Question

当表B上没有记录时，如何从两个表A和B中加入和汇总聚合数据。我只想为表A中的所有记录显示零，表B中没有匹配的记录和其他标准。在这种情况下，我想返回模型值为“cars”的项目总数。当表B为空时会发生此问题。

这是我的代码段：

SELECT 10 as No,'Total' as Label, 
                SUM(CASE WHEN (a.year = b.year) THEN 1 ELSE 0 END) AS Value,
                (a.Year) as Year
FROM A a LEFT JOIN B b ON a.year = b.year
WHERE (isDeleted = 0 OR isdeleted is null) and b.model='cars'
GROUP BY a.year
order by YearMonth asc

Answer 1

问题在于：

WHERE (isDeleted = 0 OR isdeleted is null) and b.model='cars'

如果b为空，则b.model将为null，而您的where子句将填写这些记录。您应该将该条件移至JOIN子句：

FROM A a LEFT JOIN B b ON a.year = b.year and b.model='cars'
WHERE (isDeleted = 0 OR isdeleted is null)

请注意，您的SUM可能只是COUNT，因为不计算空值：

COUNT(b.year) AS Value,

Answer 2

将b.model ='cars'从WHERE子句移动到ON子句（否则执行常规内连接。）

SELECT 10 as No,
       'Total' as Label, 
       SUM(CASE WHEN (a.year = b.year) THEN 1 ELSE 0 END) AS Value,
       (a.Year) as Year
FROM A a LEFT JOIN B b ON a.year = b.year and b.model='cars'
WHERE (isDeleted = 0 OR isdeleted is null) 
GROUP BY a.year
order by YearMonth asc

SQL Join返回空记录

2 个答案: