问题
I have a large result set coming from a pretty complex SQL query. Among the values are a string which represents a location (that will later help me determine the page location that the value came from), an int which is a priority number calculated for each row based on other values from the row, and another string which contains a value I must remember for display later.
The problem is that the sql query is so complex (it has UNIONS, JOINS, and complex calculations with aliases) that I can't logically fit anything else into it without messing with the way it works.
Suffice it to say, though, after the query is done and the calculations performed, I need something that perhaps aggregate functions might solve, but that IS NOT an option, as all the columns do not come from other aggregate functions.
I have been wracking my brain for days now as to how I can iterate through the results, store a pair of values in a list (or two separate lists tied together somehow) where one value is the sum of all the priority values for each location and the other value is a distinct location value (i.e., as the results are looped through, it will not create another list item with the same location value that has been used before, HOWEVER, it does still need the sum of all of the other priority values from locations that ARE identical). Also, the results need to be ordered by priority in Descending order (hence the problem with using two lists).
EXAMPLE:
EDIT: I forgot, the preserved value should be the value from the row with the highest priority from the sql query.
If I had the following results:
location priority value
--------------------------------------------------------------------------------
page1 1 some text!
page2 3 more text!
page2 4 even more text!
page3 3 text again
page3 1 text
page3 1 still more text!
page4 6 text
If I was able to do what I wanted I would be able to achieve something like this after iteration (and in this order):
location priority value
--------------------------------------------------------------------------------
page2 7 even more text!
page4 6 text
page3 5 text again
page1 1 some text!
I have done research after research after research but absolutely nothing really even gets close to solving this dilemma.
Is what I'm asking too tough for even the powerful C# language?
THINGS I HAVE CONSIDERED:
Looping through the sql results and checking each location for repeats, adding together all priority values as I go, and storing these two plus value in two or three separate lists.
Why I still need help
I can't use a foreach because the logic didn't pan out, and I can't use a for loop because I can't access an IEnumerable (or whatever type it is that stores what's returned from Database.Open.Query() by index. (this makes sense, of course). Also, I need to sort on priority, but can't get one list out of sync with the others.
Using LINQ to select and store what I need
Why I still need help
I don't know LINQ (at all!) mainly because I don't understand lambda expressions (no matter HOW MUCH I read up about it).
Using an instantiated class to store the name/value pairs
Why I still need help
Not only do I expect sorting on this sort of thing to be impossible, and while I do now how to use .cs files in my C#.net webpages with WebMatrix environment, I have mainly only ever used static classes and would also need a little refresher course on constructors and how to set this up appropriately.
Somehow fitting this functionality into the already sizeable and complex SQL query
Why I still need help
While this is probably where I would ideally like this functionality to be, I stress again that this IS NOT AN OPTION. I have tried using aggregate functions, but only get an error saying how not all the other columns come from aggregate functions.
Making another query based on values from the first query's result set
Why I still need help
I can't select distinct results based on only one column (i.e., location) alone.
Assuming I could get the loop logic correct, storing the values in a 3 dimensional array
Why I still need help
I can't declare the array, because I do not know all of its dimensions before I need to use it.
回答1:
Your post has amazed me in a number of ways like saying to 'mostly using static classes' and 'expecting instantiate a class/object to be impossible'.. really strange things you say. I can only respond in a quote from Charles Babbage:
I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.
Anyways.. As you say you find lambdas hard, let's trace the problem in the classic 'manual' way.
Let's assume you have a list of ROWS that contains LOCATIONS and PRIORITIES.
List<DataRow> rows = .... ; // datatable, sqldatareader, whatever
You say you need:
- list of unique locations
- a "list" of locations paired up with summed up priorites
Let's start with the first objective.
To gather a list of unique 'values', a HashSet is just perfect:
HashSet<string> locations = new HashSet<string>();
foreach(var row in rows)
locations.Add( (string)rows["LOCATION"] );
well, and that's all. After that, the locations hashset will only remember all the unique locations. The "Add" does not result in duplicate elements. The HashSet checks and "uniquifies" all values that are put inside it. Small tricky thing is the hashset does not have the [index] operator. You'll have to enumerate the hashset to get the values:
foreach(string loc in locations)
{
Console.WriteLine(loc);
}
or convert/rewrite it to a list:
List<string> locList = new List<string>(locations);
Console.WriteLine(locList[2]); // of course, assuming there were at least three..
Let's get to the second objective.
To gather a list of values related to some thing behaving like a "logical key", a Dictionary<Key,Val> may be useful. It allows you to store/associate a "value" with some "key", ie:
Dictionary<string, double> dict = new Dictionary<string, double>();
dict["mamma"] = 123.45;
double d = dict["mamma"]; // d == 123.45
dict["mamma"] += 101; // possible! double e = dict["mamma"]; // d == 224.45
However, it has a behavior of happily throwing exceptions when you try to read from an unknown key:
Dictionary<string, double> dict = new Dictionary<string, double>();
dict["mamma"] = 123.45;
double d = dict["daddy"]; // throws KeyNotBlarghException
dict["daddy"] += 101; // would throw too! tries to read the old/current value!
So, one have to be very careful with it with "keys" that it does not yet know. Fortunatelly, you can always ask the dictionary if it already knows a key:
Dictionary<string, double> dict = new Dictionary<string, double>();
dict["mamma"] = 123.45;
bool knowIt = dict.ContainsKey("daddy"); // == false
So you can easily check-and-initialize-when-unknown:
Dictionary<string, double> dict = new Dictionary<string, double>();
bool knowIt = dict.ContainsKey("daddy"); // == false
if( !knowIt )
dict["daddy"] = 5;
dict["daddy"] += 101; // now 106
So.. let's try summing up the priorities location-wise:
Dictionary<string, double> prioSums = new Dictionary<string, double>();
foreach(var row in rows)
{
string location = (string)rows["LOCATION"];
double priority = (double)rows["PRIORITY"];
if( ! prioSums.ContainsKey(location) )
// make sure that dictionary knows the location
prioSums[location] = 0.0;
prioSums[location] += priority;
}
And, really, that's all. Now the prioSums will know all locations and all sums of priorities:
var sss = prioSums["NewYork"]; // 9123, assuming NewYork was some location
However, that'd be quite useless to have to hardcode all locations. Hence, you also can ask the dictionary about what keys does it curently know
foreach(string key in prioSums.Keys)
Console.WriteLine(key);
and you can immediatelly use it:
foreach(string key in prioSums.Keys)
{
Console.WriteLine(key);
Console.WriteLine(prioSums[key]);
}
that should print all locations with all their sums.
You might already noticed an interesting thing: the dictionary can tell you what keys has it remembered. Hence, you do not need the HashSet from the first objective. Simply by summing up the priorities inside the Dictionary, you get the uniquized list of location by free: just ask the dict for its keys.
EDIT:
I noticed you've had a few more requests (like sort-descending or find-highest-prio-value), but I think I'll leave them for now. If you understand how I used a dictionary to collect the priorities, then you will easily build a similar Dictionary<string,string> to collect the highest-ranking value for a location. And the 'descending order' is done very easily if only you take the values out of dictionary and sort them as a i.e. List.. So I'll skip that for now.. This text got far tl;dr already I think :)
回答2:
LINQ is really the tool to use for this kind of problems.
Suppose you have a variable pages which is an IEnumerable<Page>, where Page is a class with properties location, priority and value you could do
var query = from page in pages
group page by page.location into grp
select new { location = grp.Key,
priority = grp.Sum(page => page.priority),
value = grp.OrderByDescending(page => page.priority)
.First().value
}
You say you don't understand LINQ, so let me try to begin explain this statement.
The rows are group by location, which results in 4 groups of pages of which page.location is the key:
location priority value
--------------------------------------
page1 1 some text!
page2 3 more text!
4 even more text!
page3 1 text
1 still more text!
3 text again
page4 6 text
The select loops through these 4 groups and for each group it creates an anonymous type with 3 properties:
location: the key of the grouppriority: the sum of priorities in one groupvalue: the firstvaluein one group when its pages are sorted by priority in descending order.
The lamba expressions are a way to express which property should be used for a LINQ function like Sum. In short they say "transform page to page.priority": page => page.priority.
You want these new rows in descending order of priority, so finally you can do
result = query.OrderByDescending(x => x.priority).ToList();
The x is just an arbitrary placeholder representing one item in the collection in hand, query (likewise in the query above page could have been any word or character).
来源：https://stackoverflow.com/questions/17604201/is-there-any-way-to-loop-through-my-sql-results-and-store-certain-name-value-pai