public void insertIncome(DateTime date, string name, string cath, long sum)
        { 
            IncomeAdap.InsertCommand = new SqlCommand("INSERT INTO Income VALUES(@date, @name, @cath, @sum);" +
                                                      "SELECT * FROM Income WHERE Дата = @date AND " +
                                                      "[Член семьи] = @name AND [Категория дохода] = @cath AND " +           
                                                      "[Сумма, руб] = @sum", dbConnection);
            IncomeAdap.InsertCommand.Parameters.Add(new SqlParameter("@date", date));
            IncomeAdap.InsertCommand.Parameters.Add(new SqlParameter("@name", name));
            IncomeAdap.InsertCommand.Parameters.Add(new SqlParameter("@cath", cath));
            IncomeAdap.InsertCommand.Parameters.Add(new SqlParameter("@sum", sum));
            IncomeAdap.InsertCommand.ExecuteNonQuery();            
        }

CREATE TRIGGER uniteIncome
ON dbo.Income
AFTER INSERT AS 
BEGIN
	DECLARE @summ bigint = 0, @date date
	SELECT @summ = dbo.Income.[Сумма, руб], @date=dbo.Income.Дата FROM dbo.Income
	IF ((SELECT dbo.Common.Дата FROM dbo.Common) = @date)
	BEGIN
		UPDATE dbo.Common
		SET dbo.Common.Доход += @summ, dbo.Common.Оборот += @summ
		WHERE dbo.Common.Дата = @date
	END
	ELSE
		INSERT INTO dbo.Common VALUES (@date, @summ, 0, @summ)
END

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
The statement has been terminated.