Question

我有以下查询来计算数据库中的设备数量。我尝试添加WHERE子句以仅选择设备并使用名称“iphone”进行计数。

SELECT device, count( * ) AS count, sum( 100 ) / total AS percentage
FROM devices WHERE device='apple'
CROSS JOIN (
SELECT count( * ) AS total
FROM devices
)x
GROUP BY 1

我尝试构造不同的查询，但它们都会导致mysql错误。 WHERE子句的正确位置是什么？

Answer 1

WHERE在 FROM之后和GROUP BY之前

Answer 2

SELECT device, count( * ) AS count, sum( 100 ) / total AS percentage
FROM devices 
INNER JOIN (
SELECT count( * ) AS total
FROM devices
)x
WHERE device='apple'
GROUP BY 1

Answer 3

顺便说一句，您可以将此查询重写为：

SELECT 'apple' as device, sum(device = 'apple') AS count,
       100 * sum(device = 'apple') / count(*) AS percentage
FROM devices ;

这会删除额外的表扫描以计算总数和join（这非常便宜，因为x中只有一行。

Answer 4

分组之前和之后的地方并加入：

SELECT device, count( * ) AS count, sum( 100 ) / total AS percentage
FROM devices 
CROSS JOIN (
SELECT count( * ) AS total
FROM devices
)x
WHERE device='apple'
GROUP BY 1

带内连接的WHERE子句

4 个答案: