Order of the sql operation

Question

hi experts,
I want to clear my basic concepts here.
I want to know the order of the sql operation.
actually the problem with me that i have read from different blog and websites
about the order of operation of the sqkl queries , hence i am confused here.
what i think the order of operation iis
-----------MY assumptuion---------------------
      From
       |
      where
       |
      group
       |
      having
       |
      select
       |
      order by
is my assupmtion is correct?
if yes , then if suppose there is rownum clause in my selection criteria,at which timing
will it apply?
on some site i have seen below criteria
1. The FROM/WHERE clause goes first.
2. ROWNUM is assigned and incremented to each output row from the FROM/WHERE clause.
3. SELECT is applied.
4. GROUP BY is applied.
5. HAVING is applied.
6. ORDER BY is applied.
is this correct or my assumption is correct?
thanks a lot in advance..!!
regards,
prashant

Answer 1

Hi Prashant,
Instead of memorizing, why not check the query plan -
-->-- Creating a test table
CREATE TABLE test_tbl as
  SELECT 'A' name, 100 sal FROM dual UNION ALL
  SELECT 'A' name, 120 sal FROM dual UNION ALL
  SELECT 'B' name, 66 sal FROM dual UNION ALL
  SELECT 'C' name, 20 sal FROM dual UNION ALL
  SELECT 'C' name, 50 sal FROM dual UNION ALL
  SELECT 'C' name, 60 sal FROM dual UNION ALL
  SELECT 'D' name, 90 sal FROM dual UNION ALL
  SELECT 'D' name, 110 sal FROM dual;
-->-- Query explain plan
EXPLAIN PLAN for
SELECT name, SUM(sal) sm
FROM test_tbl
GROUP BY name
HAVING Sum(sal) > 150
ORDER BY sm;
-->-- Fetching the plan from cursor pool
SELECT *
FROM TABLE(dbms_xplan.display);
-->-- Query plan
PLAN_TABLE_OUTPUT                                                               
Plan hash value: 3401269832                                                     
| Id  | Operation            | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT     |          |     8 |   128 |     5  (40)| 00:00:01 |
|   1 |  SORT ORDER BY       |          |     8 |   128 |     5  (40)| 00:00:01 |
|*  2 |   FILTER             |          |       |       |            |          |
|   3 |    HASH GROUP BY     |          |     8 |   128 |     5  (40)| 00:00:01 |
|   4 |     TABLE ACCESS FULL| TEST_TBL |     8 |   128 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):                             
     2 - filter(SUM("SAL")>150)                                                                                       
Now, how to read the plan?
Start reading the plan - with the line "Operation" indented towards extreme right. Line-4 in this case.
Note: If two lines are indented similarly i.e. on same vertical line... read normally as-in order.
Like this:
| Id  | Operation            | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT     |            |     8 |   128 |     5  (40)| 00:00:01 |
|   1 |  SORT ORDER BY       |            |     8 |   128 |     5  (40)| 00:00:01 |
|*  2 |   FILTER             |            |       |       |            |          |
|   3 |    HASH GROUP BY     |            |     8 |   128 |     5  (40)| 00:00:01 |
|   4 |     TABLE ACCESS FULL| TEST_TBL_1 |     8 |   128 |     3   (0)| 00:00:01 |
|   5 |     TABLE ACCESS FULL| TEST_TBL_2 |     8 |   128 |     3   (0)| 00:00:01 |
Read: line 4 and then line 5, since both are on same vertical level.
Final sequence:
1- (Line 4) Table Access : FROM clause
2- (Line 3) GROUP BY
3- (Line 2) Filtering : WHERE clause
4- (Line 1) Sorting : ORDER BY clause
5- (Line 0) SELECT
Hope this helps.
-- Ranit