为每行计算where子句。不存在以下行：

type = 'cleaning' AND type = 'painting'

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因为sum看不到行，所以它返回null。

你

SELECT
    SUM(expense) AS total 
FROM expenses 
WHERE id = 1 
AND type = 'cleaning' 
AND type = 'painting';

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查询表明您要对SUM同时等于cleaning和painting的所有记录的expense进行SUM。
由于Type要么不匹配这两个值中的任何一个，要么只匹配其中一个，而您的条件要求它匹配这两个值，因此没有记录匹配该条件，因此SUM计算空集的和。
试试看：

SELECT
    SUM(expense) AS total 
FROM expenses 
WHERE id = 1 
AND type IN ('cleaning', 'painting');

型
或

SELECT
    SUM(CASE WHEN Type = 'cleaning' THEN expense ELSE 0 END) AS total_cleaning,
    SUM(CASE WHEN Type = 'painting' THEN expense ELSE 0 END) AS total_painting
FROM expenses 
WHERE id = 1;

型
而不是.

这是因为根据查询，您正在搜索一个既有油漆又有清洁的类型。
你可以尝试使用OR来获得油漆和清洁的总和。

SELECT SUM(expense) AS total FROM expenses WHERE id = 1 AND (type = 'cleaning' OR type = 'painting');

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