LINQ to SQL produces incorrect TSQL when using UNION or CONCAT

September 10, 2008·Updated: April 26, 2026·1 min

.NET Microsoft Programming System.Data.Linq LINQ to SQL SQL Server

When a LINQ to SQL query contains a Union or Concat with a second query, and the second query references a column twice, a SqlException will occur.

var a = from address in dc.Addresses
select new {
    ID = address.AddressID,
    Address1 = address.AddressLine1,
    Address2 = address.AddressLine2,
};
var b = from address in dc.Addresses
select new {
    ID = address.AddressID,
    Address1 = address.AddressLine1,
    Address2 = address.AddressLine1, // notice AddressLine1 repeated
};
var q = a.Take(10).Union (b.Take(10));
q.ToArray ();

SqlException: All the queries in a query expression containing a UNION operator must have the same number of expressions in their select lists.

SELECT [t2].[AddressID] AS [ID], [t2].[AddressLine1] AS [Address1], [t2].[AddressLine2] AS [Address2]
FROM (
SELECT TOP (10) [t0].[AddressID], [t0].[AddressLine1], [t0].[AddressLine2]
FROM [Person].[Address] AS [t0]
UNION
SELECT TOP (10) [t1].[AddressID], [t1].[AddressLine1]
FROM [Person].[Address] AS [t1]
) AS [t2]

Notice the third SELECT statement is only selecting two columns instead of the required three.

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