Question

here is the sqlonline

There is a band.

Each member plays an instrument.

each instrument costs X$

I would like to create a report : how much each member ‘costs’ to the band manager.

create table #Inst(id int, name nvarchar(max) , price int )
insert into  #Inst values  (1,'guitar',20), 
                           (2,'bass',10),
                           (3,'drums',30),
                           (4,'piano',40) 
  
  create table #Players(id int, name nvarchar(max) , instId int )
  insert into  #Players values (1,'john',1), 
                               (2,'john',4),
                               (3,'paul',2),
                               (4,'paul',2),
                               (5,'george',1),
                               (6,'ringo',3)

What have I tried ?

select name , totalCostToTheband=   
              (select sum(price) from #inst i where i.id= #players.instId ) 
from #players

but this yields duplicate names :

I want to see 4 records which summarize each name’s costs.

p.s. :
I could have created a sum() over the whole query , but i want to know if there is a better , elegant solution.

Answer 1

In the current schema you should do

SELECT 
    p.name , 
    sum(price) as totalCostToTheband
FROM
    #players p 
    INNER JOIN #inst i ON i.id = p.instId
GROUP BY
    p.name

---

But when you have many-to-many relations you normally do a middle table with the links

create table #Inst(id int, name nvarchar(max) , price int )
create table #Players(id int, name nvarchar(max) )
create table #instplayer(id int, inst_id, player_id )

This saves from duplicating the players inside the #players table, and also allows for players with the same name to be in the same band 😉

Read http://sqlrelationship.com/many-to-many-relationship/ for more info