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.

The corresponding expressions in the select lists of the component queries of a compound query must match in number and must be in the same data type group (such as numeric or character).

If component queries select character data, then the data type of the return values are determined as follows:

• If both queries select values of data type CHAR of equal length, then the returned values have data type CHAR of that length. If the queries select values of CHAR with different lengths, then the returned value is VARCHAR2 with the length of the larger CHAR value.

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

If component queries select numeric data, then the data type of the return values is determined by numeric precedence:

• If any query selects values of type BINARY_DOUBLE, then the returned values have data type BINARY_DOUBLE.

• If no query selects values of type BINARY_DOUBLE but any query selects values of type BINARY_FLOAT, then the returned values have data type BINARY_FLOAT.

• If all queries select values of type NUMBER, then the returned values have data type NUMBER.

In queries using set operators, Oracle does not perform implicit conversion across data type groups. Therefore, if the corresponding expressions of component queries resolve to both character data and numeric data, Oracle returns an error.

Examples

The following query is valid:

SELECT 3 FROM DUAL
   INTERSECT
SELECT 3f FROM DUAL;

This is implicitly converted to the following compound query:

SELECT TO_BINARY_FLOAT(3) FROM DUAL
   INTERSECT
SELECT 3f FROM DUAL;

The following query returns an error:

SELECT '3' FROM DUAL
   INTERSECT
SELECT 3f FROM DUAL;

Restrictions on the Set Operators

The set operators are subject to the following restrictions:

• 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 the 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.

UNION Example

The following statement combines the results of two queries with the UNION operator, which eliminates duplicate selected rows. This statement shows that you must match data type (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
       1700 Contracting
       1700 Control And Credit
...

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
ORDER BY product_id;

SELECT location_id  FROM locations 
UNION ALL 
SELECT location_id  FROM departments
ORDER BY location_id;

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 unique rows returned by both queries:

SELECT product_id FROM inventories
INTERSECT
SELECT product_id FROM order_items
ORDER BY product_id;

MINUS Example

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

SELECT product_id FROM inventories
MINUS
SELECT product_id FROM order_items
ORDER BY product_id;