How to count data based on specific condition in Oracle - sql

Question

Let's say I have a table of persons with ids and names as follows
[Person]
ID NAME
================
1 Michael
2 Michelle
3 Emma
4 Evan
5 Ellen
6 Gary
I want to count the number of persons based on the first characters of their names.
Here's the output I expect
NUMBER_OF_PERSONS
=================
2 //M = Michael and Michelle
3 //E = Emma, Evan and Ellen
1 //G = Gary
How do I achieve this in Oracle?
And here's my query
select count(id) as number_of_person
from person
where substr(name) in (select distinct substr(name,1,1) from person);

Answer 1

You can acheive that purpose using below solution.
with Person (ID, NAME ) as (
select 1, 'Michael' from dual union all
select 2, 'Michelle' from dual union all
select 3, 'Emma' from dual union all
select 4, 'Evan' from dual union all
select 5, 'Ellen' from dual union all
select 6, 'Gary' from dual
)
select count(*) || ' //' || substr(NAME, 1, 1) || ' = ' ||
case
when regexp_count( listagg(NAME, ' and ') within group ( order by ID ), ' and ') > 1
then regexp_replace( listagg(NAME, ', ') within group ( order by ID ), ', ([^,]+)$', ' and \1 ', 1, 1 )
else listagg(NAME, ' and ') within group ( order by ID )
end NUMBER_OF_PERSONS
from Person
group by substr(NAME, 1, 1)
order by substr(NAME, 1, 1)
;
db<>fiddle

Answer 2

If you just want the count, you would use group by:
select substr(name, 1, 1) as first_letter,
count(*) as number_of_person
from person
group by substr(name, 1, 1) ;
If, in addition, you actually wanted the list of names, you could put that in another column, assuming there are not too many:
select substr(name, 1, 1) as first_letter,
count(*) as number_of_person,
listagg(name, ', ') within group (order by name) as names
from person
group by substr(name, 1, 1) ;

Answer 3

This is my solution to it:
WITH tbl AS (
SELECT 1 AS ID, 'Michael' AS NAME FROM dual UNION
SELECT 2, 'Michelle' FROM dual UNION
SELECT 3, 'Emma' FROM dual UNION
SELECT 4, 'Evan' FROM dual UNION
SELECT 5, 'Ellen' FROM dual UNION
SELECT 6, 'Gary' FROM dual
)
SELECT COUNT(1)
, SUBSTR(names.name,1,1)
, REGEXP_REPLACE((listagg(names.name,', ') WITHIN GROUP (ORDER BY names.name)), ',([^,]*)$', ' and \1')
FROM tbl names
GROUP BY SUBSTR(names.name,1,1);