Applying functions to list elements selectively

Question

I've got two lists that contain an element identifier and a number:

list1 = Transpose[{{"c", "a", "b"}, {2, 1, 1}}]
list2 = Transpose[{{"a", "b", "c", "e", "f"}, {2, 2, 2, 2, 2}}]
listdesired = Transpose[{{"a", "b", "c", "e", "f"}, {1, 1, 0, 2, 2}}]

I'd like to know an efficient method to produce listdesired from list1 and list2. In this case, the desired output is found by subtracting the integer in list1 from the item in list2 that has the same identifier ('a', 'b', 'c', ...).

Update

The solution that worked for me is reproduced below since it is currently in a comment and I would like the answer to persist. The answer that has been accepted works just fine as well.

KeyValueMap[List]@*Subtract @@ KeyUnion[{Rule @@@ #2, Rule @@@ #}, 0 &] &[list1, list2]

Answer 1

In the case of a subtraction, you can do :

list1 = Transpose[{{"c", "a", "b"}, {2, 1, 1}}];
list2 = Transpose[{{"a", "b", "c", "e", "f"}, {2, 2, 2, 2, 2}}];

Merge[{Rule @@@ list2, (#1 -> -#2) & @@@ list1}, Total] //KeyValueMap[List] 

{{"a", 1}, {"b", 1}, {"c", 0}, {"e", 2}, {"f", 2}}

Note that the second argument of Merge,Total, can' t do itself
the subtraction, because it doesn' t know where a unique data is
coming from (list1 or list2 ?)

More precisely, it depends on what you expect if there is an identifier in list1 that is not in list2.

Answer 2

Use Associations:

a = AssociationThread[{"a", "b", "c", "e", "f"}, {2, 2, 2, 2, 2}];
b = AssociationThread[{"c", "a", "b"}, {2, 1, 1}];
Merge[{a, -b}, Total]

<|"a" -> 2, "b" -> 2, "c" -> 2, "e" -> 2, "f" -> 2|>

<|"c" -> 2, "a" -> 1, "b" -> 1|>

<|"a" -> 1, "b" -> 1, "c" -> 0, "e" -> 2, "f" -> 2|>

Oh, now I see that this is basically what andre314 proposed (minus some massaging). But this actually shows that Association might provide a better representation of your data as your data are key-values lists.