Need to assign tasks to staff, the one with the least should be the automatic candidate

As per my comment, the first time you run this the task should be assigned to 004 - PAT as they currently do not have any tasks at all assigned to them.

This code will do what you want:

UPDATE [tasks] SET AssignedTo = 
	(select TOP 1 idno
	from [Master] M left outer join [tasks] T on M.idno=T.AssignedTo
	group by idno
	order by COUNT(t.taskID) )
WHERE AssignedTo IS NULL

Things to note:
- the use of TOP 1 to only get a single idno
- The ORDER BY so the one with the least assignments filters to the top
- I used a table like this

creeate table [tasks]
(
	taskID int identity(1,1),
	Task nvarchar(150),
	AssignedTo int
)

- Note the WHERE clause so I only update the unassigned records

The first time you run this code you may get a warning about NULL values

Quote:

Warning: Null value is eliminated by an aggregate or other SET operation.

This can be ignored and will not appear once all entries on the [Master] table have at least one Task assigned to them

To be clear I used the following test data

insert into [Master] values
('JAMES'),   -- Id = 1 (Autogenerated)
('PAUL'),    -- Id = 2
('MAY'),     -- Id = 3
('PAT')      -- Id = 4

insert into [tasks] values
('John Smith',1),       -- assigned to JAMES
('Sarah Adams',1),      -- assigned to JAMES
('Michael Jones',1),    -- assigned to JAMES
('Godfred Arthur',2),   -- assigned to PAUL
('David Lawson',2),     -- assigned to PAUL
('Jennifer Bruce',3),   -- assigned to MAY
('new item',null)       -- assigned to NO-ONE!

So the starting point is:
JAMES has 3 tasks assigned
PAUL has 2 tasks assigned
MAY has 1 task assigned
PAT has no tasks assigned to them.
There are 7 tasks on the table, all but one of them are assigned to someone.

Extrapolating the code above this section

--select TOP 1 idno
SELECT idno, name, count(t.taskID)
	from [Master] M left outer join [tasks] T on M.idno=T.AssignedTo
	group by idno, name
	order by COUNT(t.taskID)

gives the results

4       PAT	0
3	MAY	1
2	PAUL	2
1	JAMES	3

So the bit I actually used

select TOP 1 idno
	from [Master] M left outer join [tasks] T on M.idno=T.AssignedTo
	group by idno
	order by COUNT(t.taskID)

will return the value 4.
That value will be assigned to all rows on the table where the AssignedTo column is null.

Importantly, as in your case in the comments, if no rows have a null value in the AssignedTo column then nothing will be updated and the counts for each row on the Master table will remain unchanged

If I add a new row

insert into tasks values ('item 2',null)

and run the code again, the second time through both MAY and PAT only have 1 task each. It's likely that MAY be assigned the new task simply because that Id is lower than PAT, but this is not guaranteed.