Union and union all in oracle Database - sql

Question

While executing the below two queries (focus on the part between two asterisks * ____ *), I am really wondering how does the position of UNION ALL changes the output. I am unable to understand.
Query 1
SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
*UNION All SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL*
UNION SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
Query Result
NAME MARKS
Jack 100
Query 2
SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
UNION SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
*UNION ALL SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL*
Query Result
NAME MARKS
Jack 100
Jack 100
Thanks :)

Answer 1

If you don't specify parantheses, the selects would get executed one after the other. All set operators minus, union, union all, intersect have the same precedence.
Oracle Documentation
In the first query, UNION is performed at the end so there would be no dup rows per the query. In the second, UNION ALL is performed at the end, so there would be dup rows per your query.

Answer 2

The difference between Union and Union all is that Union all will not eliminate duplicate rows,
first query output:
step 1:
SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
UNION All SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
result 2 rows because union all allows duplicates.
step 2:
UNION SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
this query will pick only row without duplicates from the above 2 rows and itself.
returns 1 row.
in the second query...
step 1
SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
UNION SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
returns 1 row because union selects only distinct rows.
step 2
UNION ALL SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
return 2 rows because it allows duplicates.

Answer 3

Both union and union all have the same precedence as operations. So in the absence of parentheses, your two unions would be evaluated from top to bottom. Your first query is being evaluated like this:
SELECT Name, Marks
FROM
(
SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
UNION All
SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
) t
UNION
SELECT 'Jack' AS Name, 100 AS Marks FROM DUAL
The same reasoning applies to your second query.

Answer 4

To understand this behavior,you need to be clear with the difference between UNION and UNION ALL.
Query 1: Output of first two lines will return 2 rows. However, last line(sql statement) with 'UNION' operator will remove duplicate rows,including it's own output. So, total first query will return only 1 row.
Query 2: Now, UNION is on 2nd line. After getting combined with 1st line, it will again return only 1 row. But, on the 3rd line with 'UNION ALL',it will return 2 rows. Because, 'UNION ALL' won't remove duplicates.