having some problems figuring this one out.
select *,(select top 1 chicken_nr from chicken_photo where chicken = code order by [sort]) as Photo from Chicken
Code is a column in Table Chicken
Basically getting the cover photo for this chicken.
To make it clearer, I want it to return multiple rows from table Chicken. But only a single entry from chicken_photo.
var q = from chicken in data.chickens
join photos in data.chicken_photos
on chicken.Code equals photos.chicken
where chicken.Lang==lang && chicken.photographer_nr == nr
group chicken by new {chicken.photographer,photos.Photograph_Nr,chicken.Title,chicken.Code}
This can indeed be done such that it results in only one SQL query underneath.
If you perform the subselect as you have written against Entity Framework, then the Linq query will become a single SQL query.
var q = from chicken in data.chickens
where chicken.photographer_nr == nr && chicken.Lang == lang
select new
{
chicken.photographer,
chicken.Code,
chicken.Title,
Photo = (from cp in data.chicken_photos
where cp.chicken == chicken.Code
orderby cp.Sort
select cp.Photograph_Nr).FirstOrDefault()
};
If your tables have proper primary and foreign key relationships, and proper navigation associations in Entity Framework then you can also achieve the same results this way:
var q = from chicken in data.chickens
where chicken.photographer_nr == nr && chicken.Lang == lang
select new
{
chicken.photographer,
chicken.Code,
chicken.Title,
Photo = c.chicken_photos.Select(cp => cp.Photograph_Nr).FirstOrDefault()
};
And finally, to stay completely consistent and use only lambda-expressions:
var q = data.chickens
.Where(c => chicken.photographer_nr == nr && chicken.Lang == lang)
.Select(c => new
{
c.photographer,
c.Code,
c.Title,
Photo = c.chicken_photos.Select(cp => cp.Photograph_Nr).FirstOrDefault()
}
);
I prefer relying on entity navigation, as it forces the developer to create proper navigation associations in Entity Framework and proper foreign key relationships in the database. This will almost always result in optimized SQL underneath.
It's not always up to the developer how the database is structured, so you may have to stick with the first approach and write the sub-select yourself.
I figured it out.
Pretty obvious actually, too obvious :)
var q = from chicken in data.chickens
where chicken.photographer_nr == nr && lang == chicken.Lang
select new { chicken.photographer, chicken.Code, chicken.Title,Photo = (from b in data.chicken_photos where b.chicken==chicken.Code orderby b.Sort select b.Photograph_Nr).FirstOrDefault() };
var photo = (from c in chicken_photo where c.code = chicken orderby c.sort select c.chicken_nr).Take(1).SingleOrDefault();
You should really flesh out your question more...
Related
I am trying to write a LINQ query equivalent to below SQL
SELECT DISTINCT m.*,rm.RoleId FROM dbo.Menu m
INNER JOIN dbo.RoleMenu rm on m.Id=rm.MenuId
INNER JOIN dbo.RoleUser ru on rm.RoleId=ru.RoleId
WHERE ru.UserName='dd#dd.com' and m.Url='/dashboard#/pm'
I came with the below query which is not returning the expected output
var auth = _context.RoleUsers.Where(
x => x.Role.MenuRoles.FirstOrDefault().Menu.Url == pagePermissions.Url
&& x.UserName == pagePermissions.UserName).Count()
May I know a better way to do this?
Your sql looks at all the menus related to a role user, but your Linq is only looking at the first one. I think you want x.Role.MenuRoles.Any(mr => mr.Menu.Url == pagePermissions.Url). But then you're also doing a Count on the matching users instead of selecting the menus that match that url. A closer translation would be.
var results = (from m in _context.Menus
from rm in m.RoleMenus
from ru in rm.RoleUsers
where m.Url == pagePermissions.Url
&& u.UserName == pagePermissions.UserName
select new { Menu = m, rm.RoleId }).Distinct();
You may have to adjust some of the navigation properties as I was just guessing at them. They usually are pluralizations of the tables, but I see in your Linq that you have MenuRoles instead of RoleMenus.
I have following table in my database and im accessing them through EF.
TestPack { id, name, type }
Sheets{ id, tp_id, date, rev }
Spool { id, sheet_id, date, rev, fabricated, installed }
which means a test pack has 1-M sheets and each sheet has 1-M spools. I want to get count of total Spools in the Test Pack, and count of spools that are fabricated, count of spools that are installed.
How do I get that through Linq query?
if I understand you right,you would like to have something like this
(from tp in ctx.TestPack
join st in ctx.Sheets on st.tp_id equals tp.id
join sl in ctx.Spool on sl.steet_id equals st.id
where tp.id == testPackId //you can change or delete this condition
select new {
Total = sl.Count() ,
FabricatedSpools = sl.Count(x=>x.fabricated == true),
InstalledSpools = sl.Count(x=>x.installed == true)
}).FisrtOrDefault();
Or maybe
(from tp in ctx.TestPack
join st in ctx.Sheets on st.tp_id equals tp.id
join sl in ctx.Spool on sl.steet_id equals st.id
where tp.id == testPackId //you can change or delete this condition
select new {
Total = sl.Count() ,
FabricatedSpools = (from s in sl
where s.fabricated == true
select s.Count()),
InstalledSpools = (from i in sl
where i.installed== true
select i.Count()),
}).FisrtOrDefault();
Not sure what you exact models are like but see below.
var testPackID = 2;//assuming
//assuming your DbContext is ctx;
var totalSpools = ctx.Spools.Count(x => x.Sheets.tp_id == testPackID);
var fabricatedSpools = ctx.Spools.Count(x => x.Sheets.tp_id == testPackID && x.fabricated);
var installedSpools = ctx.Spools.Count(x => x.Sheets.tp_id == testPackID && x.installed);
Sample data and query
I had generated the queries in SQL Server and hope you can do in LINQ. if you want specifically in LINQ let me know.
And can you please clarify whether you want result in 3 or all the 3 results in one.
Hope this helps.
Thank you.
There are three tables in the database that are relevant. Advocate, Vendor, and Advocate_Vendor.
The Advocate_Vendor table being the many to many link, has a vendorId and an advocateId.
My end goal is to get back a List<Advocate> object...a collection of advocates that belong to one Vendor. I wrote this:
var list = new List<Advocate>();
foreach (var vendorAdvocates in db.Advocate_Vendors)
{
if (vendorAdvocates.VendorId == vendorId)
{
list.Add(db.Advocates.SingleOrDefault(a => a.AdvocateId == vendorAdvocates.AdvocateId));
}
}
And then this:
var list = (from vendorAdvocates in db.Advocate_Vendors
where vendorAdvocates.VendorId == vendorId
select db.Advocates.SingleOrDefault(a =>
a.AdvocateId == vendorAdvocates.AdvocateId)).ToList();
Is this the best way? Seems wrong, like maybe there could be a more streamlined way to do this using a 'contains' keyword or something that looks a bit more readable...get all the vendor's advocates
thanks
Using a join between Advocate_Vendors and Advocates would be the right way of doing it.
var list = (from vendorAdvocates in db.Advocate_Vendors
join advocates in db.Advocates
on vendorAdvocates.AdvocateId equals advocates.AdvocateId
where vendorAdvocates.VendorId == vendorId
select advocates).ToList();
var list = (from vendorAdvocates in db.Advocate_Vendors
from advocate in db.Advocates
where vendorAdvocates.VendorId == vendorId &&
vendorAdvocates.AdvocateId = advocate.Id
select advocate)
.ToList();
If you set up your foreign keys and navigation properties properly, it should be possible to write this way, or something like it:
var list = (from vendorAdvocates in db.Advocate_Vendors
where vendorAdvocates.VendorId == vendorId
select db.Advocate).ToList();
i'm very new to linq to sql and in need of a little assistance.
Basically i'm building a message board in C#. I have 3 database tables - basic info is as follows.
FORUMS
forumid
name
THREADS
threadid
forumid
title
userid
POSTS
postid
threadid
text
userid
date
Basically I want to bring back everything I need in one query. I want to list a page of THREADS (for a particular FORUM) and also display the number of POSTS in that THREAD row and when the last POST was for that THREAD.
At the moment i'm getting back all THREADS and then looping through each the result set and making calls to the POST table seperately for the POST count for a Thread and the Latest Post in that thread but obviously this will cause problems in terms of hitting the database as the Message Board gets bigger.
My Linq To SQL so far:
public IList<Thread> ListAll(int forumid)
{
var threads =
from t in db.Threads
where t.forumid == forumid
select t;
return threads.ToList();
}
basicaly i now need to get the number of POSTS in each thread and the date of the last post in each thread.
Any help would be most appreciated :)
EDIT
Hi guys. Thanks for tyour help so far. Basically i'm almost there. However, I left an important part out of my initial question in the fact that I need to retrieve the user name of the person making the last POST. Therefore I need to join p.userid with u.userid on the USERS table. So far I have the following but just need to amend this to join the POST table with the USER table:
public IList<ThreadWithPostInfo> ListAll(int forumid)
{
var threads = (from t in db.Threads
where t.forumid == forumid
join p in db.Posts on t.threadid equals p.threadid into j
select new ThreadWithPostInfo() { thread = t, noReplies = j.Count(), lastUpdate = j.Max(post => post.date) }).ToList();
return threads;
}
UPDATE:
public IList<ThreadWithPostInfo> ListAll(int forumid)
{
var threads = (from t in db.Threads
from u in db.Users
where t.forumid == forumid && t.hide == "No" && t.userid == u.userid
join p in db.Posts on t.threadid equals p.threadid into j
select new ThreadWithPostInfo() { thread = t, deactivated = u.deactivated, lastPostersName = j.OrderByDescending(post => post.date).FirstOrDefault().User.username, noReplies = j.Count(), lastUpdate = j.Max(post => post.date) }).ToList();
return threads;
}
I finally figured that part of it out with thanks to all of you guys :). My only problem now is the Search Results method. At the moment it is like this:
public IList<Thread> SearchThreads(string text, int forumid)
{
var searchResults = (from t in db.Threads
from p in db.Posts
where (t.title.Contains(text) || p.text.Contains(text)) && t.hide == "No"
&& p.threadid == t.threadid
&& t.forumid == forumid
select t).Distinct();
return searchResults.ToList();
}
Note that I need to get the where clause into the new linq code:
where (t.title.Contains(text) || p.text.Contains(text)) && t.hide == "No"
so incorporating this clause into the new linq method. Any help is gratefully received :)
SOLUTION:
I figured out a solution but I don't know if its the best one or most efficient. Maybe you guys can tell me because i'm still getting my head around linq. James I think your answer was closest and got me to near to where I wanted to be - thanks :)
public IList<ThreadWithPostInfo> SearchThreads(string text, int forumid)
{
var searchResults = (from t in db.Threads
from p in db.Posts
where (t.title.Contains(text) || p.text.Contains(text)) && t.hide == "No"
&& p.threadid == t.threadid
&& t.forumid == forumid
select t).Distinct();
//return searchResults.ToList();
var threads = (from t in searchResults
join p in db.Posts on t.threadid equals p.threadid into j
select new ThreadWithPostInfo() { thread = t, lastPostersName = j.OrderByDescending(post => post.date).FirstOrDefault().User.username, noReplies = j.Count(), lastUpdate = j.Max(post => post.date) }).ToList();
return threads;
}
May be Too many database calls per session ....
Calling the database,. whether to query or to write, is a remote call, and we want to reduce the number of remote calls as much as possible. This warning is raised when the profiler notices that a single session is making an excessive number of calls to the database. This is usually an indication of a potential optimization in the way the session is used.
There are several reasons why this can be:
A large number of queries as a result of a Select N + 1
Calling the database in a loop
Updating (or inserting / deleting) a large number of entities
A large number of (different) queries that we execute to perform our task
For the first reason, you can see the suggestions for Select N + 1. Select N + 1 is a data access anti-pattern where the database is accessed in a suboptimal way. Take a look at this code sample :
// SELECT * FROM Posts
var postsQuery = from post in blogDataContext.Posts
select post;
foreach (Post post in postsQuery)
{
//lazy loading of comments list causes:
// SELECT * FROM Comments where PostId = #p0
foreach (Comment comment in post.Comments)
{
//print comment...
}
}
In this example, we can see that we are loading a list of posts (the first select) and then traversing the object graph. However, we access the collection in a lazy fashion, causing Linq to Sql to go to the database and bring the results back one row at a time. This is incredibly inefficient, and the Linq to Sql Profiler will generate a warning whenever it encounters such a case.
The solution for this example is simple. Force an eager load of the collection using the DataLoadOptions class to specify what pieces of the object model we want to load upfront.
var loadOptions = new DataLoadOptions();
loadOptions.LoadWith<Post>(p => p.Comments);
blogDataContext.LoadOptions = loadOptions;
// SELECT * FROM Posts JOIN Comments ...
var postsQuery = (from post in blogDataContext.Posts
select post);
foreach (Post post in postsQuery)
{
// no lazy loading of comments list causes
foreach (Comment comment in post.Comments)
{
//print comment...
}
}
next is updating a large number of entities is discussed in Use Statement Batching, and can be achieved by using the PLinqO project, which is a set of extensions on top of Linq to Sql. How cool would it be to store items in cache as a group. Well, guess what! PLINQO is cool! When storing items in cache, just tell PLINQO the query result needs to belong to a group and specify the name. Invalidating cache is where the coolness of grouping really shows up. No coupling of cache and actions taken on that cache when they are in a group. Check out this example :
public ActionResult MyTasks(int userId)
{
// will be separate cache for each user id, group all with name MyTasks
var tasks = db.Task
.ByAssignedId(userId)
.ByStatus(Status.InProgress)
.FromCache(CacheManager.GetProfile().WithGroup("MyTasks"));
return View(tasks);
}
public ActionResult UpdateTask(Task task)
{
db.Task.Attach(task, true);
db.SubmitChanges();
// since we made an update to the tasks table, we expire the MyTasks cache
CacheManager.InvalidateGroup("MyTasks");
}
PLinqO supports the notion of query batching, using a feature called futures, which allow you to take several different queries and send them to the database in a single remote call. This can dramatically reduce the number of remote calls that you make and increase your application performance significantly.
cmiiw ^_^
public IList<Thread> ListAll(int forumid)
{
var threads =
from t in db.Threads
where t.forumid == forumid
select new
{
Thread = t,
Count = t.Post.Count,
Latest = t.Post.OrderByDescending(p=>p.Date).Select(p=>p.Date).FirstOrDefault()
}
}
Should be something like that
I think what you're really looking for is this:
var threadsWithPostStats = from t in db.Threads
where t.forumid == forumid
join p in db.Posts on t.threadid equals p.threadid into j
select new { Thread = t, PostCount = j.Count(), LatestPost = j.Max(post => post.date) };
Per your comment and updated question, I'm adding this restatement:
var threadsWithPostsUsers = from t in db.Threads
where t.forumid == forumid
join p in db.Posts on t.threadid equals p.threadid into threadPosts
let latestPostDate = threadPosts.Max(post => post.date)
join post in db.Posts on new { ThreadID = t.threadid, PostDate = latestPostDate } equals new { ThreadID = post.threadid, PostDate = post.date} into latestThreadPosts
let latestThreadPost = latestThreadPosts.First()
join u in db.Users on latestThreadPost.userid equals u.userid
select new { Thread = t, LatestPost = latestThreadPost, User = u };
Wouldn't hurt to get familiar with group by in LINQ and aggregates (Max, Min, Count).
Something like this:
var forums = (from t in db.Threads
group t by t.forumid into g
select new { forumid = g.Key, MaxDate = g.Max(d => d.ForumCreateDate) }).ToList();
Also check out this article for how to count items in a LINQ query with group by:
LINQ to SQL using GROUP BY and COUNT(DISTINCT)
LINQ aggregates:
LINQ Aggregate with Sub-Aggregates
I'm just wondering if anyone can offer any advice on how to improve my query.
Basically, it'll be merging 2 rows into 1. The only thing the rows will differ by is a 'Type' char column ('S' or 'C') and the Value. What I want to do is select one row, with the 'S' value and the 'C' value, and calculate the difference (S-C).
My query works, but it's pretty slow - it takes around 8 seconds to get the results, which is not ideal for my application. I wish I could change the database structure but I can't sadly!
Here is my query:
var sales = (from cm in dc.ConsignmentMarginBreakdowns
join sl in dc.SageAccounts on new { LegacyID = cm.Customer, Customer = true } equals new { LegacyID = sl.LegacyID, Customer = sl.Customer }
join ss in dc.SageAccounts on sl.ParentAccount equals ss.ID
join vt in dc.VehicleTypes on cm.ConsignmentTripBreakdown.VehicleType.Trim() equals vt.ID.ToString() into vtg
where cm.ConsignmentTripBreakdown.DeliveryDate >= dates.FromDate && cm.ConsignmentTripBreakdown.DeliveryDate <= dates.ToDate
where (customer == null || ss.SageID == customer)
where cm.BreakdownType == 'S'
orderby cm.Depot, cm.TripNumber
select new
{
NTConsignment = cm.NTConsignment,
Trip = cm.ConsignmentTripBreakdown,
LegacyID = cm.LegacyID,
Costs = dc.ConsignmentMarginBreakdowns.Where(a => a.BreakdownType == 'C' && a.NTConsignment == cm.NTConsignment && a.LegacyID == cm.LegacyID && a.TripDate == cm.TripDate && a.Depot == cm.Depot && a.TripNumber == cm.TripNumber).Single().Value,
Sales = cm.Value ?? 0.00m,
Customer = cm.Customer,
SageID = ss.SageID,
CustomerName = ss.ShortName,
FullCustomerName = ss.Name,
Vehicle = cm.ConsignmentTripBreakdown.Vehicle ?? "None",
VehicleType = vtg.FirstOrDefault().VehicleTypeDescription ?? "Subcontractor"
});
A good place to start when optimizing Linq to SQL queries is the SQL Server Profiler. There you can find what SQL code is being generated by Linq to SQL. From there, you can toy around with the linq query to see if you can get it to write a better query. If that doesn't work, you can always write a stored procedure by hand, and then call it from Linq to SQL.
There really isn't enough information supplied to make an informed opinion. For example, how many rows in each of the tables? What does the generated T-SQL look like?
One thing I would suggest first is to take the outputted T-SQL, generate a query plan and look for table or index scans.