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Set operations (SQL)
==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 aren't perfectly normalized.〔"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 ) (URL accessed on July 25, 2006)〕 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 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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