20041012oracle学习笔记，注意minus的应用

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.

[b]Note:[/b] 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;