I have a query that creates a result with two columns of ints.
from_id | to_id
---------------
51 | 100
3 | 21
... | ...
Let's call this result map_ids
Then, can I do something like
SELECT * FROM some_table st WHERE st.id IN map_ids.to_id
to only select stuff from some_table that has an ID that matches the result of the to_id in the first query?
I want to avoid using a subquery because the query that generate map_ids is quite long and is actually used twice in the real select from some_table.
It sounds like you're looking for subquery factoring. You can put your first query into a with clause and then join to that in the main body:
WITH map_ids (from_id, to_id) as (
... you first query ...
)
SELECT columns
FROM map_ids mi
JOIN some_table st ON st.id = mi.to_id
You can refer to map_ids again later, e.g. in another branch of a union.
You can do something like where st.id in (select to_id from map_ids) but joining is probably clearer (and maybe more efficient, depending on what the optimiser does with the real query).
Related
I have two query that looks close to the same but Oracle have very different performance.
Query A
Create Table T1 as Select * from FinalView1 where CustomerID in ('A0000001','A000002')
Query B
Create Table T1 as Select * from FinalView1 where CustomerID in (select distinct CustomerID from CriteriaTable)
The CriteriaTable have 800 rows but all belongs to Customer ID 'A0000001' and 'A000002'.
This means the subquery: "select distinct CustomerID from CriteriaTable" also only returns the same two elements('A0000001','A000002') as manually entered in query A
Following is the query under the FinalView1
create or replace view FinalView1_20200716 as
select
Customer_ID,
<Some columns>
from
Table1_20200716 T1
INNER join Table2_20200716 T2 on
T1.Invoice_number = T2.Invoice_number
and
T1.line_id = T2.line_id
left join Table3_20200716 T3 on
T3.id = T1.Customer_ID
left join Table4_20200716 T4 on
T4.Shipping_ID = T1.Shipping_ID
left join Table5_20200716 Table5 on
Table5.Invoice_ID = T1.Invoice_ID
left join Table6_20200716 T6 on
T6.Shipping_ID = T4.Shipping_ID
left join First_Order first on
first.Shipping_ID = T1.Shipping_ID
;
Table1_20200716,Table2_20200716,Table3_20200716,Table4_20200716,Table5_20200716,Table6_20200716 are views to the corresponding table with temporal validity feature. For example
The query under Table1_20200716
Create or replace view Table1_20200716 as
select
*
from Table1 as for period of to_date('20200716,'yyyymmdd')
However table "First_Order" is just a normal table as
Following is the performance for both queries (According to explain plan):
Query A:
Cardinality: 102
Cost : 204
Total Runtime: 5 secs max
Query B:
Cardinality:27921981
Cost: 14846
Total Runtime:20 mins until user cancelled
All tables are indexed using those columns that used to join against other tables in the FinalView1. According to the explain plan, they have all been used except for the FirstOrder table.
Query A used uniquue index on the FirstOrder Table while Query B performed a full scan.
For query B, I was expecting the Oracle will firstly query the sub-query get the result into the in operator, before executing the main query and therefore should only have minor impact to the performance.
Thanks in advance!
As mentioned from my comment 2 days ago. Someone have actually posted the solution and then have it removed while the answer actually work. After waiting for 2 days the So I designed to post that solution.
That solution suggested that the performance was slow down by the "in" operator. and suggested me to replace it with an inner join
Create Table T1 as
Select
FV.*
from
FinalView1 FV
inner join (
select distinct
CustomerID
from
CriteriaTable
) CT on CT.customerid = FV.customerID;
Result from explain plan was worse then before:
Cardinality:28364465 (from 27921981)
Cost: 15060 (from 14846)
However, it only takes 17 secs. Which is very good!
I 've got this problem:
I have a select statement, which is rather time consuming.
I have to join the result with itself.
I want to do something like this:
Select table1.*, table2.Consumption
from (heavy select statement) table1 left outer join
(same heavy statement) table2
on table1."id" = table2."id" and table1."Year" -1 = table2."Year"
I don't want to catch the same data 2 times. I would rather like to do something like table1 table2. Is this possible?
I need this for an application, which executes querys but isn't able to use create or something like this, otherwise i would store the data in a table.
You can use a common table expression (CTE) and materialize the results of the heavy select statement:
WITH heavy AS ( SELECT /*+ MATERIALIZE */ ... (heavy select statemenet) )
Select table1.*, table2.Consumption
from heavy table1 left outer join
heavy table2
on table1."id" = table2."id" and table1."Year" -1 = table2."Year"
I have a query like:
select
qsn.code,
(select prs.display_name from prs where prs.id = qsn.fk_prs) display_name,
(select prs.address from prs where prs.id = qsn.fk_prs) address,
(select prs.tel from prs where prs.id = qsn.fk_prs) tel
from
qsn
where
qsn.register_date between :x1 and :x2
When I look at the execution plan of the query, it queries prs table 3 times (each time using INDEX UNIQUE SCAN).
I wonder if I can query the prs table once using WITH clause? How can I write the query that way.
I shall mention that because each of the tables have millions of record, joining them makes the query so slow.
using with clause your query goes like this:
with abc as (select id,
display_name ,
address ,
tel
from prs)
select
qsn.code,
abc.display_name,
abc.address,
abc.tel
from qsn
inner join abc
on qsn.fk_prs = abc.id
where qsn.register_date between :x1 and :x2 ;
ps: not tested.
Use a join:
select qsn.code, prs.display_name, prs.address, prs.tel
from qsn
left join prs on prs.id = qsn.fk_prs
where qsn.register_date between :x1 and :x2
The issue I face that I need after selecting multiple rows,to loop over each row and fetch those who have some related information to the first row.
Example:
select NAME,ENGLISH_GRADE,FRANCE_GRAE
from (some complex query that have order by and returns 100 rows) WHOLE_ROWS
where
//Here I need to loop over WHOLE_ROWS and make something like that:
//if(currentRow.ENGLISH_GRADE==WHOLE_ROWS(0).ENGLISH_GRADE)
//fetch this row
Basically, you need to join your query to itself. You can do with with a subquery factoring clause:
WITH complex_query AS
( ... complex query here ... )
SELECT
FROM complex_query cq1
WHERE cq1.english_grade = ( SELECT english_grade FROM cq1
WHERE rownum = 1 )
Here is a SQL Fiddle. You could also do this with analytics, but those seem to me more difficult to understand.
I have one table, 'a', with id and timestamp. Another table, 'b', has N multiple rows referring to id, and each row has 'type', and "some other data".
I want a LINQ query to produce a single row with id, timestamp, and "some other data" x N. Like this:
1 | 4671 | 46.5 | 56.5
where 46.5 is from one row of 'b', and 56.5 is from another row; both with the same id.
I have a working query in SQLite, but I am new to LINQ. I dont know where to start - I don't think this is a JOIN at all.
SELECT
a.id as id,
a.seconds,
COALESCE(
(SELECT b.some_data FROM
b WHERE
b.id=a.id AND b.type=1), '') AS 'data_one',
COALESCE(
(SELECT b.some_data FROM
b WHERE
b.id=a.id AND b.type=2), '') AS 'data_two'
FROM a first
WHERE first.id=1
GROUP BY first.ID
you didn't mention if you are using Linq to sql or linq to entities. However following query should get u there
(from x in a
join y in b on x.id equals y.id
select new{x.id, x.seconds, y.some_data, y.type}).GroupBy(x=>new{x.id,x.seconds}).
Select(x=>new{
id = x.key.id,
seconds = x.Key.seconds,
data_one = x.Where(z=>z.type == 1).Select(g=>g.some_data).FirstOrDefault(),
data_two = x.Where(z=>z.type == 2).Select(g=>g.some_data).FirstOrDefault()
});
Obviously, you have to prefix your table names with datacontext or Objectcontext depending upon the underlying provider.
What you want to do is similar to pivoting, see Is it possible to Pivot data using LINQ?. The difference here is that you don't really need to aggregate (like a standard pivot), so you'll need to use Max or some similar method that can simulate selecting a single varchar field.