当我LEFT JOIN表2，我怎么返回0，如果该值为NULL？

Question

我正在尝试设置0到任何空值在LEFT JOIN，使用COALESCE（）或IFNULL（），SQLite中。 我知道这个问题已经被问了很多，但是，我坚持。

BUDGET 
amount category 
2  food 
2  book 
2  movie 

EXPENSE 
amount category 
2  food 
2  book 

EXPECTED RESULT 
budsum budcat expsum 
2  food 2 
2  book 2 
2  movie 0 

这是我所做的查询。

SELECT SUM(budget.amount) AS budsum, budget.category AS budcat, 
SUM(coalesce(expense.amount,0)) AS expsum 
FROM budget 
LEFT JOIN expense 
ON budcat=expense.category 
GROUP BY budcat; 

SELECT SUM(budget.amount) AS budsum, budget.category AS budcat, 
SUM(coalesce(expense.amount,0)) AS expsum 
FROM budget 
LEFT JOIN expense 
ON budcat=expense.category 
GROUP BY budcat; 

Answer 1

改为在sum上使用​​3210。

select SUM(budget.amount) as budsum, 
    budget.category as budcat, 
    coalesce(SUM(expense.amount), 0) as expsum 
from budget 
left join expense on budget.category = expense.category 
group by budget.category;