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SQLite GROUP BY on result of union not using indexes of constituent tables


SQLite GROUP BY on result of union not using indexes of constituent tables

By : Bhaskar Kudalkar
Date : November 21 2020, 11:01 PM
this will help It might be possible that the database would be able to apply the grouping to each table – in theory. However, the problem is that the software must be able to prove that this transformation is always valid.
With the current optimizer implementation, this query cannot be flattened because it would violate rule 17.
code :


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How to get union of 3 tables in sqlite?

How to get union of 3 tables in sqlite?


By : user2351917
Date : March 29 2020, 07:55 AM
I wish did fix the issue. A SELECT in parentheses is a subquery, but compound queries must be constructed of 'normal' queries.
Using parentheses to group compound operators is correct, but then you have to add a separate SELECT at the top level:
code :
SELECT * FROM (SELECT * FROM a
               INTERSECT
               SELECT * FROM b
               INTERSECT
               SELECT * FROM c)
UNION 
SELECT * FROM (SELECT * FROM a
               INTERSECT
               SELECT * FROM b)
Simple TSQL union view won't respect indexes of tables

Simple TSQL union view won't respect indexes of tables


By : user2676160
Date : March 29 2020, 07:55 AM
Hope that helps I have 2 tables with identical structure & a view that unions them together. The view, however, isn't using the indexes on the underlying tables. Is there some way to make SQL Server respect the indexes without knowing which particular indexes will be needed by the caller? The plan for the view should be the same as the plan for a query that mimics the view's query... , The solution is to use the following view:
code :
create View Both as 
   select * from Uno 
     union all 
   select * from Dos
select *
from (select * from Uno
     union
     select * from Dos)
where SomeInt > 1
Query one left table and two right tables UNION result without union operator

Query one left table and two right tables UNION result without union operator


By : Demented Given
Date : March 29 2020, 07:55 AM
Hope this helps The equivalent of a UNION operation would be a FULL OUTER JOIN on all fields of both tables:
code :
SELECT Left_Table.KeyValue, Left_Table.Data_Left1, Left_Table.Data_Left2, 
       UN.KeyValue AS KeyValue2, UN.Data_Right1, UN.Data_Right2
FROM Left_Table LEFT OUTER JOIN (
SELECT COALESCE(t1.KeyValue, t2.KeyValue) AS KeyValue, 
       COALESCE(t1.Data_Right1, t2.Data_Right1) AS Data_Right1,
       COALESCE(t1.Data_Right2, t2.Data_Right2) AS Data_Right2
FROM Right_Table_1 AS t1
FULL OUTER JOIN Right_Table_2 AS t2  
  ON t1.KeyValue = t2.KeyValue AND 
     t1.Data_Right1 = t2.Data_Right1 AND 
     t1.Data_Right2 = t2.Data_Right2) AS UN
  ON Left_Table.KeyValue = UN.KeyValue
union multiple tables and group with aggregation (SQlite)

union multiple tables and group with aggregation (SQlite)


By : R3dPand4
Date : March 29 2020, 07:55 AM
I wish this help you SQLite does not support full outer joins, so get a list of all parts first, and then look up their prices with left outer joins:
code :
SELECT part,
       table1.price AS priceTable1,
       table2.price AS priceTable2,
       min(ifnull(table1.price, 'inf'),
           ifnull(table2.price, 'inf')) AS minPrice
FROM (SELECT part FROM table1
      UNION
      SELECT part FROM table2)
LEFT JOIN table1 USING (part)
LEFT JOIN table2 USING (part);
Join 2 tables via UNION group by a given field and get only the latest of the group

Join 2 tables via UNION group by a given field and get only the latest of the group


By : Joey
Date : March 29 2020, 07:55 AM
wish help you to fix your issue As Gordon said, if group by your data, that reduce the data to one row, but if you only use order by clause, you will still get duplicated data, it will bring all the data from union table.
The group by clause has a having clause too but it seems to only work with numbers, like count(*) > 2 and other stuff.
code :
select email_from, group_concat(email_body), date from (
  select email_from, email_body, date from incoming union all
  select email_to as email_from, email_body, date from outgoing
) as t_union group by email_from order by date desc;
select email_from, email_body, date from (
  select email_from, email_body, date from incoming union all
  select email_to as email_from, email_body, date from outgoing
) as t_union_1 where date in (
  select max(date) from (
    select email_from, email_body, date from incoming union all
    select email_to as email_from, email_body, date from outgoing
  ) as t_union group by email_from order by date desc
);
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