帶內連接的WHERE子句

Question

我有以下查詢來計算我的數據庫中的設備數量。 我嘗試添加WHERE子句以僅選擇設備並使用名稱“iphone”進行計數。

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 

我試圖構造不同的查詢，但它們都導致mysql錯誤。 WHERE子句的正確位置是什么？

Answer 1

WHERE 后談到FROM之前GROUP BY

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

順便說一下，您可以將此查詢重寫為：

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

這將刪除額外的表掃描以計算總計和join （這非常便宜，因為x只有一行。

Answer 4

在分組之前和之后去哪里並加入：

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