There are two tables in two database
Promotion: code, source, broker
BrokerNo: Id, Name, BrokerNo
What I want to query are all BrokerNo.Name records satisfying the condition of Promotion.broker=BrokerNo.BrokerNo and Promotion.source="asdf"
How to write the query statement using LINQ?
from promotion in Promotion
from brokerNo in BrokerNo
where promotion.source == "asdf" && promotion.broker == brokerNo.BrokerNo
select brokerNo.Name
Related
The table in question has ~30mio records. Using Entity Framework I write a LINQ Query like this:
dbContext.MyTable.FirstOrDefault(t => t.Col3 == "BQJCRHHNABKAKU-KBQPJGBKSA-N");
Devart DotConnect for Oracle generates this:
SELECT
Extent1.COL1,
Extent1.COL2,
Extent1.COL3
FROM MY_TABLE Extent1
WHERE (Extent1.COL3 = :p__linq__0) OR ((Extent1.COL3 IS NULL) AND (:p__linq__0 IS NULL))
FETCH FIRST 1 ROWS ONLY
The query takes about four minutes, obviously a full table scan.
However, handcrafting this SQL:
SELECT
Extent1.COL1,
Extent1.COL2,
Extent1.COL3
FROM MY_TABLE Extent1
WHERE Extent1.COL3 = :p__linq__0
FETCH FIRST 1 ROWS ONLY
returns the expected match in 200ms.
Question: Why is it so? I would expect the query optimizer to note that the right part is false if the parameter is not null, so why doesn't the first query hit the index?
Please set UseCSharpNullComparisonBehavior=false explicitly:
var config = Devart.Data.Oracle.Entity.Configuration.OracleEntityProviderConfig.Instance;
config.QueryOptions.UseCSharpNullComparisonBehavior = false;
If this doesn't help, send us a small test project with the corresponding DDL script so that we can investigate the issue.
Is there ability to make a multiple update with subquery on Symfony3 with Doctrine query builder or DQL?
For example, I want to run this query:
UPDATE tableA
SET fieldA2 = max_field2
FROM (SELECT
field1,
max(field2) AS max_field2
FROM table
GROUP BY field1) AS subquery
WHERE subquery.field1 = tableA.field1;
I can't understand how to use $entityManager->createQuery()->update with FROM subquery.
As far as I know, it's not possible through DQL.
You need to go through a loop.
foreach($entities as entity)
{
$em->flush();
}
Else, you will consider Batch processing, or just use plain SQL. So it might be usefull to check DBAL.
Is it possible to write a statement in sqlite.swift that will generate the equivalent sql:
SELECT foods.name, food_types.name FROM foods, food_types
WHERE foods.type_id=food_types.id LIMIT 10;
I can't figure out how to query from multiple Table objects at once.
Thanks!
Your original query passes two tables to the FROM clause, creating an implicit join. SQLite.swift's query builder language currently only supports explicit joins.
Check out the documentation under Joining Other Tables for more information on joining tables.
In your case:
let foods = Table("foods")
let food_types = Table("food_types")
let name = Expression<String>("name")
let id = Expression<Int64>("id")
let type_id = Expression<Int64>("type_id")
let query = foods
.select(foods[name], food_types[name])
.join(food_types, on: foods[type_id] == food_types[id])
.limit(10)
I figured it out. The foreign key is a column for all the tables I'm trying to join, but there are foreign key members that are not common among all the tables so I believe sql then generates a a cross join vs an inner join... That leads to all the extra rows in the query. I confirmed this by using the sql that sqlite.swift generates on the db directly.
I have a collection of IDs in memory, and I would like to fetch only rows from a DB matching those IDs.
In SQL, I could either write a query like SELECT * FROM mytable WHERE id IN (1,3,5,10) or do a join between tables.
My problem is that EF can't build a query where I join my EF-data with my local array or list.
(I'm using EF4.1, but I'm guessing the problem/solution would be similar in older versions, as well as with Linq-to-SQL.)
You can use Contains() with your ID collection myIDs to generate the equivalent WHERE id IN .. query:
var results = context.mytable.Where(x => myIds.Contains(x.Id));
I have two entities: Master and Details.
When I query them, the resulting query to database is:
SELECT [Extent2]."needed columns listed here", [Extent1]."needed columns listed here"
FROM (SELECT * [Details]."all columns listed here"...
FROM [dbo].[Details] AS [Details]) AS [Extent1]
LEFT OUTER JOIN [dbo].[Master] AS [Extent2] ON [Extent1].[key] = [Extent2].[key]
WHERE [Extent1].[filterColumn] = #p__linq__0
My question is: why not the filter is in the inner query? How can I get this query? I've tried a lot of EF and Linq expressions.
What I need is something like:
SELECT <anything needed>
FROM Master LEFT JOIN Details ON Master.key = Details.Key
WHERE filterColumn = #param
I'm having a full sequential scan in both tables, and in my production environment, I have milions of rows in each table.
Thanks a lot !!
Sometimes The entity Framework does not produce the best query. You can do a few of the following to optimize.
Modify the linq statement (test with
LINQPad)
Create a stored proc and map the stored proc to return an entity
Create a view that handles the join and map the view to a new
entity