如何优化查询以提高性能？

以下是一条查询语句：

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

针对此查询，可以通过以下方法进行优化：

添加索引

查询中频繁连接的字段station_id未添加索引，这会导致查询性能低下。为station_id字段添加索引可以显着提高查询速度。

使用子查询优化

查询中包含多个子查询，这会导致查询执行效率低下。可以使用连接子查询的方式来优化查询：

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;

通过连接子查询，可以将多个子查询合并为一个，从而提高查询效率。

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