What is the difference between UNION and UNION ALL ?
Both UNION and UNION ALL is
used to select information from structurally similar tables. That means
corresponding columns specified in the union should have same data type. For
example, in the above query, if FIRST_NAME is DOUBLE and
LAST_NAME is STRING above query wont work. Since the data type of both the
columns are VARCHAR, union is made possible. Difference between UNION and UNION
ALL is that , UNION query return only distinct values.
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