MySQL 查询使用表中的行子集计算比率

Question

I have a simple table SeatTracking that tracks the number of seats used in a given room. The table has three columns: roomID, type, value.

Here's an example of the data:

roomID	type	value
1001	occupied	20
1001	vacant	10
1002	occupied	5
1002	vacant	95

I know how to write a query returning the total number of seats in each room.

SELECT roomID,SUM(value) AS total_seats FROM SeatTracking GROUP BY roomID;

I also know how to write a query returning the total number of vacant seats in each room.

SELECT roomID,SUM(CASE type WHEN 'vacant' THEN value END) AS vacant_seats FROM SeatTracking GROUP BY roomID;

Here's the part I'm struggling with - I want to return a vacancy rate for each room. Can someone please point me in the right direction here? Thank you!

I tried the below but I get an error - "you have an error in your SQL syntax:

SELECT roomID, (SUM(CASE type WHEN 'vacant' THEN value END)/SUM(CASE type WHEN 'occupied' THEN value END))*100 AS vacancy_rate 
FROM SeatTracking GROUP BY roomID;

I also tried being more explicit, but I get an error unknown column vacant_seats in field list:

SELECT roomID, SUM(CASE type WHEN 'vacant' THEN value END) AS vacant_seats, 
SUM(CASE type WHEN 'occupied' THEN value END) AS occupied_seats,
(vacant_seats/occupied_seats)*100 AS vacancy_rate 
FROM SeatTracking GROUP BY roomID;

Answer 1

SELECT 
    (SUM
        (
            CASE type
            WHEN 'vacant' THEN value
            END
        ) / SUM(
            CASE type
            WHEN 'occupied' THEN value
            END
        ) * 100
    ) AS vacancy_rate
FROM
    SeatTracking;

Simplest query that will work. Query output here