Execution sequence of connect by and group by

Question

Hi All,
I know 'connect by' precedes 'where clause'. And I can use a test case to test this.
But I can't figure out which comes first, connect by and group by.
Can someone give me some tips on this, pleaes?
And another confusion of mine is execution sequence of select and order by.
In my opinion, since order by can read the alias of selected column, I think the sequence is that oracle select rows first and then sort them.
Please correct or confirm my statement.
Best regards,
Leon

Answer 1

Hi, Leon,
user12064076 wrote:
Hi All,
I know 'connect by' precedes 'where clause'. And I can use a test case to test this.
But I can't figure out which comes first, connect by and group by. Can you figure out if WHERE precedes GROUP BY?
SELECT    job
,         COUNT (*)   AS cnt
FROM      scott.emp
WHERE     sal     < 2500
GROUP BY  job
;Aggregate functions, like COUNT, are evaluated as the GROUP BY clause is being performed. After the GROUP BY clause, only the ABC's (<b>A</b>ggregates, Group <b>B</b>Y expressions, <b>C</b>onstants, and <b>D</b>eterministic combinations of these) can be referenced.
Can someone give me some tips on this, pleaes? Consider this query:
SELECT     LEVEL
,     COUNT (*)     AS cnt
FROM     scott.emp
START WITH     mgr     IS NULL
CONNECT BY     mgr     = PRIOR empno
GROUP BY     LEVEL
;Output:
`    LEVEL        CNT
         1          1
         2          3
         4          2
         3          8
And another confusion of mine is execution sequence of select and order by.
In my opinion, since order by can read the alias of selected column, I think the sequence is that oracle select rows first and then sort them.
Please correct or confirm my statement. Yes, the ORDER BY clause is evaluated after the SELECT clause. The ability to reference column aliases in the ORDER BY clause hints at this. Also, you can say
ORDER BY  1