Understanding SQL Group By, Like a Boss...

The Oracle Documentation helps, but essentially the group by clause allows you to return one row for a column that might have duplicates.  For instance if you groups all the member_ids in a post table together you could perfom some kind of calculation on them.  How many there are, tally up a column for them, etc.  

Group By Examples

How about we count the toal number of posts for users?    

select 
MEMBER_ID, count(MEMBER_ID)
from USER_POSTS
group by MEMBER_ID;

 MEMBER_ID                        COUNT(MEMBER_ID)
---------- ---------------------------------------
1404220137                                      14
1404782755                                      12
1414338077                                      13
1414369197                                      12

Ok now what if we want to total up the edits for each member? 

select 
MEMBER_ID, count(EDIT_COUNT)
from USER_POSTS
group by MEMBER_ID;

 MEMBER_ID                    COUNT(EDIT_COUNT)
---------- ---------------------------------------
1414481042                                      29
1414490498                                      21
1414490548                                      19
1414490340                                       2
1414490514                                      19
1414492023                                       3

Ok, cool. Getting the hang of it. How about the post with the most edits for each member?

select 
MEMBER_ID, max(EDIT_COUNT)
from USERS_POSTS
group by MEMBER_ID;

 MEMBER_ID                      MAX(EDIT_COUNT)
---------- ---------------------------------------
1404081638                                        2
1404804526                                        0
1414390433                                        4
1414413987                                        10
1414358158                                        1
1414381507                                        6

Having Clause Examples

Now lets see how to add the HAVING clause into our statement.  The where clause does not work in a group by function.  So you can basically think of HAVING like a WHERE clause for your GROUP BY queires.

Let's only grab users who have actually editied a post.

select 
MEMBER_ID
from USER_POSTS
group by MEMBER_ID
having sum(EDIT_COUNT) > 0;

 MEMBER_ID
----------
1405226398
1405263540
1405251685
1405216407