Set operations (SQL)

Set operationsinSQLis a type of operations which allow the results of multiple queries to be combined into a singleresult set.

Set operators in SQL includeUNION,INTERSECT,andEXCEPT,whichmathematicallycorrespond to the concepts ofunion,intersectionandset difference.

UNION operator

InSQLtheUNIONclause combines the results of two SQL queries into a singletableof all matchingrows.The two queries must result in the same number ofcolumnsand compatibledata typesin order to unite. Any duplicate records are automatically removed unlessUNION ALLis used.

UNIONcan be useful indata warehouseapplications where tables are not perfectlynormalized.A simple example would be a database having tablessales2005andsales2006that have identical structures but are separated because of performance considerations. AUNIONquery could combine results from both tables.

Note thatUNION ALLdoes not guarantee the order of rows. Rows from the second operand may appear before, after, or mixed with rows from the first operand. In situations where a specific order is desired,ORDER BYmust be used.

Note thatUNION ALLmay be much faster than plainUNION.

Examples

Given these two tables:

sales2005

person	amount
Joe	1000
Alex	2000
Bob	5000

sales2006

person	amount
Joe	2000
Alex	2000
Zach	35000

Executing this statement:

SELECT*FROMsales2005
UNION
SELECT*FROMsales2006;

yields this result set, though the order of the rows can vary because noORDER BYclause was supplied:

person	amount
Joe	1000
Alex	2000
Bob	5000
Joe	2000
Zach	35000

Note that there are two rows for Joe because those rows are distinct across their columns. There is only one row for Alex because those rows are not distinct for both columns.

UNION ALLgives different results, because it will not eliminate duplicates. Executing this statement:

SELECT*FROMsales2005
UNIONALL
SELECT*FROMsales2006;

would give these results, again allowing variance for the lack of anORDER BYstatement:

person	amount
Joe	1000
Joe	2000
Alex	2000
Alex	2000
Bob	5000
Zach	35000

The discussion offull outer joinsalso has an example that usesUNION.

INTERSECT operator

The SQLINTERSECToperator takes the results of two queries and returns only rows that appear in both result sets. For purposes of duplicate removal theINTERSECToperator does not distinguish betweenNULLs.TheINTERSECToperator removes duplicate rows from the final result set. TheINTERSECT ALLoperator does not remove duplicate rows from the final result set, but if a row appears X times in the first query and Y times in the second, it will appear$\min(X,Y)$times in the result set.

Example

The following exampleINTERSECTquery returns all rows from the Orders table where Quantity is between 50 and 100.

SELECT*
FROMOrders
WHEREQuantityBETWEEN1AND100

INTERSECT

SELECT*
FROMOrders
WHEREQuantityBETWEEN50AND200;

EXCEPT operator

The SQLEXCEPToperator takes the distinct rows of one query and returns the rows that do not appear in a second result set. For purposes of row elimination and duplicate removal, theEXCEPToperator does not distinguish betweenNULLs.TheEXCEPT ALLoperator does not remove duplicates, but if a row appears X times in the first query and Y times in the second, it will appear$\max(X-Y,0)$times in the result set.

Notably, the Oracle platform provides aMINUSoperator which is functionally equivalent to theSQL standardEXCEPT DISTINCToperator.

Example

The following exampleEXCEPTquery returns all rows from the Orders table where Quantity is between 1 and 49, and those with a Quantity between 76 and 100.

Worded another way; the query returns all rows where the Quantity is between 1 and 100, apart from rows where the quantity is between 50 and 75.

SELECT*
FROMOrders
WHEREQuantityBETWEEN1AND100

EXCEPT

SELECT*
FROMOrders
WHEREQuantityBETWEEN50AND75;

Example

The following example is equivalent to the above example but without using theEXCEPToperator.

SELECTo1.*
FROM(
SELECT*
FROMOrders
WHEREQuantityBETWEEN1AND100)o1
LEFTJOIN(
SELECT*
FROMOrders
WHEREQuantityBETWEEN50AND75)o2
ONo1.id=o2.id
WHEREo2.idISNULL

See also

• Union (set theory)
• Join (SQL)
• SQL:2003
• Select (SQL)

External links

• MSDN documentation on UNION in Transact-SQL for SQL Server
• Naming of select list items in set operations
• UNION in MySQL with Examples
• UNION in MySQL
• UNION Clause in PostgreSQL
• SQL UNION and UNION ALL
• Sort order within UNION statement
• Designing a data flow that loads a warehouse table
• Oracle 11g documentation for UNION (ALL), INTERSECT and MINUS
• SQL Set Operators