Oracle “Partition By” Keyword

Question

Can someone please explain what the partition by keyword does and give a simple example of it in action, as well as why one would want to use it? I have a SQL

Answer 1

I think, this example suggests a small nuance on how the partitioning works and how group by works. My example is from Oracle 12, if my example happens to be a compiling bug.

I tried :

SELECT t.data_key
,      SUM ( CASE when t.state = 'A' THEN 1 ELSE 0 END) 
OVER   (PARTITION BY t.data_key) count_a_rows
,      SUM ( CASE when t.state = 'B' THEN 1 ELSE 0 END) 
OVER   (PARTITION BY t.data_key) count_b_rows
,      SUM ( CASE when t.state = 'C' THEN 1 ELSE 0 END) 
OVER   (PARTITION BY t.data_key) count_c_rows
,      COUNT (1) total_rows
from mytable t
group by t.data_key  ---- This does not compile as the compiler feels that t.state isn't in the group by and doesn't recognize the aggregation I'm looking for

This however works as expected :

SELECT distinct t.data_key
,      SUM ( CASE when t.state = 'A' THEN 1 ELSE 0 END) 
OVER   (PARTITION BY t.data_key) count_a_rows
,      SUM ( CASE when t.state = 'B' THEN 1 ELSE 0 END) 
OVER   (PARTITION BY t.data_key) count_b_rows
,      SUM ( CASE when t.state = 'C' THEN 1 ELSE 0 END) 
OVER   (PARTITION BY t.data_key) count_c_rows
,      COUNT (1) total_rows
from mytable t;

Producing the number of elements in each state based on the external key "data_key". So, if, data_key = 'APPLE' had 3 rows with state 'A', 2 rows with state 'B', a row with state 'C', the corresponding row for 'APPLE' would be 'APPLE', 3, 2, 1, 6.