Final answer:
Two SELECT statements must have the same number of columns and compatible data types for set operations like UNION, INTERSECT, or EXCEPT.
Step-by-step explanation:
For two SELECT statements to be combined via set operations such as UNION, INTERSECT, or EXCEPT, they must satisfy certain conditions. Firstly, the number of columns and their order in both SELECT statements must be the same. This ensures that the corresponding columns are compatible for the set operation. Secondly, the data types of the columns need to be compatible so that the database can correctly combine the data without error. These conditions ensure that the operations combine the statements correctly without any ambiguity or data type conflict, resulting in a set of rows as accurate output.
For example, using the concept of set theory, the set A AND B represents the intersection of sets, which contains only the elements common to both sets. Conversely, A OR B represents the union of the sets, containing all elements that appear in either set A or B. Similar principles apply when using set operations with SQL SELECT statements.