Set operations (SQL) explained

Set operations in SQL is a type of operations which allow the results of multiple queries to be combined into a single result set.^[1]

Set operators in SQL include UNION, INTERSECT, and EXCEPT, which mathematically correspond to the concepts of union, intersection and set difference.

UNION operator

In SQL the UNION clause combines the results of two SQL queries into a single table of all matching rows. The two queries must result in the same number of columns and compatible data types in order to unite. Any duplicate records are automatically removed unless UNION ALL is used.

UNION can be useful in data warehouse applications where tables are not perfectly normalized.^[2] A simple example would be a database having tables sales2005 and sales2006 that have identical structures but are separated because of performance considerations. A UNION query could combine results from both tables.

Note that UNION ALL does 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 BY must be used.

Note that UNION ALL may be much faster than plain UNION.

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 * FROM sales2005UNIONSELECT * FROM sales2006;

yields this result set, though the order of the rows can vary because no ORDER BY clause 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 ALL gives different results, because it will not eliminate duplicates. Executing this statement:

SELECT * FROM sales2005UNION ALLSELECT * FROM sales2006;

would give these results, again allowing variance for the lack of an ORDER BY statement:

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

The discussion of full outer joins also has an example that uses UNION.

INTERSECT operator

The SQL INTERSECT operator takes the results of two queries and returns only rows that appear in both result sets. For purposes of duplicate removal the INTERSECT operator does not distinguish between [[Null (SQL)|NULLs]]. The INTERSECT operator removes duplicate rows from the final result set. The INTERSECT ALL operator 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 example INTERSECT query returns all rows from the Orders table where Quantity is between 50 and 100.

SELECT *FROM OrdersWHERE Quantity BETWEEN 1 AND 100

INTERSECT

SELECT *FROM OrdersWHERE Quantity BETWEEN 50 AND 200;

EXCEPT operator

The SQL EXCEPT operator 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, the EXCEPT operator does not distinguish between [[Null (SQL)|NULLs]]. The EXCEPT ALL operator 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 a MINUS operator which is functionally equivalent to the SQL standard EXCEPT DISTINCT operator.^[3]

Example

The following example EXCEPT query 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 *FROM OrdersWHERE Quantity BETWEEN 1 AND 100

EXCEPT

SELECT *FROM OrdersWHERE Quantity BETWEEN 50 AND 75;

Example

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

SELECT o1.*FROM (SELECT * FROM Orders WHERE Quantity BETWEEN 1 AND 100) o1LEFT JOIN (SELECT * FROM Orders WHERE Quantity BETWEEN 50 AND 75) o2ON o1.id = o2.idWHERE o2.id IS NULL

See also

• Union (set theory)
• Join (SQL)
  • 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

Notes and References

1. Web site: The UNION [ALL], INTERSECT, MINUS Operators]. Oracle. 14 July 2016.
2. "a UNION ALL views technique for managing maintenance and performance in your large data warehouse environment ... This UNION ALL technique has saved many of my clients with issues related to time-sensitive database designs. These databases usually have an extremely volatile current timeframe, month, or day portion and the older data is rarely updated. Using different container DASD allocations, tablespaces, tables, and index definitions, the settings can be tuned for the specific performance considerations for these different volatility levels and update frequency situations." Terabyte Data Warehouse Table Design Choices - Part 2 (accessed on July 25, 2006)
3. "E071-03, EXCEPT DISTINCT table operator: Use MINUS instead of EXCEPT DISTINCT" Web site: Oracle Compliance To Core SQL:2003. Docs.oracle.com. 7 July 2022.