Selecting employee based availability Query SQL Server 2012

I have two tables, one for employees and another of their current and past activities. I need a function/query that selects an employee for the next activity based on type. here is sample data:

Table Employees

EmployeesID UserName    Type    Available
1           Bill    Clerical    1
2           Ann     Clerical    1
3           John    Technical   1
4           Jack    Technical   0
5           Mary    Clerical    0
6           Sam     Technical   0
7           Mark    Clerical    1
8           Andy    Clerical    1
9           Rick    Clerical    1
10          Nancy   Clerical    1

Activities

ActivityID  EmployeesID Status  Task_Datetime
101                2    Complete    6/11/2016 10:00
102                1    Complete    6/11/2016 10:12
103                3    Complete    6/11/2016 10:24
104                4    Complete    6/11/2016 10:36
105                7    Complete    6/11/2016 10:48
106                6    Complete    6/11/2016 11:00
107                5    Complete    6/11/2016 11:12
108                8    Complete    6/11/2016 11:24
109               10    Complete    6/11/2016 11:36
110                9    Complete    6/11/2016 11:48
111                5    Complete    6/11/2016 12:00
112                8    Complete    6/11/2016 12:12
113                3    Complete    6/11/2016 12:24
114                4    Complete    6/11/2016 12:36
115                1    Complete    6/11/2016 12:48
116                6    Complete    6/11/2016 13:00
117                7    Complete    6/11/2016 13:12
118                2    Complete    6/11/2016 13:24
119                9    Complete    6/11/2016 13:36
120               10    In Progress 6/11/2016 13:48
121                1    In Progress 6/11/2016 14:00
122                2    Complete    6/11/2016 14:12
123                3    Complete    6/11/2016 14:24
124                4    Complete    6/11/2016 14:36
125                6    In Progress 6/11/2016 14:48

Need to assign to available employee matching type without an activity in progress if all are in progress, return 0 if more than one available, assign to the one that was assigned

so if next assign is clerical, do not take 1 or 10 since in progress, but take last first completed clerical of 2,5,7,8,9 (other unoccupied ones) which is 8 (skipping 5 since is not available)

if next assign is technical, it should go to John and if john is not available, should return null.

Wrote something like this so far:

function: NextEmployee('Clerical')

Select EmployeeID from Employees e , Activities a where e.available= 1 and e.type='Clerical' and