如何从MySQL的多个表中获取值？

Question

如何从各种表中获取行并执行求和运算？

例如表1：树

-------------------
id        name     
-------------------
1         tree1    
-------------------
2         tree2    
-------------------
3         tree3    
-------------------
4         tree4    
-------------------

表2：叶子

--------------------------------
tree_id        name        value
--------------------------------
1              test1          5
--------------------------------
1              test2          3   
--------------------------------
2              test3          7
--------------------------------
4              test4          9
-------------------------------- 

所需输出：

select tree.id,tree.name,sum(leaf.value) as total from tree,leaf where leaf.tree_id=tree.id

Array([0] => Array([id]=>1, [name]=>tree1, [total]=>8),
[1] => Array([id]=>2, [name]=>tree2, [total]=>7)
[2] => Array([id]=>3, [name]=>tree3, [total]=>0)
[3] => Array([id]=>4, [name]=>tree4, [total]=>9))

Answer 1

SELECT tree.id, tree.name, sum(leaf.value) as total
FROM tree
LEFT JOIN leaf
  ON leaf.tree_id = tree.id
GROUP BY tree.id

Answer 2

SELECT tree.id, tree.name, sum(leaf.value) as total
FROM tree
left  join leaf
  ON leaf.tree_id = tree.id group by tree.id