Sams Teach Yourself SQL in 24 Hours, Third Edition By Ronald R. Plew, Ryan K. Stephens
Table of Contents
Hour 15. Combining Multiple Queries into One
You have been introduced to compound queries. All SQL statements previous to this hour have consisted of a single query. Compound queries allow multiple individual queries to be used together as a single query to achieve the data result set desired as output. The compound query operators discussed included UNION, UNION ALL, INTERSECT, and EXCEPT (MINUS). UNION returns the output of two single queries without displaying duplicate rows of data. UNION ALL simply displays all output of single queries, regardless of existing duplicate rows. INTERSECT is used to return identical rows between two queries. EXCEPT (the same as MINUS) is used to return the results of one query that do not exist in another query. Compound queries provide greater flexibility when trying to satisfy the requirements of various queries, which, without the use of compound operators, could result in very complex queries.
