如何优化包含子查询的复杂SQL语句？

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如何优化复杂sql语句

原sql语句如下：

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

优化方法

1. 添加索引

为表air_conditioner_equipment的字段station_id添加索引，可以加快根据该字段进行查询的速度。

1. 使用子查询优化

优化后sql语句如下：

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;

优化后的sql语句使用子查询ace来预先计算所需的聚合值，然后再使用left join将其与主表连接。这样可以减少需要进行的子查询数量，从而提高查询效率。

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