如何優化查詢以提高性能？
以下是一條查詢語句：
select es.id, es.station_name, es.province, es.city, es.district, es.detailed, ( select count(*) from air_conditioner_equipment where station_id = es.id ) as air_conditioning_count, ( select sum( cold_power ) from air_conditioner_equipment where station_id = es.id) as cold_power_total, ( select sum( hot_power) from air_conditioner_equipment where station_id = es.id) as hot_power_total, es.create_time, es.state from energy_station es left join user_station us on us.station_id = es.id left join user_info u on u.id = us.user_id
針對此查詢，可以通過以下方法進行優化：
添加索引
查詢中頻繁連接的字段station_id未添加索引，這會導致查詢性能低下。為station_id字段添加索引可以顯著提高查詢速度。
使用子查詢優化
查詢中包含多個子查詢，這會導致查詢執行效率低下?？梢允褂眠B接子查詢的方式來優化查詢：
SELECT es.id, es.station_name, es.province, es.city, es.district, es.detailed, SUM(ace.air_conditioning_count) AS air_conditioning_count, SUM(ace.cold_power) AS cold_power_total, SUM(ace.hot_power) AS hot_power_total, es.create_time, es.state FROM energy_station es LEFT JOIN user_station us ON us.station_id = es.id LEFT JOIN user_info u ON u.id = us.user_id LEFT JOIN ( SELECT station_id, COUNT(*) as air_conditioning_count, SUM(cold_power) as cold_power, SUM(hot_power) as hot_power FROM air_conditioner_equipment GROUP BY station_id ) ace ON ace.station_id = es.id GROUP BY es.id;
通過連接子查詢，可以將多個子查詢合并為一個，從而提高查詢效率。
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