The UNION removes duplicate records.
Both relations or table must have same number of column.
Example
T1 U T2
T1
Rows=100
Columns=10
T2
Rows=50
Columns=10
T1 U T2
Rows=150
Columns=10
When no T2 rows same.
T1 U T2
Rows=100
Columns=10
When T2 rows same.
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