Union all oracle

What is the difference between Union and Union all? You can combine multiple queries using the set operators UNION , UNION ALL , INTERSECT, and MINUS. All set operators have equal precedence. If a SQL statement contains multiple set operators, then Oracle Database evaluates them from the left to right unless parentheses explicitly specify another order. This Oracle tutorial explains how to use the Oracle UNION ALL operator with syntax and examples.

It is different from UNION operator in a way that it does not remove duplicate rows between the various SELECT statements. Oracle Union All operator is used to combine the result sets of two or more SELECT statements. It combines the both SELECT statement result sets and return as it is.

It does not remove duplicate rows between the various SELECT statements ( all rows are returned). Each SELECT statement within the UNION ALL must have the same number of fields in the result sets with similar data types. In this Oracle UNION operator example, if a supplier_id appeared in both the suppliers and order_details table, it would appear once in your result set. The Oracle UNION operator removes duplicates. If you do not wish to remove duplicates, try using the Oracle UNION ALL operator.

UNION ALL The UNION ALL command is equal to the UNION comman except that UNION ALL selects all values. The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table. WITH Clause with union all - Oracle 12c. SQL: how to use UNION and order by a. The UNION operator is used to combine the result-set of two or more SELECT statements.

We should use use UNION ALL , if we are sure that all the records returned are unique from your union (as in our query 2), it gives faster. UNION will eliminate duplicate rows and UNION ALL will display all rows. UNION and UNION ALL used to combine ( set operation ) two or more query.

The UNION ALL is combining the sets. A UNION is wastefull because the sets could not be the same, because the ordered field is different. Ask Question Asked years, months ago. Can anyone explain the following query? In the last few years, The Big Three have worked specifically on enhancing their ability to do UPDATE, DELETE, and INSERT statements on views based on a UNION ALL operator.

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. A UNION is highly optimized and really fast, except in cases where one query finishes long before the other, and Oracle must wait to get the whole result set before starting sorting. We tried pushing the SQL into a WITH clause, and some other subquery rewrites, but no perfomance boost.

Currently Derby uses sorting to eliminate duplicates from a UNION. You can use UNION ALL to avoid sorting, but UNION ALL will return duplicates. You most probably also want to use UNION ALL which avoids removing duplicates between the two queries and is faster than a plain UNION.

The UNION ALL operator returns from all the participating queries, including the duplicate rows. This means that if we will execute the same query then we will get all the rows returned from both the tables along with the duplicate one. Let me demonstrate this to you. After the UNION , it would appear once in the result set because Oracle UNION operator removes duplicate sets.

We have made union all query to get data from all these queries and each query involves multiple joins and approximately returns 100k records. All the queries uses a common temp table that holds the user details. This table would be loaded at the start and queries will make use of this table.