LINQ to SQL query to determine if value starts with numeric

Question

I have a project where I query users by first letter:

repository.GetAll().Where(q => q.BrukerIdent.StartsWith(letter.ToString())).ToList();

Answer 1

I suspect @abatishchev's revised answer will help here, but if you are doing this lots, or this is an important query, then I strongly suggest a refactor. For example:

create table Test (
    id int not null identity(1,1) primary key clustered,
    name nvarchar(20) not null,
    firstChar as (case when LEN(name) = 0 then null
            else SUBSTRING(name,1,1) end) persisted
    )
go
create nonclustered index Test_firstChar on Test (firstChar)

I can now do a very efficient first-character match simply by testing firstChar. Which could be an integer instead if the numeric ones are particularly important.

Answer 2

If this is for LINQ-to-SQL you could use SqlMethods.Like method here:

var result = repository
    .GetAll()
    .Where(q => SqlMethods.Like(q.BrukerIdent, "[0-9]%"))
    .ToList();

Answer 3

Perhaps not so readable, but the following would probably work:

q.BrukerIdent >= "0" && q.BrukerIdent < ":"

Maybe there are some exotic collation sequences where this would fail, but I expect it would work in most situations.

Answer 4

repository.GetAll().Where(q => Char.IsNumber(q.BrukerIdent[0]))

MSDN

---

var numbers = Enumerable
                     .Range(0, 10)
                     .Select(i => i.ToString(CultureInfo.InvariantCulture));

// var numbers = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 );
// var numbers = HashSet<int> { ... };

var q = from b in repository.GetAll()
        where numbers.Contains(b.BrukerIdent.FirstOrDefault())) //[0]
        select b;