I've got a database table with the following schema:
id
screenshot
language
projects_ID
I'm need to write a LINQ query, that, given an 'id', 'projects_ID', and 'language', will return the 'id' of the next row that has a matching 'projects_ID' and 'language'.
In English, my query would be:
select the next 'id' from screenshots after the current 'id' where
'projects_ID' == current 'projects_ID' and language == current 'language'
Is this possible to do this via LINQ?
Thanks.
Sounds like you probably want:
var next = table.Where(item => item.id > lastId &&
item.projects_ID == projects_ID &&
item.language == language)
.OrderBy(item => item.id)
.FirstOrDefault();
Or as a query expression:
var next = (from item in table
where item.id > lastId &&
item.projects_ID == projects_ID &&
item.language == language
orderby item.id
select item).FirstOrDefault();
The result will be null if there are no matches (e.g. if you're already looking at the last ID). This is assuming by "next" you mean "matching item with the lowest ID greater than the current one".
Related
I have a following problem with LINQ to Entities. I am selecting records from DB, group them and then order them by Id descending. Then i want to select First item but only if Quantity is != 0. I think because i have to use FirstOrDefault i am getting wrong results but am not sure how to correct it.
DB
Now this query will give me ID's 2 and 1 grouped and 4 "grouped". What i need is to select FIRST items only if Quantity is != 0 and am not sure how to get that.
data = DbContext.Items.Where(x.WarehouseId == 1)
.GroupBy(x => x.ItemCode, (key, g) => g.OrderByDescending(y => y.Id).FirstOrDefault())
.OrderBy(parameters.SortExpression)
.Skip(parameters.Start)
.Take(parameters.Length);
If WarehouseID is 1 i need to get only row with ID 4 back. Any help is appreciated.
Edit: First i need to groupBy ItemCode, then i will have two groups for my case above. That's 1 and 2 for first group, and 4 for second group. Then i order them by Id descending and i get (2, 1), (4). Then i need to select first from group but only if Quantity is != 0. If Quantity is zero i don't want to select anything from a group.
Image to clarify what i need. I'm stuck on last step Take FristFrom each group only if quantity is != 0
EDIT: This is, what you want to do:
var data = DbContext.Items
.Where(p => p.WarehouseId == 1)
.GroupBy(p => p.ItemCode, (key, items) => new { Key = key, Items = items, ItemsCountWithQuantityZero = items.Where(p => p.Quantity == 0).Count() })
.Where(p => p.ItemsCountWithQuantityZero == 0)
.Select(p => p.Items.OrderByDescending(q => q.Id).First());
How can I select documents based on values within the embedded documents? For instance, I have class r that has a list of i classes. I want to select all r's that have i.name=="foo".
If you want to select all r's that have at least one i with i.Name == "foo" you can use this query:
var result = collectionOfRs.Where(r => r.ListOfIs.Any(i => i.Name == "foo"));
If you want to select all r's that have all i's with i.Name == "foo" you can use this one:
var result = collectionOfRs.Where(r => r.ListOfIs.All(i => i.Name == "foo"));
I have a long query that returns
Item {
DateTime entryDate
.....
}
I like to combine the result of this query with another table
Value {
DateTime date,
double value
}
such that if entryDate >= CUTOFF, then take the value on CUTOFF, else take the value on entryDate. In other words, I'd want to achieve:
SELECT Item.*, Value.value WHERE
MIN( Item.entryDate, CUTOFF ) == Value.date
Excuse my syntax, but that's the idea.
EDIT: After some trial and error, I came up with this linq-to-sql query:
from iValue in Values
join iItem in ... (long query)
let targetDate = iItem.EntryDate > CUTOFF ? iItem.EntryDate : CUTOFF
where iValue.Date == targetDate
select new
{
iItem,
targetDate,
iValue
}
Thanks for your help.
yourLongQuery.Where(y => y.Item.entryDate == Value.date || CUTOFF == Value.date)
.Select(x => new {
entrydate = (x.Item.entryDate < CUTOFF ? x.Item.entryDate : CUTOFF),
/*rest of x.Item properties here */ ,
x.Value.date,
x.Value.value
});
Filter the query, Combine the two items into one item and modify the first item
Given that you've returned your data from your Item query and that the Value table is relatively small then this is a nice way to go:
var lookup = values.ToLookup(v => v.date, v => v.value);
var query =
from i in items
let c = i.entryDate < CUTOFF ? i.entryDate : CUTOFF
let v = lookup[c].FirstOrDefault()
select new
{
Item = i,
Value = v,
};
The ToLookup extension is very useful and often overlooked.
such that if entryDate >= CUTOFF, then take the value on CUTOFF, else
take the value on entryDate. In other words, I'd want to achieve:
SELECT Item.*, Value.value WHERE MAX( Item.entryDate, CUTOFF ) ==
Value.date
This is contradictory - if entryDate >= CUTOFF then take the value on CUTOFF imples that you want MIN(Item.entryDate, CUTOFF), not MAX.
Having said that, you just want to select the value.Value that matches each item of your query. Each item should look up the relevant value which matches your MAX (or, I believe, MIN) statement.
query.Select(item =>
{
var matchingValue = Values.Single(v =>
v.date == Min(item.entryDate, CUTOFF));
return new { item, matchingValue.value };
});
This will return an IQueryable of anonymous { Item, double } objects.
If you require this to be a executed as a single SQL statement you'll need to do some refactoring. A good start is to swap matchingValue into a single statement.
query.Select(item => new { item, context.Values.Single(v =>
v.date == Min(item.entryDate, CUTOFF)).value });
I don't have a Visual Studio in front of me to confirm, but I am not sure that the Math.Min function is mapped in LINQ-to-SQL. Let's assume it's not.
query.Select(item => new { item, context.Values.Single(v =>
v.date == (item.entryDate < CUTOFF ? item.entryDate : CUTOFF)).value });
I believe that will resolve to a single query if you execute it with a .ToList() but can't confirm until I have some tools in front of me. Test it with SQL profiler to be sure.
I tried to use the suggestion provided here for using In operator in linq but, i am not able to convert my requirement into LINQ statement.
Below is the SQL query which i need to convert to Linq
select *
from navigator_user_field_property
where user_id = 'albert'
and field_id in (
select field_id
from navigator_entity_field_master
where entity_id = 1
and use_type = 0)
order by field_id
I want this to be converted to a Efficient Linq.
Most of the answers deal with the predetermined list of string array which is not working in my case.
Thanks
Looks like a join to me:
var query = from navigator in db.NavigatorUserFieldProperties
where navigator.UserId == "albert"
join field in db.NavigatorEntityFieldMasters
.Where(f => f.EntityId == 1 && f.UseType == 0)
on navigator.FieldId equals field.FieldId
select navigator;
Note that this will return the same value multiple times if there are multiple fields with the same ID - but I suspect that's not the case.
You could do a more literal translation like this:
var query = from navigator in db.NavigatorUserFieldProperties
where navigator.UserId == "albert" &&
db.NavigatorEntityFieldMasters
.Where(f => f.EntityId == 1 && f.UseType == 0)
.select(f => f.FieldId)
.Contains(navigator.FieldId)
select navigator;
... and that may end up translating to the same SQL... but I'd personally go with the join.
Here is an efficient and readable LINQ query:
var fields =
from field in db.navigator_entity_field_masters
where field.entity_id == 1 && field.user_type == 0
select field;
var properties =
from property in db.navigator_user_field_properties
where property.user_id == "albert"
where fields.Contains(property.field)
select property;
Look mama!! Without joins ;-)
Given a date, what is the most efficient way to query the last record before that date, any record that equals that date, and the next one after that date.
It should be functionally equivalent to a query like this:
from asset in Assets
where asset.Id == assetId
select new {
Previous = (from a in a.Orders where a.Date < myDate orderby a.Date descending select a).FirstOrDefault(),
Current = (from a in a.Orders where a.Date == myDate select a).SingleOrDefault(),
Next = (from a in a.Orders where a.Date > myDate orderby a.Date select a).FirstOrDefault()
}
As is, this query runs three queries, and presumably has to sort the dataset by myDate three times to do it.
Some similar questions:
How do I get 5 records before AND after a record with a specific ID? (just uses two queries)
How do I get records before and after given one? Not in Linq, and therefore hard for me to take advantage of (my team will get annoyed).
To provide the "most efficient" query depends on what you mean by efficient.
If you want a single query to the database, a single sort of orders by date and finally fast look-ups by date then I suggest the following might be the most efficient. :-)
var orders =
(from a in Assets
where a.Id == assetId
from o in a.Orders
orderby o.Date
select o).ToArray();
var previous = orders.LastOrDefault(o => o.Date < myDate);
var current = orders.SingleOrDefault(o => o.Date == myDate);
var next = orders.FirstOrDefault(o => o.Date > myDate);
This should query the database once for the orders associated with the required asset Id, sort them by date, and return them as an array in memory. Since this is in memory it is now blindingly fast to look for the current, previous & next records for the specified date.
Does your Orders table have a sequential ID field? If so, you might be able to do it with:
from asset in Assets
where asset.Id == assetID
let current = asset.Orders.Where(x => x.Date == myDate).FirstOrDefault()
where current != null
let previous = asset.Orders.Where(x => x.id == current.id - 1).FirstOrDefault()
let next = asset.Orders.Where(x => x.id == current.id + 1).FirstOrDefault()
select new {
Previous = previous,
Current = current,
Next = next
};
If it doesn't, then it'd be a bit more code:
from asset in Assets
where asset.Id == assetID
let current = asset.Orders.Where(x => x.Date == myDate).FirstOrDefault()
where current != null
let previous = asset.Orders.Where(x => x.Date < current.Date).OrderByDescending(x => x.Date).FirstOrDefault()
let next = asset.Orders.Where(x => x.Date > current.Date).OrderBy(x => x.Date).FirstOrDefault()
select new {
Previous = previous,
Current = current,
Next = next
};
That should get compiled into a single SQL query that utilizes sub-queries. IE: the database server will execute multiple queries, but your client program is only submitting one.
Edit One other idea that would work if your Order table had sequential IDs:
var sample = (from asset in Assets
where asset.Id == assetID
let current = asset.Orders.Where(x => x.Date == myDate).FirstOrDefault()
where current != null
from order in asset.Orders
where order.Id == current.id - 1
select order)
.Take(3)
.ToArray();
var Previous = sample[0];
var Current = sample[1];
var Next = sample[2];
Other Answers, for example, SkipWhile etc. very very slow. Good luck ^^
//Current Record
var query
= (from item in db.Employee
where item.UserName.Equals(_username)
select item).SingleOrDefault();
//Next Record
var query
= (from item in db.Employee
where item.UserName.CompareTo(_username) > 0
select item).FirstOrDefault();
//Previous Record
var query
= (from item in db.Employee
where item.UserName.CompareTo(_username) < 0
orderby item.UserName Descending
select item).FirstOrDefault();
Almost the same, but the SQL query plan might be different.
var q =
from asset in Assets
where asset.Id == assetID
select new
{
Previous = asset.Orders.where(a => a.Date == asset.Orders.Where(x => x.Date < myDate).Max(x => x.Date)).FirstOrDefault(),
Current = asset.Orders.Where(x => x.Date == myDate).FirstOrDefault(),
Next = asset.Orders.where(a => a.Date == asset.Orders.Where(x => x.Date > myDate).Min(x => x.Date)).FirstOrDefault()
};