By default, SQL Server is case insensitive. You can define a column as case sensitive by applying a case-sensitive collation:
declare @t table (state varchar(4) check (state in ('ok', 'computer')))
insert @t values ('OK') -- No error, case insensitive by default
go
declare @t table (state varchar(4) collate SQL_Latin1_General_CP1_CS_AS
check (state in ('ok', 'computer')))
insert @t values ('OK') -- The INSERT statement conflicted with the CHECK constraint
Note the CS in SQL_Latin1_General_CP1_CS_AS, which stands for Case Sensitive. The AS stands for Accent Sensitive.