将子查询转换为sqlite中的左连接

Question

我有一个针对使用几个子查询的 SQLite 数据库运行的查询。为了适应一些新的要求，我需要将其翻译为使用连接。以下是原始查询的结构版本：

SELECT c.id AS category_id, b.budget_year,
(
    SELECT sum(actual)
    FROM lines l1
    WHERE status = 'complete'
    AND category_id = c.id
    AND billing_year = b.budget_year
) AS actual
(
    SELECT sum(planned)
    FROM lines l2
    WHERE status IN ('forecasted', 'in-progress')
    AND category_id = c.id
    AND billing_year = b.budget_year
) AS rough_proposed
FROM categories AS c
LEFT OUTER JOIN budgets AS b ON (c.id = b.category_id)
GROUP BY c.id, b.budget_year;

下一个查询是我第一次尝试将其转换为使用LEFT OUTER JOINs：

SELECT c.id AS category_id, b.budget_year, sum(l1.actual) AS actual, sum(l2.planned) AS planned
FROM categories AS c
LEFT OUTER JOIN budgets AS b ON (c.id = b.category_id)
LEFT OUTER JOIN lines AS l1 ON (l1.category_id = c.id
    AND l1.billing_year = b.budget_year
    AND l1.status = 'complete')
LEFT OUTER JOIN lines AS l2 ON (l2.category_id = c.id
    AND l2.billing_year = b.budget_year
    AND l2.status IN ('forecasted', 'in-progress'))
GROUP BY c.id, b.budget_year;

但是，actual 和 rough_proposed 列比预期的要大很多。我不是 SQL 专家，我很难理解这里发生了什么。有没有一种直接的方法可以将子查询转换为联接？

Answer 1

您的两个查询都有问题。但是，第一个查询隐藏了问题，而第二个查询使其可见。

这是怎么回事：您加入 lines 两次 - 一次以 l1 身份加入，一次以 l2 身份加入。当同时存在实际行和预测/进行中行时，分组前的查询将多次具有同一行。发生这种情况时，每行都会被计算多次，从而导致值膨胀。

第一个查询隐藏了这一点，因为它没有将聚合应用到 actual 和 rough_proposed 列。 SQLite 为每个组选择第一个条目，它具有正确的值。

您可以通过仅加入行一次并有条件地计算金额来修复您的查询，如下所示：

SELECT
    c.id AS category_id
,   b.budget_year
,   SUM(CASE WHEN l.status = 'complete' THEN l.actual END) AS actual
,   SUM(CASE WHEN l.status IN ('forecasted', 'in-progress') THEN l.planned END) AS planned
FROM categories AS c
LEFT OUTER JOIN budgets AS b ON (c.id = b.category_id)
LEFT OUTER JOIN lines AS l ON (l.category_id = c.id AND l1.billing_year = b.budget_year)
GROUP BY c.id, b.budget_year

在这个新查询中，lines 中的每一行只被引入一次；将其计入 actual/planned 列之一的决定是在嵌入在 SUM 聚合函数中的条件表达式中做出的。