What is the linq equivalent of the below sql query

Question

select Productid from categories where `categoryname` in `('abc','def','ghi')`; 

I have tried this:

var res = from catg in db.Categories where catg.CategoryId.ToString().Contains(SelectedProducts) select catg;

But this doesnt seem to work...

Answer 1

Assuming SelectedProducts is an array of product ids (integers):

var cats = db.Categories.Where(o => SelectedProducts.Contains(o.CategoryId));
var pids = cats.Select(o => o.ProductId);

Reason: SQL IN operator is implemented oppositely in LINQ to SQL. The question highlights a common mistake in LINQ developers trying to translate from SQL, expecting an [attribute] [operator] [set] syntax.

Using an abstract set language we can highlight syntax differences

• SQL uses a "Element is included in Set" syntax
• LINQ uses a "Set contains Element" syntax

So any IN clause must be reverted using the Contains operator. It will translate to attribute IN (SET) anyways.

Answer 2

You need to use Contains on SelectedProducts

var res = from catg in db.Categories where 
SelectedProducts.Contains(catg.categoryname) select catg.Productid;

Using method notation

var res = db.Categories.Where(catg => SelectedProducts
            .Contains(catg.categoryname)).Select(catg.Productid);

Answer 3

The equivalence of a SQL IN with IEnumerable.Contains() :

var res = from catg in db.Categories 
          where new[] {"abc","def","ghi"}.Contains(catg.categoryname) 
          select catg.Productid

Or lambda

db.Categories.Where(x => new[] {"abc","def","ghi"}.Contains(x.categoryname)).Select(c => c.ProductId);