Q.What does the UNION operator do? State the conditions two SELECT statements must satisfy to be combined with UNION, and explain the difference between UNION and UNION ALL.

UNION stacks the results of two queries; this asks what it does, its requirements, and how it differs from UNION ALL.

The UNION operator combines the results of two SELECT queries by placing their rows one below the other — that is, it stacks rows (row-wise), unlike a join, which combines columns (column-wise).

Conditions for UNION: the two SELECT statements must return the same number of columns, and the corresponding columns must have compatible (matching) data types. For example, combining a list of current and former employee names:

SELECT Name FROM Employee
UNION
SELECT Name FROM FormerEmployee;
``` …

Do not confuse UNION (stacks rows; needs matching columns) with a join (widens rows by combining columns). Also remember plain UNION removes duplicates automa …