WHERE clause with inner join

Question

I have the following query to count the amount of devices in my database. I try to add a WHERE clause to only select the device and count with the name "iphone".

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 

I tried to construct different queries, but they all result in a mysql error. What is the correct position of the WHERE clause?

Answer 1

WHERE 后谈到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

By the way, you can rewrite this query as:

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

This removes the extra table scan for calculating the total and the join (which is really cheap because there is only one row in x .

Answer 4

Where goes before group by and fater from and join:

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