Answer Posted / lalitha

join:

The SQL JOIN clause is used whenever we have to select data
from 2 or more tables.
To be able to use SQL JOIN clause to extract data from 2
(or more) tables, we need a relationship between certain
columns in these tables.

union:

The purpose of the SQL UNION command is to combine the
results of two queries together. In this respect, UNION is
somewhat similar to JOIN in that they are both used to
related information from multiple tables. One restriction
of UNION is that all corresponding columns need to be of
the same data type. Also, when using UNION, only distinct
values are selected (similar to SELECT DISTINCT).

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