I'm learning Linq and using MVC. I have written a SQL query which I need to convert to a LINQ query.
select TokenID,TokenAsset,packet from TokenTable where id = 6 and packet = ''
and TokenID not in (select TokenID from TokenTable where id=6 and packet <> '')
group by TokenID,TokenAsset,Packet
I kindly ask help to convert the above query to a LINQ query. I know that the SQL query isn't efficient. It would better if you can help me to fix it.
Try this one:
var result = Tokens.Where(x=>x.Id==6 &&
x.Packet=="" &&
!Tokens.Exists(y=>y.TokenID==x.TokenID &&
y.Id==6 &&
y.Packet!="")
)
.GroupBy(x=>x.ID)
.ThenGroupBy(x=>x.TokenAsset)
.ThenGroupBy(x=>x.Packet);
Note I suppose that collection Tokens holds all the tokens you have.
Firstly your SQL query can just be
select distinct TokenID, TokenAsset, packet
from TokenTable
where id = 6 and packet = ''
the group by is not that useful since there are no aggregated columns. All selected columns are in the group by clause. Use distinct to achieve the same.
the secondary AND condition for tokenid is also redundant. It is exclusive to the first condition and hence doesn't change the result.
use this LINQ query:
var results = dbcontext.TokenTables
.Where(t => t.id == 6 && t.Packet == "")
.Select(t => new { t.TokenId, t.TokenAsset, t.Packet }).Distinct();
project only columns you need for performant calls by avoiding extra data transfer.
Related
I have two tables: tableA and tableB, both have an attribute "CommissionNumber" which contains strings in the form of D123456789 (one letter followed by a fixed number of digits).
I need to find the commissionnumbers in table A, that are not in table B.
In SQL, this could look like:
SELECT *
FROM tableA
WHERE CommissionNumber NOT IN
(
select CommissionNumber from tableB
)
Which gives me no results. However, if i try this:
var tableA= dbContext.tableA.Select(x => x.CommissionNumber).ToList();
var tableB= dbContext.tableB.Select(x => x.CommissionNumber).ToList();
IEnumerable<string> missingFiles = tableA.Except(tableB);
I get 92 hits. I don't understand what's wrong, my SQL query of the use of the .Except function.
Any ideas?
You have created two LINQ queries which retrieves all data from two tables into the memory and applied Except. It is wrong if you care about performance.
If you want to create SQL in the same way, LINQ query should be written accordingly. LINQ equivalent for IN operator is Contains.
var query = dbContext.tableA
.Where(x => !dbContext.tableB.Select(b => b.CommissionNumber).Contains(x.CommissionNumber));
Or by EXISTS which have analogue Any
var query = dbContext.tableA
.Where(x => !dbContext.tableB.Any(b => b.CommissionNumber == x.CommissionNumber));
Also the same result you can achieve by LEFT JOIN
var query =
from a in dbContext.tableA
join b in dbContext.tableB on a.CommissionNumber equals b.CommissionNumber into gj
from b in gj.DefaultIfEmpty()
where b.CommissionNumber == null
select a;
gsharp was on the right track. I had some case sensitivity issues in my data which was ignored in the native SQL query but taken seriously by EF core.
Just assume a simple example:I have two table and use this query:
select *
from People
where PersonGuid = (select PersonGuid from Sellers where Guid = '')
How can I write this query with linq?
I tried this:
var person = from p in loginContext.Person where p.PersonGuid =
(loginContext.Seller.Where(s => s.Guid == sellerGuid).FirstOrDefaultAsync());
but that's wrong.
What is the right way to write it?
If you were to rewrite your SQL query as
select *
from People AS PE
JOIN
Sellers AS Sellers
ON PE.PersonGuid = SE.PersonGuid
WHERE
SE.Guid = ''
then the LINQ becomes a little more obvious:
var person = (from p in loginContext.Person
join s in loginContext.Seller on p.PersonGuid equals s.PersonGuid
where s.guid == ""
select p)
.FirstOrDefault();
Although your SQL would suggest FirstOrDefault is not required.
Warning - not tested.
You could probably try using LINQ JOIN clause.
It should be something like:
from p in loginContext.Person
join s in loginContext.Seller on new { p.PersonGuid = s.Guid }
where s.Guid = sellerGuid
You can know more about LINQ JOIN here -> Microsoft C# Reference
Hope it helps!
I will assume that select PersonGuid from Sellers where Guid = '' will provide a single value no matter what, otherwise your SQL statement would fail.
Accepted answer works, but if you want a solution that keeps the spirit of the SQL query, you can rely on Any function. Something along the following:
var person = await loginContext.Person
.Where(p => loginContext.Sellers
.Any(s => s.PersonGuid == p.PersonGuid && s.Guid = ""))
.FirstOrDefaultAsync();
I expect this to be transformed into an SELECT ... WHERE EXISTS which is similar to your initial query.
var statusId = db.WorkOrder.Where(w => w.OrderType.Name == "ReadAudit" && w.WorkOrderMapping.MeterOldTag == meterTag && w.OrderStatusId != 80)
.OrderByDescending(w => w.CreationDatetime)
.Select(r => r.OrderStatusId)
.FirstOrDefault();
That produces this crazy sql:
SELECT TOP (1)
[Project1].[OrderStatusId] AS [OrderStatusId]
FROM ( SELECT
[Filter1].[OrderStatusId] AS [OrderStatusId],
[Filter1].[CreationDatetime] AS [CreationDatetime]
FROM (
SELECT [Extent1].[OrderStatusId] AS [OrderStatusId],
[Extent1].[CreationDatetime] AS [CreationDatetime],
[Extent3].[MeterOldTag] AS [MeterOldTag]
FROM [dbo].[WorkOrder] AS [Extent1]
INNER JOIN [dbo].[OrderType] AS [Extent2] ON [Extent1].[OrderTypeKey] = [Extent2].[OrderTypeKey]
LEFT OUTER JOIN [dbo].[WorkOrderMapping] AS [Extent3] ON [Extent1].[WorkOrderKey] = [Extent3].[WorkOrderMappingKey]
WHERE (80 <> [Extent1].[OrderStatusId])
AND (N'ReadAudit' = [Extent2].[Name])
) AS [Filter1]
WHERE ([Filter1].[MeterOldTag] = #p__linq__0) OR (([Filter1].[MeterOldTag] IS NULL) AND (#p__linq__0 IS NULL))
) AS [Project1]
ORDER BY [Project1].[CreationDatetime] DESC
And I'm told that its hitting the database pretty hard:
Table 'WorkOrder'. Scan count 30, logical reads 84403
Table 'WorkOrderMapping'. Scan count 9, logical reads 16516
The EF query doesn't seem that complicated. Is there a way to get the generated SQL to be more efficient?
The only thing "crazy" about that SQL Query is the predicate on MeterOldTag. It's written like that because EF, by default, writes queries to emulate C# comparison semantics for LINQ queries. If you want a simple equality comparison in the databse, set UseDatabaseNullSemantics for your DbContext.
It is just how Entity Framework works.
If you want more control, you can write the query yourself via Entity Framework Raw Queries, see here and below a (not complete) example how your query might look like.
DbRawSqlQuery<Int32> query = db.Database.SqlQuery<Int32>("SELECT OrderStatusId FROM ... ");
var statusId = query.FirstOrDefault();
Edit:
Also have a look at the actual query plan that gets executed for this query in order to find if whether the appropriate indexes are present on the tables involved.
Consider moving the constants RealAuditand 80 to variables, like below:
var orderType = "RealAudit";
var orderStatusId = 80;
var statusId = db.WorkOrder.Where(w => w.OrderType.Name == orderType
&& w.WorkOrderMapping.MeterOldTag == meterTag
&& w.OrderStatusId != orderStatusId
)
.OrderByDescending(w => w.CreationDatetime)
.Select(r => r.OrderStatusId)
.FirstOrDefault();
By doing so, these will appear as SQL parameters in the query, something like:
#p__linq__1 <> [Extent1].[OrderStatusId])
AND (#p__linq__2 = [Extent2].[Name])
This allows that a single query plan can be used for all variations of this query, whereas now you get a queryplan per separate value of the the MeterOldTag argument.
Hi guys I have this SQL query (MSSQL), I'm doing a query where the result of joins are giving me the "top" row of newest row by date without having duplicates of results, you can find here information of what I'm doing http://goo.gl/Uv0FR The thing is this, I accomplished already the SQL query, Is Working as I'm expecting, I'm getting 1 row for each IDKEY uses in the clause "where pi.PlazaIe in ('','') without duplication
Select * from PlazaI pi
join (
Select * from PlazaE pe where
NOT EXISTS(SELECT 1 FROM PlazaE pe1
WHERE pe.Id_plaza = pe1.Id_plaza AND pe1.Fecha > pe.Fecha AND pe1.Fecha < GETDATE() and pe1.Id_Emp != 0)
) pe on pe.Id_plaza = pieepo.Id_plaza
join Emp e on pe.Id_Emp = e.Id_Emp
join View ct on ct.Id_Nodo = pe.id_nodo
where pi.PlazaIe in ('value1','value2')
The PROBLEM is when I'm trying to convert from SQL to LINQ is just can't make to happened. (I'm new in this world of Linq)
the following is my linq query.
var q1 = (from pe in db.PlazaEmpleados
where !db.PlazaEmpleados.Any
(
pe1 => (pe1.Id_plaza.Equals(pe.Id_plaza) && pe1.Fecha > pe.Fecha && pe1.Id_Emp != 0 && pe1.Fecha > DateTime.Now)
) select pe);
var q2 = (from pi in db.Context
join pe in (q1) on pi.Id_plaza equals pe.Id_plaza
select new EmpVO
{
Id_Nodo = pe.id_nodo,
Id_plaza = pi.PlazaSome,
Num_Plaza = pi.Id_plaza,
});
When I run this linq2sql query I'm getting duplicate results instead of just 1 for each value. So the thing is, I would like to know if someone can convert in a good way the SQL query to LINQ Query or point me where is the error.
thanks in advance.
Your check for the Date is different:
LINQ:
pe1.Fecha > DateTime.Now
SQL:
pe1.Fecha < GETDATE()
Isnt your LINQ supposed to be:
pe1.Fecha < DateTime.Now
I didn't find answer which resolve my problem, so what I finally did is to use the
db.ExecuteQuery<ObjectVO>(sqlQuery);
I know this is not the best practice and also don't resolve the question why my sql query and my linq query don't get the same result set, but non of the previous answer did.
The other thing is my query grown in complexity (new business logic requirement) have to join 7 table and search for Max dates and movement is some of them, so now is more complicated to transform the query to a linq to sql.
Thanks for the support.
this part:
var q1 = from pe in db.PlazaEmpleados
where !db.PlazaEmpleados.Any
(pe1 =>
pe1.Id_plaza.Equals(pe.Id_plaza) &&
pe1.Fecha > pe.Fecha &&
pe1.Id_Emp != 0 &&
pe1.Fecha < DateTime.Now
)
select pe;
In SQL you first use PlazaI then PlazaE- in Linq you both times use PlazaEmpleados.
Put your SQL query to stored procedure an add it to context. Then just call:
var q = context.MyProcedure(new object[] {"value1","value2"});
Anybody know how to write a LINQ to SQL statement to return every nth row from a table? I'm needing to get the title of the item at the top of each page in a paged data grid back for fast user scanning. So if i wanted the first record, then every 3rd one after that, from the following names:
Amy, Eric, Jason, Joe, John, Josh, Maribel, Paul, Steve, Tom
I'd get Amy, Joe, Maribel, and Tom.
I suspect this can be done... LINQ to SQL statements already invoke the ROW_NUMBER() SQL function in conjunction with sorting and paging. I just don't know how to get back every nth item. The SQL Statement would be something like WHERE ROW_NUMBER MOD 3 = 0, but I don't know the LINQ statement to use to get the right SQL.
Sometimes, TSQL is the way to go. I would use ExecuteQuery<T> here:
var data = db.ExecuteQuery<SomeObjectType>(#"
SELECT * FROM
(SELECT *, ROW_NUMBER() OVER (ORDER BY id) AS [__row]
FROM [YourTable]) x WHERE (x.__row % 25) = 1");
You could also swap out the n:
var data = db.ExecuteQuery<SomeObjectType>(#"
DECLARE #n int = 2
SELECT * FROM
(SELECT *, ROW_NUMBER() OVER (ORDER BY id) AS [__row]
FROM [YourTable]) x WHERE (x.__row % #n) = 1", n);
Once upon a time, there was no such thing as Row_Number, and yet such queries were possible. Behold!
var query =
from c in db.Customers
let i = (
from c2 in db.Customers
where c2.ID < c.ID
select c2).Count()
where i%3 == 0
select c;
This generates the following Sql
SELECT [t2].[ID], [t2]. --(more fields)
FROM (
SELECT [t0].[ID], [t0]. --(more fields)
(
SELECT COUNT(*)
FROM [dbo].[Customer] AS [t1]
WHERE [t1].[ID] < [t0].[ID]
) AS [value]
FROM [dbo].[Customer] AS [t0]
) AS [t2]
WHERE ([t2].[value] % #p0) = #p1
Here's an option that works, but it might be worth checking that it doesn't have any performance issues in practice:
var nth = 3;
var ids = Table
.Select(x => x.Id)
.ToArray()
.Where((x, n) => n % nth == 0)
.ToArray();
var nthRecords = Table
.Where(x => ids.Contains(x.Id));
Just googling around a bit I haven't found (or experienced) an option for Linq to SQL to directly support this.
The only option I can offer is that you write a stored procedure with the appropriate SQL query written out and then calling the sproc via Linq to SQL. Not the best solution, especially if you have any kind of complex filtering going on.
There really doesn't seem to be an easy way to do this:
How do I add ROW_NUMBER to a LINQ query or Entity?
How to find the ROW_NUMBER() of a row with Linq to SQL
But there's always:
peopleToFilter.AsEnumerable().Where((x,i) => i % AmountToSkipBy == 0)
NOTE: This still doesn't execute on the database side of things!
This will do the trick, but it isn't the most efficient query in the world:
var count = query.Count();
var pageSize = 10;
var pageTops = query.Take(1);
for(int i = pageSize; i < count; i += pageSize)
{
pageTops = pageTops.Concat(query.Skip(i - (i % pageSize)).Take(1));
}
return pageTops;
It dynamically constructs a query to pull the (nth, 2*nth, 3*nth, etc) value from the given query. If you use this technique, you'll probably want to create a limit of maybe ten or twenty names, similar to how Google results page (1-10, and Next), in order to avoid getting an expression so large the database refuses to attempt to parse it.
If you need better performance, you'll probably have to use a stored procedure or a view to represent your query, and include the row number as part of the stored proc results or the view's fields.