The UNION [ALL], INTERSECT, MINUS Operators

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 evaluates them from the left to right if no parentheses explicitly specify another order.

The corresponding expressions in the select lists of the component queries of a compound query must match in number and datatype. If component queries select character data, then the datatype of the return values are determined as follows:

• If both queries select values of datatype CHAR, then the returned values have datatype CHAR.
• If either or both of the queries select values of datatype VARCHAR2, then the returned values have datatype VARCHAR2.

Restrictions on Set Operators

• The set operators are not valid on columns of type BLOB, CLOB, BFILE, VARRAY, or nested table.
• The UNION, INTERSECT, and MINUS operators are not valid on LONG columns.
• If the select list preceding the set operator contains an expression, then you must provide a column alias for the expression in order to refer to it in the order_by_clause.
• You cannot also specify the for_update_clause with these set operators.
• You cannot specify the order_by_clause in the subquery of these operators.
• You cannot use these operators in SELECT statements containing TABLE collection expressions.

  Note: To comply with emerging SQL standards, a future release of Oracle will give the INTERSECT operator greater precedence than the other set operators. Therefore, you should use parentheses to specify order of evaluation in queries that use the INTERSECT operator with other set operators.

The following examples combine the two query results with each of the set operators.

UNION Example

The following statement combines the results with the UNION operator, which eliminates duplicate selected rows. This statement shows that you must match datatype (using the TO_CHAR function) when columns do not exist in one or the other table:

SELECT location_id, department_name "Department", 
   TO_CHAR(NULL) "Warehouse"  FROM departments
   UNION
   SELECT location_id, TO_CHAR(NULL) "Department", warehouse_name 
   FROM warehouses;

LOCATION_ID Department            Warehouse
----------- --------------------- --------------------------
       1400 IT
       1400                       Southlake, Texas
       1500 Shipping
       1500                       San Francisco
       1600                       New Jersey
       1700 Accounting
       1700 Administration
       1700 Benefits
       1700 Construction
...

UNION ALL Example

The UNION operator returns only distinct rows that appear in either result, while the UNION ALL operator returns all rows. The UNION ALL operator does not eliminate duplicate selected rows:

SELECT product_id FROM order_items
UNION
SELECT product_id FROM inventories;

SELECT location_id  FROM locations 
UNION ALL 
SELECT location_id  FROM departments;

A location_id value that appears multiple times in either or both queries (such as '1700') is returned only once by the UNION operator, but multiple times by the UNION ALL operator.

INTERSECT Example

The following statement combines the results with the INTERSECT operator, which returns only those rows returned by both queries:

SELECT product_id FROM inventories
INTERSECT
SELECT product_id FROM order_items;

MINUS Example

The following statement combines results with the MINUS operator, which returns only rows returned by the first query but not by the second:

SELECT product_id FROM inventories
MINUS
SELECT product_id FROM order_items;