Unions, Intersections, and Differences

The results of a Select statement are in the form of a table. This can be a subset of a single table, or the result of joining several tables. The exact set of rows is chosen by using various Where clauses. The use of Boolean operation such as and, or, and not allows a sort of union (or), intersection (and), and difference (not). For example  

Select logp From logp Where temp = 25 And ref Like %Hansch% Or ref Like %Yalkowsky%  

The careful use of the Boolean operations and, or, and not along with parentheses will produce the desired set of rows from any table. When data are in separate tables that are related to one another, this approach also works well when the two tables are joined together using the SQL Join clause. 

When data are selected from tables that are not related to one another, a different approach is used. The SQL operators union, intersect, and exclude allow set operations on tables or the sets of rows resulting from a select statement. For example  

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