Joing multiple with one master table - mysql

Question

I have six tables- Project,Equipment,Fish,Staff and the junction tables - Project_Equipment,Project_Fish and Project_Staff.I want to retrieve Project total cost.
So, I wrote the statement like follows,
SELECT P.ProjectID, (SUM(E.EquipPrice*PE.EQuantity)+SUM(F.FishPrice*PF.FQuantity)+SUM(PS.Salary)) as ProjectCost
FROM Equipment as E INNER JOIN Project_Equipment as PE
ON E.EquipID=PE.EquipID
INNER JOIN Project as P
ON PE.ProjectID=P.ProjectID
INNER JOIN Project_Fish as PF
ON P.ProjectID=PF.ProjectID
INNER JOIN Fish as F
ON PF.FishID=F.FishID
INNER JOIN Project_Staff as PS
ON P.ProjectID=PS.ProjectID
INNER JOIN Staff as S
PS.StaffID=S.StaffID
GROUP BY ProjectID
But, I got the price with twice of correct amount.

Answer 1

Your query will end up with a lot of duplicate results. Consider the following simpler case:
Table: Project
ProjectID EQuantity
1 1
2 1
Table: Equipment
EquipID EPrice
1 1
2 1
Table: Fish
FishID
1
2
Table: Project_Equipment
ProjectID EquipID
1 1
1 2
2 1
2 2
Table: Project_Fish
ProjectID FishID
1 1
1 2
2 1
2 2
Now, let's look at the results of a Project_Equipment query only:
SELECT p.projectid, e.eprice, pe.equantity FROM project p
INNER JOIN project_equipment pe ON pe.projectid=p.projectid
INNER JOIN equipment e ON e.equipid=pe.equipid
ProjectID EPrice EQuantity
1 1 1 // a
1 1 1 // b
2 1 1 // c
2 1 1 // d
That's as expected; a list of the price and quantity of each piece of equipment used by each project. But what do you think will happen when we INNER JOIN that to project_fish? That first result has project 1 in it twice, and project 2 in it twice, so we end up with every combination of that result and project_fish!
SELECT p.projectid, e.eprice, pe.equantity, f.fishid FROM project p
INNER JOIN project_equipment pe ON pe.projectid=p.projectid
INNER JOIN equipment e ON e.equipid=pe.equipid
INNER JOIN project_fish pf ON pf.projectid=p.projectid
ProjectID EPrice EQuantity FishID
1 1 1 1 // from a above
1 1 1 2 // from a above
1 1 1 1 // from b above
1 1 1 2 // from b above
2 1 1 1 // from c above
2 1 1 2 // from c above
2 1 1 1 // from d above
2 1 1 2 // from d above
This duplication will continue with every inner join. The amount your price will be off won't always be 2x, it will actually depend on the number of combinations of all your joins.
So, you can't really do what you are trying to do with this particular query. Instead you'll have to calculate the cost of each relationship separately. Then you sum all those together. You can do this by selecting each one separately and calculating the cost into a ProjectID and ProjectCost column, using UNION to concatenate those altogether, then once again grouping the results by ProjectID and summing the individual ProjectCost subtotals.
I explained that poorly, but think of it as subtotaling equipment, fish, and salary costs, then sticking all those subtotals into one table and summing that. E.g.:
SELECT x.projectid, SUM(x.ProjectCost) FROM
(
SELECT p.projectid, SUM(e.eprice * pe.equantity) ProjectCost FROM project p
INNER JOIN project_equipment pe ON pe.projectid=p.projectid
INNER JOIN equipment e ON e.equipid=pe.equipid
GROUP BY p.projectid
UNION
SELECT p.projectid, SUM(f.fprice * pf.fquantity) ProjectCost FROM project p
INNER JOIN project_fish pf ON pf.projectid=p.projectid
INNER JOIN fish f ON f.fishid=pf.fishid
GROUP BY p.projectid
UNION
SELECT p.projectid, SUM(s.salary) ProjectCost FROM project p
INNER JOIN project_staff ps ON ps.staffid=p.projectid
INNER JOIN staff s ON s.staffid=ps.staffid
GROUP BY p.projectid
) x
GROUP BY x.projectid
Each of the subqueries produces a projectid column and a ProjectCost column. Run the subquery (between the parens) by itself to see the results. The outer query then adds the subtotals for the projects.
Sorry, btw, I renamed your EquipPrice and FishPrice columns to EPrice and FPrice when I was testing.