ConcatとUnionのちがい
LINQのConcatはSQLのUNION ALL、LINQのUionはSQLのUNIONに変換されました。
Concat
var query = (from e in db.Employee select e.EmployeeName) .Concat (from d in db.Department select d.DepartmentName);
SELECT [t2].[EmployeeName] FROM ( SELECT [t0].[EmployeeName] FROM [dbo].[Employee] AS [t0] UNION ALL SELECT [t1].[DepartmentName] FROM [dbo].[Department] AS [t1] ) AS [t2]