我有2张桌子：
T1 (id, flag1)
T2 (id, amount, date, flag2, t1_id);
我有以下查询：
SELECT T1.id, ROUND(COALESCE(SUM(T2.amount), 0), 2) AS spent_amount
FROM T1
LEFT JOIN T2 ON T2.t1_id = T1.id
WHERE T2.date <= '2014-01-01' AND T2.flag2 = 't' AND T1.flag1 = 't'
GROUP BY T1.id
问题是我想在结果中有一行，例如：id = 123, spent_amount = 0如果我有一个入口T1，但其中没有连接的行T2。
T2上有WHERE子句，它将过滤掉所有NULL：
SELECT T1.id, ROUND(COALESCE(SUM(T2.amount), 0), 2) AS spent_amount
FROM T1
LEFT JOIN T2
ON T2.t1_id = T1.id
AND T2.date <= '2014-01-01'
AND T2.flag2 = 't'
WHERE T1.flag1 = 't'
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我来说两句