The UNION operator in SQL is used to combine the result sets of two or more SELECT statements into a single result set. It appends the rows from one table to another. For this operation to work, the tables must have the same number of columns, and the corresponding columns must have compatible data types. In this scenario, both the Managers and Non-managers tables share an identical structure, making UNION the correct method to create a single, consolidated report of all employees by stacking the rows from both tables vertically.

A. Group: This is used with aggregate functions (e.g., COUNT, SUM) to summarize data into groups, not to combine tables.

B. Join: This combines tables horizontally based on a related column, merging columns from multiple tables into a single row.

D. Nested: This refers to a subquery (a query within another query) and describes a query's structure, not the specific operation for combining these tables.

1. Microsoft SQL Server Documentation. (2023). UNION (Transact-SQL). Microsoft Docs. In the "Arguments" section

it is stated that UNION "Specifies that multiple result sets are to be combined and returned as a single result set." The basic example shows combining two tables with identical column structures.

2. PostgreSQL 16 Documentation. (2023). 7.4. Combining Queries. In this section

the documentation states

"The UNION operator computes the set union of the rows returned by the involved SELECT statements... The two SELECT statements that represent the direct operands of the UNION must produce the same number of columns

and corresponding columns must be of compatible data types."

3. Ullman

J. D.

& Widom

J. (n.d.). CS145 Introduction to Databases Course Notes: More SQL. Stanford University. In the section on "Union

Intersection

and Difference of Relations

" the notes specify that UNION is the standard set-union operator for SQL

requiring the tables to be "union-compatible

" which the tables in the question are.