mysql 分组统计总数 没有的默认为0

a表：
id room_id
1 room1
2 room2
3 room3
b表
id client_name room_name
1 xiaowang room1
2 xiangzhang room1
3 xiangli room2

统计这三个房间的人数，没有的默认为0

SELECT
a.room_id,
IFNULL(t1.count1, 0)
FROM
a
LEFT JOIN
(SELECT
room_name,
COUNT(*) AS count1
FROM
b
GROUP BY room_name) t1
ON a.`room_id` = t1.room_name

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