like ('-')
                )
                or (
                    -- Any single parenthesis followed by a digit or negative sign.
                    NumberOfParentheses = 1 
                    and (
                        substring(DefaultValue, 2, 1) like ('[0-9]') 
                        and substring(DefaultValue, 2, 1) like ('-')
                    )
                )
            then
                0
            else 1
        end as FollowsTheRules
    from c1
)
select *
from c2
--where FollowsTheRules = 0
order by FollowsTheRules asc, NumberOfParentheses desc, DefaultValue asc
;

I checked a few databases and these rules held up. However, I'd be interested to see if others see the same results.