UNION and UNION ALL used to combine ( set operation ) two or more query results. UNION will eliminate duplicate rows and UNION ALL will display all rows.
SQL> select * from table_a;
No
–
1
2
2
SQL> select * from table_b;
No
—–
2
3
SQL> select * from table_a UNION select * from table_b;
No
—
1
2
3
SQL> select * from table_a UNION ALL select * from table_b;
No
—
1
2
2
2
3
Things to remember writing UNION queries
1. Number of columns in each UNION query must match
g :- select col1,col2 from table_a UNION select col1 from table_b; — This will not work;
Instead you can replace column with null clause to match the number of columns
2. Data types must match
Eg :- select ‘a’ from table_a UNION select 1 from table_b; — This will not work;
3. For large data set queries UNION might have performance issues. So use it very carefully.
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Nice Exp; …Keep it up
Thanks
very nice………
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very nice
Thanks
Good logic,better to understand
Thanks
Hi….
which one is execute very quick..means fast..
union all, bcoz it will NOT eliminate duplicates