Quick question about GROUP BY

Question

Hello guys,
I was wondering if anybody can shed the light on this? It's not very important. I'm just being curious here.
Imagine I have the following table:
CREATE TABLE my_table
  n NUMBER(15)
INSERT INTO my_table SELECT level FROM dual CONNECT BY LEVEL <= 10;I know I can do this
SELECT 'MY_TABLE',
       (SELECT COUNT(*) FROM my_table)
  FROM dual;To get the name of the table and how many records the table has. Because DUAL table has only one record it will return the correct result.
But I don't understand why the following query returns the correct result:
SELECT 'MY_TABLE',
       (SELECT COUNT(*) FROM my_table)
  FROM my_table
GROUP BY NULL;If I remove the GROUP BY clause, I will have 10 records because my original table has 10 records. But how is it possible to GROUP BY NULL? Actually, you can group by on anything (1, 'HELLO'...) and it will return the correct result.
So, can anyone tell me how does this make sense??
Thanks in advance,

Answer 1

Hi,
user13117585 wrote:
Hello guys,
I was wondering if anybody can shed the light on this? It's not very important. I'm just being curious here.
Imagine I have the following table:
CREATE TABLE my_table
n NUMBER(15)
INSERT INTO my_table SELECT level FROM dual CONNECT BY LEVEL <= 10;I know I can do this
SELECT 'MY_TABLE',
(SELECT COUNT(*) FROM my_table)
FROM dual;To get the name of the table and how many records the table has. Because DUAL table has only one record it will return the correct result.The fact that dual has only one row explains why the result set has one row. How many columns and what is in those columns doesn't depend on dual in any way.
But I don't understand why the following query returns the correct result:
SELECT 'MY_TABLE',
(SELECT COUNT(*) FROM my_table)
FROM my_table
GROUP BY NULL;If I remove the GROUP BY clause, I will have 10 records because my original table has 10 records. But how is it possible to GROUP BY NULL? Actually, you can group by on anything (1, 'HELLO'...) and it will return the correct result.
So, can anyone tell me how does this make sense??Right. "GROUP BY x" means the output will have 1 row for each distinct value of x. (NULL is treated like a value in this case.) So how many distinct values of NULL are there, or how many distinct values of 1, or 'HELLO', or any constant? There's only 1 value, constants, as the name implies, never change values. So if you GROUP BY any constant, the result set will have exactly one row.
Thanks in advance,