Error When executing a sql statement - sql

Question

When i am trying to execute the following command i get the following output error.
("INSERT INTO Leaves (EmpID,TotalLeavesRemaining,Year)
SELECT EmpID,TotalLeavesRemaining,'" + year + "'
FROM Leaves
WHERE Year = '" + yearbefore + "' ", con)
Additional information: Conversion from string
INSERT INTO Leaves (EmpID,TotalL" to type 'Double' is not valid.
When i run the command directry through the sql management studio the query does its job !

Answer 1

If you really need to concatenate SQL string then convert your year to string or join using & instead of +
("INSERT INTO Leaves (EmpID,TotalLeavesRemaining,Year)
SELECT EmpID,TotalLeavesRemaining,'" + year.ToString + "'
FROM Leaves
WHERE Year = '" + yearbefore.ToString + "' ", con)

Answer 2

If you put single quotes around your year number it makes it a string. But apparently your year column is of type double. (Why? Shouldn't' it be of type int?). Remove the single quotes!
And also use command parameters. This makes it safer, less error prone and also faster, as the SQL-Server can cache the query and reuse it without having to recompile it and to create an execution plan again.
using (var cmd new SqlCommand(#"INSERT INTO Leaves (EmpID, TotalLeavesRemaining, Year)
SELECT EmpID,TotalLeavesRemaining, #year
FROM Leaves
WHERE Year = #yearbefore", con)) {
cmd.Parameters.AddWithValue("#year", 2015);
cmd.Parameters.AddWithValue("#yearbefore", 2014);
...
}
Note that you don't need any quotes around parameters, even for string parameters. They free you from escaping strings, formatting numbers and dates.
If you want to use multiline strings, use verbatim strings introduced with an #. These string literals can span multiple lines and don't interpret the backslash (\) as an escape character. Single quotes can be escaped by double single quotes.