Add Unique Records

You can use the MERGE statement in SQL Server to achieve this. Here's how you can modify your code to use MERGE:

string constr = ConfigurationManager.ConnectionStrings["Cnn"].ConnectionString;
using (SqlConnection con = new SqlConnection(constr))
{
    string query = @"
        MERGE INTO Category AS target
        USING (VALUES (@CategoryId, @CategoryName)) AS source (CategoryId, CategoryName)
        ON target.CategoryId = source.CategoryId
        WHEN NOT MATCHED THEN
            INSERT (CategoryId, CategoryName)
            VALUES (source.CategoryId, source.CategoryName);";
    
    using (SqlCommand cmd = new SqlCommand(query))
    {
        cmd.Connection = con;
        cmd.Parameters.AddWithValue("@CategoryId", data.id);
        cmd.Parameters.AddWithValue("@CategoryName", data.snippet.title);
        con.Open();
        cmd.ExecuteNonQuery();
        con.Close();
    }
}

When creating a table you can specify fields in tables as being unique. It is an option for fields in tables. You can alter the existing table to specify that specific fields must be unique.

The code provided here is good, it is good to avoid attempting to add a row that should not be added but if you never want duplicates of a field then the table should be created approriately.

Try below code:

string constr = ConfigurationManager.ConnectionStrings["Cnn"].ConnectionString;
using (SqlConnection con = new SqlConnection(constr))
{
    con.Open();

    // Check if the category already exists
    bool exists = false;
    string checkQuery = "SELECT COUNT(*) FROM Category WHERE CategoryId = @CategoryId OR CategoryName = @CategoryName";
    using (SqlCommand checkCmd = new SqlCommand(checkQuery, con))
    {
        checkCmd.Parameters.AddWithValue("@CategoryId", data.id);
        checkCmd.Parameters.AddWithValue("@CategoryName", data.snippet.title);
        
        exists = (int)checkCmd.ExecuteScalar() > 0;
    }

    if (!exists)
    {
        // Only insert if the category does not already exist
        string query = "INSERT INTO Category (CategoryId, CategoryName) VALUES (@CategoryId, @CategoryName)";
        using (SqlCommand cmd = new SqlCommand(query, con))
        {
            cmd.Parameters.AddWithValue("@CategoryId", data.id);
            cmd.Parameters.AddWithValue("@CategoryName", data.snippet.title);
            
            cmd.ExecuteNonQuery();
        }
    }

    con.Close();
}

Thanks