加入3表问题

Question

I am trying to solve this problem:

"Our data: We have three Tabels in the database :

Plant – describes the possible plants
Planted – whatever the gardener planted
Picked – the crop that was provided after the plantation."

What I am trying to do is join to 3 tables like that:

SELECT plant.PlantID, plant.name, planted.Seeds, picked.Amount
FROM plant 
INNER JOIN planted ON plant.PlantID = planted.PlantFK
INNER JOIN picked ON picked.PlantFK = plant.PlantID

So I could get data about specific PlantID how many seeds was planted and how many was picked. But for some reason Im getting multiple plantids. Because the database has more then 1 entry about the plant/ picked about the same PlantID..

How can I fix that join?

Answer 1

Just use an aggregate function SUM or COUNT with a GROUP BY plant.PlantID, plant.name like so:

SELECT 
  plant.PlantID, 
  plant.name, 
  SUM(DISTINCT planted.Seeds) 'Planted', 
  SUM(DISTINCT picked.Amount) 'Picked' 
FROM plant 
INNER JOIN planted ON plant.PlantID = planted.PlantFK
INNER JOIN picked ON picked.PlantFK = plant.PlantID
GROUP BY   plant.PlantID, plant.name;

SQL Fiddle Demo

If you want to get the plants that have no seeds, and never picked from, you have to use LEFT JOIN with IFNULL() to get zeros instead of NULL s like so:

SELECT 
  plant.PlantID, 
  plant.name, 
  SUM(IFNULL(DISTINCT planted.Seeds, 0)) 'Planted', 
  SUM(IFNULL(DISTINCT picked.Amount, 0)) 'Picked' 
FROM plant 
LEFT  JOIN planted ON plant.PlantID = planted.PlantID
LEFT  JOIN picked ON picked.PlantID = plant.PlantID
GROUP BY   plant.PlantID, plant.name;

Updated SQL Fiddle Demo