Oracle union all

You can combine multiple queries using the set operators UNION , UNION ALL, INTERSECT, and MINUS. All set operators have equal precedence. Le tabelle usate per gli esempi per gli operatori UNION e UNION ALL.

In un precedente articolo dal titolo “Gli operatori set” abbiamo introdotto gli operatori insiemistici e le loro caratteristiche. The UNION operator is a set operator that combines result sets of two or more SELECT statements into a single result set.

This SQL UNION ALL example would return the supplier_id multiple times in the result set if that same value appeared in both the suppliers and orders table. The SQL UNION ALL operator does not remove duplicates. Utilizzando UNION eventuali valori duplicati (cioè record restituiti da più di una query) vengono automaticamente filtrati, mentre con UNION ALL questo non accade (quindi vengono moltrati anche risultati duplicati qualora uno stesso record fosse restituito da più di una query). In diesem SQL UNION ALL-Beispiel wird die supplier_id mehrere Male in der Ergebnismenge zurückgegeben, wenn derselbe Wert sowohl in der Tabelle suppliers als auch in der Tabelle orders enthalten ist. Der SQL UNION ALL Operator entfernt keine Duplikate.

Wenn Sie Duplikate entfernen möchten, verwenden Sie den UNION Operator. Union ：对两个结果集进行并集操作，不包括重复行，同时进行默认规则的. UNION merges the contents of two structurally-compatible tables into a single combined table.

Difference: The difference between UNION and UNION ALL is that UNION will omit duplicate records whereas UNION ALL will include duplicate records. Currently Derby uses sorting to eliminate duplicates from a UNION. You can use UNION ALL to avoid sorting, but UNION ALL will return duplicates.

It combines the both SELECT statement result sets and return as it is. Ask Question Asked years, months ago. Rimuovere (o meno) i duplicati nelle UNION. Di default le UNION effettuano una pulizia del et dai risultati duplicati (di default opera come se fosse una SELECT DISTINCT). Qualora si desideri mostrare tutti i risultati estratti, invece, sarà necessario utilizzare UNION ALL.

It is different from UNION operator in a way that it does not remove duplicate rows between the various SELECT statements. UNION It returns a union of two select statements. It is returning unique (distinct) values of them. SQL: how to use UNION and order by a specific select ? Browse other questions tagged sql oracle select union or ask your own question. Oracle оператор UNION ALL не удалит дубликаты.

UNION or UNION ALL statements that only consist of serial or remote branches are not processed concurrently unless specifically using the PQ_CONCURRENT_ UNION hint. The DOP of this SQL statement is at most the number of serial and remote inputs. Try generationg explain plan, or use sql trace with tkprof to select proper indexing, UNION ALL is not so costly operator.

Unio operator only concatenetes result i will suggest you tunning your queries one by one. Again, the UNION above does not get the same as the join above. If you find a case where the UNION seems to be faster, post a script that people can use to re-create the sitution, and the execution plans for each. A UNION returns a distinct combination. So if there are members of the second select which match members of the first select the count is going to be thrown off.

To illustrate, if the first select were to return ( a,b,c ) and the second query returned (a,z,x) the correct count should be (a,b,c,z,x). However preferring UNION ALL vs UNION will always be true, and portable SQL should avoid depending on optimizations that may not be in all databases.