Is there a way to do an insert/select with Linq that translates to this sql:
INSERT INTO TableA (...)
SELECT ...
FROM TableB
WHERE ...
Yes #bzlm covered it first, but if you prefer something a bit more verbose:
// dc = DataContext, assumes TableA contains items of type A
var toInsert = from b in TableB
where ...
select new A
{
...
};
TableA.InsertAllOnSubmit(toInsert);
dc.SubmitChanges();
I kind of prefer this from a review/maintenance point of view as I think its a bit more obvious what's going on in the select.
In response to the observation by #JfBeaulac :
Please note that this will not generate the SQL shown - so far as I'm aware it's not actually possible to generate directly using Linq (to SQL), you'd have to bypass linq and go straight to the database. Functionally its should achieve the same result in that it will perform the select and will then insert the data - but it will round-trip the data from the server to the client and back so may not be optimal for large volumes of data.
context
.TableA
.InsertAllOnSubmit(
context
.TableB
.Where( ... )
.Select(b => new A { ... })
);
Related
I have a mySQL database table t1 with a bigint field called filter which I need to 'and' to a given query parameter in order to filter out entries.
table t1:
filter bigint unsigned
info varchar(200)
As a pure mySQL statement my query would be this:
SELECT info FROM t1 WHERE (filter & 34603933) = filter;
Of course the number 34603933 is a parameter and changes from query to query.
The question is, if loopbackjs supports such a calculus in the where condition. Can someone point me the way, or if it is not possible suggest a workaround?
In the documentation http://docs.strongloop.com/display/public/LB/Where+filter I did not see a possibility to do this, but somehow I can't really believe it, since using references to columns values in the right side of a where comparison is nothing unusual, right?
I do not believe loopback has this support built-in and bitwise and operations are not that widespread (in my experience). You might try the raw SQL interface:
var mysqlDS = app.dataSources.mysqlDS;
var bitWiseAndValue = 34603933;
mysqlDS.connector.query('SELECT info FROM t1 WHERE (filter & '+bitWiseAndValue+') = filter', function(err) {
if (err) {return console.log(err);}
// success actions
});
I'm writing a DB layer which talks to MS SQL Server, MySQL & Oracle. I need an operation which can update an existing row if it contains certain data, otherwise insert a new row; All in one SQL operation.
Essentially I need to save over existing data if it exists, or add it if it doesn't
Conceptually this is the same as upsert except it only needs to work on a single table. I'm trying to make sure I don't need to delete then insert as this has a performance impact.
Is there generic SQL to do this or do I need vendor specific solutions?
Thanks.
You need vendor specific SQL as MySQL (unlike MS and Oracle) doesn't support MERGE
http://en.wikipedia.org/wiki/Merge_(SQL)
I suspect that sooner rather than later, you're going to need a vendor specific implementation of your DB layer - SQL portability is pretty much a myth as soon as you do anything even slightly advanced.
I am pretty sure this is going to be vendor specific. For SQL Server, you can accomplish this using the MERGE statement.
If you are using SQL Server 2008, use Merge Statement. But keep in mind that if your Insert part has some condition involve, then it cannot be used. In which case you need to write your own way for accomplishing this. And in your case it has to be since you are involving MySQL which does not have a Merge Statement.
Why are you not using an ORM layer (like Entity Framework) for this purpose?
Just some pseudo code(in C#)
public int SaveTask(tblTaskActivity task, bool isInsert)
{
int result = 0;
using (var tmsEntities = new TMSEntities())
{
if (isInsert) //for insert
{
tmsEntities.AddTotblTaskActivities(task);
result = tmsEntities.SaveChanges();
}
else //for update
{
var taskActivity = tmsEntities.tblTaskActivities.Where(i => i.TaskID == task.TaskID).FirstOrDefault();
taskActivity.Priority = task.Priority;
taskActivity.ActualTime = task.ActualTime;
result = tmsEntities.SaveChanges();
}
}
return result;
}
In MySQL you have something similar to merge:
insert ... on duplicate key update ...
MySQL Reference - Insert on duplicate key update
I've discovered a very nasty gotcha with Linq-to-sql, and i'm not sure what the best solution is.
If you take a simple L2S Union statement, and include L2S code in one side, and constants in the other, then the constants do not get included in the SQL Union and are only projected into the output after the SQL, resulting in SQL errors about the number of columns not mathching for the union.
As an example:
(from d in dc.mytable where foo == "bar" select new {First = d.Foo, Second = d.Roo})
.Union(from e in dc.mytable where foo == "roo" select new {First= "", Second = e.Roo})
This will generate an error "All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists.
This is particularly insidious (and maddening) because there are obviously the same number of expressions in the list, but when you look at the SQL, you will notice that it does not generate a column for "First" in the second half of the Union. This is because "First" is inserted into the projection AFTER the query.
Ok, the easy solution is to just convert each part into Enumerables or Lists or something and then do the union in memory rather than SQL, and that's fine if you're dealing with a small amount of data. However, if you're working with a large set of data, which you then plan to further filter (in sql) before returning it this is not ideal.
I guess what i'm looking for is a way to force L2S to include the column in the SQL. Is that possible?
UPDATE:
While not an exact duplicate, this error is similar to This Question and has similar solutions. So i'm closing, but not deleting this question because it may help someone else come to posible solutions from a different way.
Unfortunately, L2S is too smart of it's own good sometimes.
I've decided that the only real solution is to use a stored proc. Hope this helps.
This is a bug in the Linq2SQL provider.
In LinqPad you can clearly see the bug.
(from d in dc.mytable where foo == "bar" select new {First = d.Foo, Second = d.Roo})
.Union(from e in dc.mytable where foo == "roo" select new {First= "", Second = e.Roo})
Will server side produce something like this:
SELECT [t2].[Foo], [t2].[Roo]
FROM (
SELECT [t0].[Foo], #p0 AS [value]
FROM [dc].[Mytable] AS [t0]
UNION ALL
SELECT [t1].[Foo], [t1].[Roo]
FROM [dc].[Mytable] AS [t1]
) AS [t2]
This will be a problem because the union will name the second column "value" instead of "Roo", which will cause the outer query to fail.
If you, however, switch the order of the two tables
(from e in dc.mytable where foo == "roo" select new {First= "", Second = e.Roo})
.Union(from d in dc.mytable where foo == "bar" select new {First = d.Foo, Second = d.Roo})
So that the constant assignment within the generated T-SQL comes in the non-first table, then things may work because T-SQL ignores the column names of subsequent tables.
Note: The first table in a union decides both column name and type. So would be smart to get LinqPad anyway.
My understanding is that the LinqToSql pseudolanguage describes a set using a syntax very similar to SQL and this will allow you to efficiently update a property on a collection of objects:
from b in BugsCollection where b.status = 'closed' set b.status = 'open'
This would update the underlying database using just one SQL statement.
Normally an ORM needs to retieve all of the rows as separate objects, update attributes on each of them and save them individually to the database (at least that's my understanding).
So, how does linq-to-sql avoid having to do this when other orms are not able to avoid it?
The syntax shown in your question is incorrect. LINQ is not intended to have side-effects; it is a query language. The proper way to accomplish what you're looking for is
var x = from b in dataContext.BugsCollection where b.status == "closed";
foreach (var y in x)
y.status = "open";
dataContext.SubmitChanges();
This would generate the single SQL statement that you're talking about. The reason it is able to accomplish this is because of deferred execution - the L2S engine doesn't actually talk to the database until it has to - in this case, because SubmitChanges() was called. L2S then sends the generated SQL statement to the database for execution.
Because LINQ to SQL uses Expression Trees to convert your Query Syntax to actual SQL...it then executes the SQL against the database (rather than pulling all of the data, executing against the in-memory data, and then writing the changes back to the database).
For example, the following Query Syntax:
var records = from r in Records
where r.Property == value
select r
Gets translated first to Lamda Syntax:
Records.Where(r => r.Property == value).Select();
And finally to SQL (via Expression Trees):
SELECT Property, Property2, Property3 FROM Record WHERE Property = #value
...granted, the example doesn't update anything...but the process would be the same for an update query as opposed to a simple select.
I have a few rows of data pulled into business objects via linq-to-sql from large tables.
Now I want to get a few rows that don't match to test my comparison functions.
Using what I thought would work I get a NotSupportedException:
Local sequence cannot be used in LINQ to SQL implementation of query operators except the Contains() operator.
Here's the code:
//This table has a 2 field primary key, the other has a single
var AllNonMatches = from c in dc.Acaps
where !Matches.Rows.Any((row) => row.Key.Key == c.AppId & row.Key.Value == c.SeqNbr)
select c;
foreach (var item in AllNonMatches.Take(100)) //Exception here
{}
The table has a compound primary key: AppId and SeqNbr.
The Matches.Rows is defined as a dictionary of keyvaluepair(appid,seqnbr).
and the local sequence it is referring to appears to be the local dictionary.
Could you provide more information on the structure and the name(s) of the table(s) plz?
Not sure what you're trying to do...
edit:
Ok.. I think I get it now...
It appears you can't merge/join local tables (dictionary) with a SQL table.
If you can, I'm afraid I don't know how to do it.
The simplest solution I can think of is to put those results in a table ("Match" for instance) with foreign keys related to your table "Acaps" and then use linq-to-sql, like:
var AllNonMatches = dc.Acaps.Where(p=>p.Matchs==null).Take(100).ToList();
Sorry I couldn't come up with any better =(
What about this:
var AllNonMatches = from c in dc.Acaps
where !(Matches.Rows.ContainsKey(c.AppId) && Matches.Rows.ContainsValue(c.SeqNbr))
select c;
That will work fine. I have also used a bitwise AND operator (&&) - I think thats the right term to help improve performance over the standard AND operator.