How to use union in oracle

The usage of UNION in Oracle is to merge multiple query result sets with the same structure into a single result set. This operator removes duplicate records unless UNION ALL is used, which merges all records, including duplicates.

UNION usage in Oracle

UNION is used in Oracle to combine multiple queries SQL operator that combines result sets into a single result set. It is used to merge tables or query results that have the same structure (column names and data types).

Syntax:

<code>SELECT ...
UNION
SELECT ...
UNION
...</code>

Usage:

• Merge tables with the same structure:

  <code>SELECT * FROM table1
  UNION
  SELECT * FROM table2;</code>

• Merge different queries:

  <code>SELECT name, age FROM students
  UNION
  SELECT name, NULL AS age FROM teachers;</code>

Note:

• Only columns with the same number and data type can be merged.
• UNION will delete duplicate records unless UNION ALL is used.
• UNION ALL will combine all records, including duplicate records.

Example:

The following table contains two tables:

table1

##1John202Mary25

id	name	age

table2

##id34Using UNION, we can merge these two tables:

name	job
Bob	teacher
Alice	student

<code>SELECT * FROM table1
UNION
SELECT id, name, NULL AS job FROM table2;</code>

Result:

id1234

name	age	job
John	20	null
Mary	25	null
Bob	null	teacher
Alice	null	student

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