當我LEFT JOIN表2，我怎麼返回0，如果該值爲NULL？

Question

我正在嘗試設置0到任何空值在LEFT JOIN，使用COALESCE（）或IFNULL（），SQLite中。 我知道這個問題已經被問了很多，但是，我堅持。

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 

這是我所做的查詢。

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

改爲在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;