I would like to write a SQL query (oracle)
to know if an operation (identified by ope.ope_operationid)
has at least an operation of a certain type (opt.opt_id), and if it does not, to show that it doesn't in the results.
For example, I have this operation LAA351BP (I know this one exists in base),
and I would like to know if it has at least an operation type which id is 3781.
If it has, print everything, if it hasn't, print the operationid
and something like 'not found' next to it
Is nvl the function to use ? It seems I can't get it to work properly.
SELECT DISTINCT ope.ope_operationid,
ser.ser_code,
opt.opt_code,
ost.ost_code
FROM od_operation ope,
od_service_type ser,
od_operation_type opt,
od_status_type ost,
od_equipment_type eqt,
WHERE ope.ser_id = ser.ser_id
AND opt.opt_id = ope.opt_id
AND ost.ost_id = ope.ost_id
AND ope.opt_id = 3781
AND ope.ope_operationid = 'LAA351BP'
Thanks
You should start using standard JOIN syntax. Apart from being more readable (at least in my opinion) it also protects you from accidental cartesian joins if you forget the actual join condition in the WHERE clause. Plus it is portable across nearly all DBMS as opposed to the clunky (+) syntax used by Oracle (which also has some limitations that the JOIN syntax does not have)
Here is the query re-written using explicit (instead of implicit) joins:
SELECT DISTINCT ope.ope_operationid,
ser.ser_code,
opt.opt_code,
ost.ost_code
FROM od_operation ope,
LEFT JOIN od_service_type ser ON ope.ser_id = ser.ser_id
LEFT JOIN od_operation_type opt ON opt.opt_id = ope.opt_id
LEFT JOIN od_status_type ost ON ost.ost_id = ope.ost_id
LEFT JOIN od_equipment_type eqt ON ????????
WHERE ope.opt_id = 3781
AND ope.ope_operationid = 'LAA351BP'
Edit
The missing join condition on od_equipment_type is exactly the reason why the JOIN syntax is preferred. If the trailing comma in the original SQL is removed, the statement would create an unwanted cartesian join which might have a severe impact on the server if the involved tables are big.
With the JOIN syntax you will always get a syntax error which prevents you from such typos. Using implicit joins will only give you an error when you leave a comma in the FROM list, but never if you miss a join condition in the WHERE
Related
I am new to entity framework. I am using a linq query that will fetch many records (upto millions) from database. There are many filter parameters in where condition and on each request the parameters may change. So i wanted to know whether compiled queries will be effective in this case or will it be a new query on each request. Here is my query:
List<FarmerDetailsReport> fdr =
(from fp in mstfp join pd in personalDetails on fp.personDetails.Id equals pd.Id
join ic in identityCertificate on fp.identityCertificate.Id equals ic.Id
join pid in pacsInsuranceData on fp.pacsInsuranceData.Id equals pid.Id into temp
from pid in temp.DefaultIfEmpty()
join bd in bankDetails on fp.bankDetails.Id equals bd.Id
join cd in contactDetails on fp.contactDetails.Id equals cd.Id
join id in incomeDetails on fp.incomeDetails.Id equals id.Id into tmp
from id in tmp.DefaultIfEmpty()
join ua in userAttributes on fp.UserId equals ua.EmailID
where ((ua.CompanyName == companyName ) && (cd.District == model.DistrictForProfileMIS ) && (cd.Block == model.BlockForProfileMIS) && (bd.bankName == model.BankForProfileMIS ) && Status == "Active")
select new FarmerDetailsReport { .......... }).ToList();
Short answer:
Yes...... well, maybe.
Long answer:
This is hard to answer as you have no control over the actual SQL that gets generated.
We had perf problems with some queries like this as the optimizer would optimize for a certain wet of filter cases (like short circuits of clauses) then when a new query was made with a massive change in parameters it would take AGES.
What we did in the end:
Don't use a big LINQ query, create a stored proc or view where you have more control over the SQL generated.
Used things like OPTION(RECOMPILE) ... looks this up it was very useful.
Have a few overloads of the query for different parameters so that the DB can optimize them separately.
Obviously this is just what we did, it might not be perfect for you. I STRONGLY suggest getting the generated SQL for each different parametrized version and going over it with your DBA (if you have one) or your team and google if you don't.
Can anyone explain to me why I get a 00904 error when I run the following
SELECT "OASM"."DT_GROUPEPG".GROUPEPGID,
"OASM"."DT_GROUPEPG".GROUPID,
"OASM"."DT_GROUPEPG".EPGID,
"OASM"."DT_GROUPEPG".ZAPID,
"OASM"."LU_EPG".LASTREADTIME,
"OASM"."LU_EPG".SERVICE_NAME,
"OASM"."LU_EPG".SOURCE_ID,
"OASM"."LU_EPG".ONID,
"OASM"."LU_EPG".TSID,
"OASM"."LU_EPG".SID,
"OASM"."LU_EPG".TYPE_ID,
"OASM"."LU_EPG".OPERATOR_ID,
"OASM"."LU_EPG".URL
FROM "OASM"."DT_GROUPEPG"
INNER JOIN "OASM"."LU_EPG"
ON "OASM"."DT_GROUPEPG".EPGID = "OASM"."LU_EPG".EPGID
ORDER BY LastReadTime;
I'm still new to Oracle, and was of the impression that, because Oracle executes blocks of statements, and not line by line, that doing this kind of query would be valid? The error currently fires at the OPERATOR_ID line, but removing/commenting it out just moves the erro up a line, until all the LU_EPG table references are removed
You can't (and don't need to) specify the schema name when referring to the columns. Also, I recommend you use table aliases (e.g. a and b in the example below):
SELECT a.GROUPEPGID,
a.GROUPID,
a.EPGID,
a.ZAPID,
b.LASTREADTIME,
b.SERVICE_NAME,
b.SOURCE_ID,
b.ONID,
b.TSID,
b.SID,
b.TYPE_ID,
b.OPERATOR_ID,
b.URL
FROM "OASM"."DT_GROUPEPG" a
INNER JOIN "OASM"."LU_EPG" b
ON a.EPGID = b.EPGID
ORDER BY b.LastReadTime;
question pretty much says it all. I have a big flinq query of the following form:
for alias1 in table1 do
for alias2 in table2 do
if alias1.Id = alias2.foreignId
using this form, how can I do a left outer join between these two tables?
I think you can use the groupJoin function available in the Query module. Here is an example using Northwind with Products as the primary table and Categories as the table with foreign key:
open System.Linq
<# Query.groupJoin
db.Products db.Categories
(fun p -> p.CategoryID.Value)
(fun c -> c.CategoryID)
(fun p cats ->
// Here we get a sequence of all categories (which may be empty)
let cat = cats.FirstOrDefault()
// 'cat' will be either a Category or 'null' value
p.ProductName, if cat = null then "(none)" else cat.CategoryName) #>
|> query
There are definitely nicer ways of expressing this using the seq { .. } syntax and by implementing join-like behavior using nested for loops. Unfortunatelly, the quotations to LINQ translator will probably not support these. (Personally, I would prefer writing the code using nested for and using if to check for empty collection).
I was just looking at some improvements in the PowerPack library as part of a contracting work for the F# team, so this will hopefully improve in the future... (but no promises!)
Perhaps you should create a view in the database that performed the left outer join, and then LINQ over that view.
I ended up created separate queries for each outer join and calling that at certain points when looping through the resultset of the outermost query.
Let's say I have an Order table which has a FirstSalesPersonId field and a SecondSalesPersonId field. Both of these are foreign keys that reference the SalesPerson table. For any given order, either one or two salespersons may be credited with the order. In other words, FirstSalesPersonId can never be NULL, but SecondSalesPersonId can be NULL.
When I drop my Order and SalesPerson tables onto the "Linq to SQL Classes" design surface, the class builder spots the two FK relationships from the Order table to the SalesPerson table, and so the generated Order class has a SalesPerson field and a SalesPerson1 field (which I can rename to SalesPerson1 and SalesPerson2 to avoid confusion).
Because I always want to have the salesperson data available whenever I process an order, I am using DataLoadOptions.LoadWith to specify that the two salesperson fields are populated when the order instance is populated, as follows:
dataLoadOptions.LoadWith<Order>(o => o.SalesPerson1);
dataLoadOptions.LoadWith<Order>(o => o.SalesPerson2);
The problem I'm having is that Linq to SQL is using something like the following SQL to load an order:
SELECT ...
FROM Order O
INNER JOIN SalesPerson SP1 ON SP1.salesPersonId = O.firstSalesPersonId
INNER JOIN SalesPerson SP2 ON SP2.salesPersonId = O.secondSalesPersonId
This would make sense if there were always two salesperson records, but because there is sometimes no second salesperson (secondSalesPersonId is NULL), the INNER JOIN causes the query to return no records in that case.
What I effectively want here is to change the second INNER JOIN into a LEFT OUTER JOIN. Is there a way to do that through the UI for the class generator? If not, how else can I achieve this?
(Note that because I'm using the generated classes almost exclusively, I'd rather not have something tacked on the side for this one case if I can avoid it).
Edit: per my comment reply, the SecondSalesPersonId field is nullable (in the DB, and in the generated classes).
The default behaviour actually is a LEFT JOIN, assuming you've set up the model correctly.
Here's a slightly anonymized example that I just tested on one of my own databases:
class Program
{
static void Main(string[] args)
{
using (TestDataContext context = new TestDataContext())
{
DataLoadOptions dlo = new DataLoadOptions();
dlo.LoadWith<Place>(p => p.Address);
context.LoadOptions = dlo;
var places = context.Places.Where(p => p.ID >= 100 && p.ID <= 200);
foreach (var place in places)
{
Console.WriteLine(p.ID, p.AddressID);
}
}
}
}
This is just a simple test that prints out a list of places and their address IDs. Here is the query text that appears in the profiler:
SELECT [t0].[ID], [t0].[Name], [t0].[AddressID], ...
FROM [dbo].[Places] AS [t0]
LEFT OUTER JOIN (
SELECT 1 AS [test], [t1].[AddressID],
[t1].[StreetLine1], [t1].[StreetLine2],
[t1].[City], [t1].[Region], [t1].[Country], [t1].[PostalCode]
FROM [dbo].[Addresses] AS [t1]
) AS [t2] ON [t2].[AddressID] = [t0].[AddressID]
WHERE ([t0].[PlaceID] >= #p0) AND ([t0].[PlaceID] <= #p1)
This isn't exactly a very pretty query (your guess is as good as mine as to what that 1 as [test] is all about), but it's definitively a LEFT JOIN and doesn't exhibit the problem you seem to be having. And this is just using the generated classes, I haven't made any changes.
Note that I also tested this on a dual relationship (i.e. a single Place having two Address references, one nullable, one not), and I get the exact same results. The first (non-nullable) gets turned into an INNER JOIN, and the second gets turned into a LEFT JOIN.
It has to be something in your model, like changing the nullability of the second reference. I know you say it's configured as nullable, but maybe you need to double-check? If it's definitely nullable then I suggest you post your full schema and DBML so somebody can try to reproduce the behaviour that you're seeing.
If you make the secondSalesPersonId field in the database table nullable, LINQ-to-SQL should properly construct the Association object so that the resulting SQL statement will do the LEFT OUTER JOIN.
UPDATE:
Since the field is nullable, your problem may be in explicitly declaring dataLoadOptions.LoadWith<>(). I'm running a similar situation in my current project where I have an Order, but the order goes through multiple stages. Each stage corresponds to a separate table with data related to that stage. I simply retrieve the Order, and the appropriate data follows along, if it exists. I don't use the dataLoadOptions at all, and it does what I need it to do. For example, if the Order has a purchase order record, but no invoice record, Order.PurchaseOrder will contain the purchase order data and Order.Invoice will be null. My query looks something like this:
DC.Orders.Where(a => a.Order_ID == id).SingleOrDefault();
I try not to micromanage LINQ-to-SQL...it does 95% of what I need straight out of the box.
UPDATE 2:
I found this post that discusses the use of DefaultIfEmpty() in order to populated child entities with null if they don't exist. I tried it out with LINQPad on my database and converted that example to lambda syntax (since that's what I use):
ParentTable.GroupJoin
(
ChildTable,
p => p.ParentTable_ID,
c => c.ChildTable_ID,
(p, aggregate) => new { p = p, aggregate = aggregate }
)
.SelectMany (a => a.aggregate.DefaultIfEmpty (),
(a, c) => new
{
ParentTableEntity = a.p,
ChildTableEntity = c
}
)
From what I can figure out from this statement, the GroupJoin expression relates the parent and child tables, while the SelectMany expression aggregates the related child records. The key appears to be the use of the DefaultIfEmpty, which forces the inclusion of the parent entity record even if there are no related child records. (Thanks for compelling me to dig into this further...I think I may have found some useful stuff to help with a pretty huge report I've got on my pipeline...)
UPDATE 3:
If the goal is to keep it simple, then it looks like you're going to have to reference those salesperson fields directly in your Select() expression. The reason you're having to use LoadWith<>() in the first place is because the tables are not being referenced anywhere in your query statement, so the LINQ engine won't automatically pull that information in.
As an example, given this structure:
MailingList ListCompany
=========== ===========
List_ID (PK) ListCompany_ID (PK)
ListCompany_ID (FK) FullName (string)
I want to get the name of the company associated with a particular mailing list:
MailingLists.Where(a => a.List_ID == 2).Select(a => a.ListCompany.FullName)
If that association has NOT been made, meaning that the ListCompany_ID field in the MailingList table for that record is equal to null, this is the resulting SQL generated by the LINQ engine:
SELECT [t1].[FullName]
FROM [MailingLists] AS [t0]
LEFT OUTER JOIN [ListCompanies] AS [t1] ON [t1].[ListCompany_ID] = [t0].[ListCompany_ID]
WHERE [t0].[List_ID] = #p0
I am in the process of writing something that will use Linq to combine results from my database, via Linq2Sql and an in-memory list of objects in order to find out which of my in-memory objects match something on the database.
I've come up with the query in both expression and query syntax.
Expression Syntax
var query = order.Items.Join(productNonCriticalityList,
i => i.ProductID,
p => p.ProductID,
(i, p) => i);
Query Syntax
var query =
from p in productNonCriticalityList
join i in order.Items
on p.ProductID equals i.ProductID
select i;
I realise that we have all the code completion goodness with expression syntax, and I do actually use that more. Mainly because it's easier to create re-usable chunks of filter code that can be chained together to form more complex filters.
But for a join the latter seems far more readable to me, but maybe that is because I am used to writing T-SQL.
So, am I missing a trick or is it just a matter of getting used to it?
I agree with the other responders that the exact question you're asking is simply a matter of preference. Personaly, I mix the two forms depending upon which is clearer for the specific query that I'm writing.
If I have one comment though, I would say that the query looks like it might load all of the items from the order. That might be fine for a single order one time, but if you're looping through lots of orders, it might be more efficient to load all of the items for all of the in one go (you might want to additionally filter by date or customer, or whatever though). If you do that, you might get better results by switching the query around:
var productIds = (from p in productNonCriticalityList
orderby p.productID
select p.ProductID).Distinct();
var orderItems = from i in dc.OrderItems
where productIds.Contains(i.ProductID)
&& // Additional filtering here.
select i;
It's a bit backwards at first glance, but it could save you from loading in all the order items and also from sending lots of queries. It works because the where productIds.Contains(...) call can be converted to where i.ProductID in (1, 2, 3, 4, 5) in SQL. Of course, you'd have to judge it based on the expected number of order items, and the number of product IDs.
It really all comes down to preference. Some people just hate the idea of query like syntax in their code. I for one appreciate the query syntax, it is declarative and quite readable. Like you said though, the chainability of the first example is a nice thing to have. I guess for my money I would keep it query until I felt I needed to begin chaining the call.
I used to feel the same way. Now I find query syntax easier to read and write, particularly when things get complicated. As much as it irked me to type it the first time, 'let' does wonderful things in ways that would not be readable in Expression Syntax.
I prefer the Query syntax when its complex and Expression syntax when its a simple query.
If a DBA were to read the C# code to see what SQL we are using, they would understand and digest the Query syntax easier.
Taking a simple example:
Query
var col = from o in orders
orderby o.Cost ascending
select o;
Expression
var col2 = orders.OrderBy(o => o.Cost);
To me, the Expression syntax is an easier choice to understand here.
Another example:
Query
var col9 = from o in orders
orderby o.CustomerID, o.Cost descending
select o;
Expression
var col6 = orders.OrderBy(o => o.CustomerID).
ThenByDescending(o => o.Cost);
Both are easy to read and understand, however if the query was
//returns same results as above
var col5 = from o in orders
orderby o.Cost descending
orderby o.CustomerID
select o;
//NOTE the ordering of the orderby's
That looks a little confusing to be as the fields are in a different order and it appears a little backwards.
For Joins
Query
var col = from c in customers
join o in orders on
c.CustomerID equals o.CustomerID
select new
{
c.CustomerID,
c.Name,
o.OrderID,
o.Cost
};
Expression:
var col2 = customers.Join(orders,
c => c.CustomerID,o => o.CustomerID,
(c, o) => new
{
c.CustomerID,
c.Name,
o.OrderID,
o.Cost
}
);
I find that Query is better.
My summary would be use whatever looks easiest and fastest to understand given the query at hand. There is no golden rule of which to use. However, if there are a lot of joins, I'd go with Query syntax.
Well, both statements are equivalent. So you could youse them both, depending on the surrounging code and what is more readable. In my project I make the decision which syntax to use dependent on those two conditions.
Personally I would write the expression syntax in one line, but this is a matter of taste.