How to select a specific person in MySQL? - mysql

Question

I created the following tables:
create table people
(
ID varchar(10),
name varchar(35),
CONSTRAINT pk_ID PRIMARY KEY (ID)
);
create table numbers
(
code varchar(10),
ID varchar(10),
number numeric,
CONSTRAINT pk_code PRIMARY KEY (code)
);
I inserted the following datas:
insert into people(ID, name)
values('fdx1','Peter');
insert into people(ID, name)
values('fdx2','Alice');
insert into people(ID, name)
values('fdx3','Louis');
insert into numbers(code, ID, number)
values('001','fdx1',1);
insert into numbers(code, ID, number)
values('002','fdx1',1);
insert into numbers(code, ID, number)
values('003','fdx2',2);
insert into numbers(code, ID, number)
values('004','fdx2',3);
insert into numbers(code, ID, number)
values('005','fdx3',4);
insert into numbers(code, ID, number)
values('006','fdx3',4);
My problem is: how to select people that has the same number. For example "Peter" and "Louis".

Answer 1

By "same number" you mean that there is only one number in numbers for the person. You can do this with group by and having:
select n.id
from numbers n
group by n.id
having min(number) = max(number);
Note: this doesn't take NULL into account. Your question doesn't specify what to do if one of the values is NULL.

Answer 2

If I understand correctly you want to see out of the 2 rows for each user in numbers who has the same number twice? If so you could do.
SELECT p.ID, p.name, p.number, COUNT(DISTINCT n.id) as num
FROM people as p
INNER JOIN numbers as n on n.ID = p.ID
GROUP BY p.ID, n.number
HAVING num > 1
The would return a list of people how have the same number in numbers more than once
In case this was not what you were looking for some other things you could do are:
To return a list of people for a specific number you could do the following
SELECT p.ID, p.name
FROM people as p
INNER JOIN numbers as n on n.ID = p.ID
WHERE n.number = [[X]]
You would replace [[X]] with the number e.g. 1 this would then return a list of people who are linked to number 1 in this case Peter
If you wanted a list of all people and their associated number you could do:
SELECT p.ID, p.name, n.number
FROM people as p
INNER JOIN numbers as n on n.ID = p.ID
ORDER BY n.number
This would return the users ID, Name and their associated number.

Answer 3

You could use this query:
select p.name, n.number, count(*) repeats
from people p
inner join numbers n on n.ID = p.ID
group by p.id, n.number
having count(*) > 1
This lists the names of the people who have a duplicate number. That number is included in the output, and the number of times it occurs.
Output:
| name | number | repeats |
|-------|--------|---------|
| Peter | 1 | 2 |
| Louis | 4 | 2 |
sql fiddle