set-operations

I assume there's no good way to do so, but if you had to, how would you implement set operations on Appengine's datastore? For example given two collections of integers, how would you store them in the datastore to get a good performance for intersect and except (all those items in A that are not in B) operations? There aren't any built in set operations in the datastore API. I see you having two options: For smallish sets (hundreds of items) You might get away with doing keys-only queries for both set A and set B, and doing the intersection in your application code. The precise definition of

I came across this example and I don't understand what it means. (SELECT drinker FROM Frequents) EXCEPT ALL (SELECT drinker FROM Likes); relations: Frequents(drinker, bar), Likes(drinker, beer) What does the ALL do in this case? How is the result different from the query below? (SELECT drinker FROM Frequents) EXCEPT (SELECT drinker FROM Likes); The SQL EXCEPT operator takes the distinct rows of one query and returns the rows that do not appear in a second result set. The EXCEPT ALL operator does not remove duplicates. For purposes of row elimination and duplicate removal, the EXCEPT operator

What's the simplest way to perform a set subtraction given two arrays in C#? Apparently this is dead easy in Ruby. Basically I just want to remove the elements from array a that are in array b : string[] a = new string[] { "one", "two", "three", "four" }; string[] b = new string[] { "two", "four", "six" }; string[] c = a - b; // not valid c should equal { "one", "three" } . b - a would yield { "six" } . If you're using Linq, you can use the Except operator like this: string [] c = a.Except(b).ToArray(); Edit: CodeInChaos makes a good point. If a contains duplicates, it will remove any

I have the following tables: work_units - self explanatory workers - self explanatory skills - every work unit requires a number of skills if you want to work on it. Every worker is proficient in a number of skills. work_units_skills - join table workers_skills - join table A worker can request the next appropriate free highest priority (whatever that means) unit of work to be assigned to her. Currently I have: SELECT work_units.* FROM work_units -- some joins WHERE NOT EXISTS ( SELECT skill_id FROM work_units_skills WHERE work_unit_id = work_units.id EXCEPT SELECT skill_id FROM workers_skills

Say I have 5 vectors: a <- c(1,2,3) b <- c(2,3,4) c <- c(1,2,5,8) d <- c(2,3,4,6) e <- c(2,7,8,9) I know I can calculate the intersection between all of them by using Reduce() together with intersect() , like this: Reduce(intersect, list(a, b, c, d, e)) [1] 2 But how can I find elements that are common in, say, at least 2 vectors? i.e.: [1] 1 2 3 4 8 It is much simpler than a lot of people are making it look. This should be very efficient. Put everything into a vector: x <- unlist(list(a, b, c, d, e)) Look for duplicates unique(x[duplicated(x)]) # [1] 2 3 1 4 8 and sort if needed. Note: In

I have two collections of type ICollection<MyType> called c1 and c2 . I'd like to find the set of items that are in c2 that are not in c1 , where the heuristic for equality is the Id property on MyType . What is the quickest way to perform this in C# (3.0)? Use Enumerable.Except and specifically the overload that accepts an IEqualityComparer<MyType> : var complement = c2.Except(c1, new MyTypeEqualityComparer()); Note that this produces the set difference and thus duplicates in c2 will only appear in the resulting IEnumerable<MyType> once. Here you need to implement IEqualityComparer<MyType> as