The function of the union operation in SQL is to combine the results of two or more SELECT queries into a single result set. It merges rows from each query, removing duplicate rows by default, so that each distinct row appears only once in the combined output
. Key points about the union operation include:
- It requires that all SELECT statements involved have the same number of columns with compatible data types and in the same order
- The default UNION operator eliminates duplicate rows, whereas UNION ALL includes all rows, including duplicates, which can improve performance
- Union is useful for creating a unified dataset from multiple tables or queries, enabling comprehensive data analysis across different sources
- It can be used in various SQL contexts such as views, subqueries, and triggers
In summary, the union operation serves to efficiently combine and consolidate data from multiple queries into a single, distinct result set for easier data management and analysis.
