Sum up the number sales ids handled by employees called Morningstar, Heather and Dean.
SUM = ADD(SUM(SalesID WHERE FirstName = 'Morningstar'), SUM(SalesID WHERE FirstName = 'Heather'), SUM(SalesID WHERE FirstName = 'Dean'));
SELECT SUM(IIF(T2.FirstName = 'Morningstar', 1, 0)) + SUM(IIF(T2.FirstName = 'Heather', 1, 0)) + SUM(IIF(T2.FirstName = 'Dean', 1, 0)) AS num FROM Sales AS T1 INNER JOIN Employees AS T2 ON T1.SalesPersonID = T2.EmployeeID
CREATE TABLE Customers
(
    CustomerID    INTEGER         not null
        primary key,
    FirstName     TEXT not null,
    MiddleInitial TEXT null,
    LastName      TEXT not null
);
CREATE TABLE Employees
(
    EmployeeID    INTEGER         not null
        primary key,
    FirstName     TEXT not null,
    MiddleInitial TEXT null,
    LastName      TEXT not null
);
CREATE TABLE Products
(
    ProductID INTEGER            not null
        primary key,
    Name      TEXT    not null,
    Price     REAL null
);
CREATE TABLE Sales
(
    SalesID       INTEGER not null
        primary key,
    SalesPersonID INTEGER not null,
    CustomerID    INTEGER not null,
    ProductID     INTEGER not null,
    Quantity      INTEGER not null,
    foreign key (SalesPersonID) references Employees (EmployeeID)
            on update cascade on delete cascade,
        foreign key (CustomerID) references Customers (CustomerID)
            on update cascade on delete cascade,
        foreign key (ProductID) references Products (ProductID)
            on update cascade on delete cascade
);
