Oracle Rownum 解析

現象：

SQL> select rownum,emp.* from emp;

    ROWNUM          A
---------- ----------
         1          1
         2          2
         3          1
         4          1

SQL> select rownum,emp.* from emp where rownum<3;

    ROWNUM          A
---------- ----------
         1          1
         2          2

SQL> select rownum,emp.* from emp where rownum=2;

未選定行

解釋（google到的，不過自己後面也理解了）：

How ROWNUM Works

ROWNUM is a pseudocolumn (not a real column) that is available in a query. ROWNUM will be assigned the numbers 1, 2, 3, 4, ... N, where N is the number of rows in the set ROWNUM is used with. A ROWNUM value is not assigned permanently to a row (this is a common misconception). A row in a table does not have a number; you cannot ask for row 5 from a table—there is no such thing.

！ROWNUM是在什麼時候被賦予每條記錄的!
Also confusing to many people is when a ROWNUM value is actually assigned. A ROWNUM value is assigned to a row after it passes the predicate phase of the query but before the query does any sorting or aggregation. Also, a ROWNUM value is incremented only after it is assigned, which is why the following query will never return a row:

 

select *from twhere ROWNUM > 1;

 

Because ROWNUM > 1 is not true for the first row, ROWNUM does not advance to 2. Hence, no ROWNUM value ever gets to be greater than 1. Consider a query with this structure:

 

select ..., ROWNUMfrom twhere <where clause>group by <columns>having <having clause>order by <columns>;

 

Think of it as being processed in this order:

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.

That is why a query in the following form is almost certainly an error:

 

select *from empwhere ROWNUM <= 5order by sal desc;

 

The intention was most likely to get the five highest-paid people—a top-N query. What the query will return is five random records (the first five the query happens to hit), sorted by salary. The procedural pseudocode for this query is as follows:

 

ROWNUM = 1for x in( select * from emp )loopexit when NOT(ROWNUM <= 5)OUTPUT record to tempROWNUM = ROWNUM+1end loopSORT TEMP

 

It gets the first five records and then sorts them. A query with WHERE ROWNUM = 5 or WHERE ROWNUM > 5 doesn't make sense. This is because a ROWNUM value is assigned to a row during the predicate evaluation and gets incremented only after a row passes the WHERE clause.

Here is the correct version of this query:

 

select *from( select *from emporder by sal desc )where ROWNUM <= 5;

 

This version will sort EMP by salary descending and then return the first five records it encounters (the top-five records). As you'll see in the top-N discussion coming up shortly, Oracle Database doesn't really sort the entire result set—it is smarter than that—but conceptually that is what takes place.