If you have TABLE A with these columns (I put the data type in parenthesis, they are not actually part of the table name):
LAST_NAME(TEXT), FIRST_NAME(TEXT), MIDDLE_NAME(TEXT), DATE_OF_BIRTH(DATE),
and TABLE B with these columns:
LAST_NAME(TEXT), FIRST_NAME(TEXT), MIDDLE_NAME(TEXT), DATE_OF_EMPLOYMENT(DATE)
and you ran this UNION query:
* What column name would the 4th column get?
* Would the data in the 4th column have much "meaning" or "context" at that point?
SQL uses the column names of the first part of the union query as the column ...
SQL Database Columns are studied.