如何使用计数函数从不同的 table 中减去 2 列？

Question

目前，我正在使用计数函数计算 2 列的值，并想减去 2 列并导出到 excel 文件。

这是我的代码。我认为粗体的表述存在逻辑错误

SELECT software.*, 
(SELECT count(lkey.k_id) FROM lkey WHERE lkey.s_id = software.s_id) AS Total, 
(SELECT count(assign.k_id) FROM assign WHERE assign.s_id = software.s_id) AS Installed, 

**(SELECT count(lkey.k_id) - count(assign.k_id) FROM lkey INNER JOIN assign 
WHERE lkey.k_id = assign.k_id GROUP BY lkey.k_id) AS Available**
FROM software";

Answer 1

我认为您需要两个 LEFT JOIN 聚合查询：

SELECT s.*, 
    COALESCE(l.total,     0) total, 
    COALESCE(a.installed, 0) installed, 
    COALESCE(l.total, 0) - COALESCE(a.installed, 0) available
FROM software s
LEFT JOIN (SELECT s_id, count(*) total     FROM lkey   GROUP BY s_id) l 
    ON l.s_id = s.s_id
LEFT JOIN (SELECT s_id, count(*) installed FROM assign GROUP BY s_id) a 
    ON a.s_id = s.s_id

在 MySQL 的最新版本中，您可以使用横向连接：

SELECT s.*, l.total, a.installed, l.total - a.installed available
FROM software s
LEFT JOIN LATERAL (SELECT count(*) total     FROM lkey l   WHERE l.s_id = s.s_id) l ON 1
LEFT JOIN LATERAL (SELECT count(*) installed FROM assign a WHERE a.s_id = s.s_id) a ON 1

如何使用计数函数从不同的 table 中减去 2 列？

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