【mysql】复杂语句的分析过程

在项目中看到前辈写的sql，尝试去理解
公司项目需要保密 就不透露表的结构和数据了，大家看思路就好

select  ci.car_number,
            ci.brand_model,
            ci.authority_oilwear,stat.staticMonth,ifnull(stat.total_mileage,0) as totalMileage, ifnull(stat.oilMass,0) as oilMass,
            ifnull(round(stat.oilMass/stat.total_mileage*100,2),0) as avgOilMass
            from t_dmv_car_info ci
        join
        ( select mT.staticMonth,curt.total_mileage,ft.oilMass from (
				SELECT 1 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 2 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 3 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 4 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 5 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 6 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 7 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 8 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 9 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 10 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 11 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 12 AS staticMonth FROM DUAL ) mT
            left join
            (
                SELECT f.car_id, DATE_FORMAT( f.business_date, '%m' ) as staticMonth, sum( fd.oil_mass ) AS oilMass
                FROM t_dmv_fee f
                    LEFT JOIN t_dmv_fee_detail fd ON fd.fee_id = f.id AND fd.fee_type = 1
                WHERE f.fee_category = 3  AND DATE_FORMAT( f.business_date, '%Y' ) ='2022' and f.car_id = ''
                group by DATE_FORMAT( f.business_date, '%m' )
            ) ft on ft.staticMonth = mT.staticMonth
            left join
            (  SELECT cur.car_id,DATE_FORMAT( cur.begin_time, '%m' ) as staticMonth,sum( cur.total_mileage ) AS total_mileage
                    FROM t_dmv_car_use_record cur
                    WHERE DATE_FORMAT( cur.begin_time, '%Y' ) = '2022' and cur.car_id = ''
                    GROUP BY DATE_FORMAT( cur.begin_time, '%m' )
                ) curt on curt.staticMonth = mT.staticMonth
        ) stat where ci.id = ''
        order by stat.staticMonth

乍一看很吓人，确实很吓人，但是慢慢看还是可以捋清楚思路的

第一步：明确需要的字段分别来做哪些表

查询某一年中每个月的 耗油量，行驶公里数，以及一些基础字段
最后字段：车牌号、官方油耗、品牌车型、月份、出车公里数、加油量、平均油耗

其中：
车辆信息表 car_info 车牌号、官方油耗、品牌车型
car_number\authority_oilwear\brand_model\

车辆使用记录表 car_use_record 出车公里数
total_mileage,

费用详情表 fee_detail 加油量
oil_mass

平均油耗= 加油量/ 出车公里数

第二步：从每个表中拿到对应的数据

由于是与月份相关的，所以从每个表中拿到数据后 构建成新表

1.补充知识点：构建一年中各个月份表

可能有更好的方法

SELECT 1 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 2 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 3 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 4 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 5 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 6 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 7 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 8 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 9 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 10 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 11 AS staticMonth FROM DUAL
		UNION ALL
		SELECT 12 AS staticMonth FROM DUAL

上方sql获得月份表，临时表名为 mT

2.拿到费用表中的数据 作为新表ft

费用表和费用详情表join
根据年份和车辆id，查找车辆id，月份，月份总耗油量
结果根据月份进行分组 结果为新表 ft

SELECT f.car_id,
 DATE_FORMAT( f.business_date, '%m' ) as staticMonth, 
 sum( fd.oil_mass ) AS oilMass
FROM t_dmv_fee f
LEFT JOIN t_dmv_fee_detail fd ON fd.fee_id = f.id AND fd.fee_type = 1
WHERE f.fee_category = 3  AND DATE_FORMAT( f.business_date, '%Y' ) ='2022' and f.car_id = ''
group by DATE_FORMAT( f.business_date, '%m' )

3.拿到车辆使用记录表中的数据 作为新表curt

车辆使用表中 根据年份和车辆id 根据开始时间月份分组 查询 车辆id，开始时间的月份，
统计月份总公里数 结果作为新表 curt

SELECT cur.car_id,
DATE_FORMAT( cur.begin_time, '%m' ) as staticMonth,
sum( cur.total_mileage ) AS total_mileage
FROM t_dmv_car_use_record cur
WHERE DATE_FORMAT( cur.begin_time, '%Y' ) = '2022' and cur.car_id = ''
GROUP BY DATE_FORMAT( cur.begin_time, '%m' )

4.以上三个表 根据staticMonth 字段left join

查询结果作为新表 stat

select mT.staticMonth,curt.total_mileage,ft.oilMass from mt
left join ft on ft.staticMonth = mT.staticMonth
left join curt on curt.staticMonth = mT.staticMonth

5.关联stat和car_info表 获取最后的结果

select  ci.car_number,
            ci.brand_model,
            ci.authority_oilwear,stat.staticMonth,
            ifnull(stat.total_mileage,0) as totalMileage, ifnull(stat.oilMass,0) as oilMass,
            ifnull(round(stat.oilMass/stat.total_mileage*100,2),0) as avgOilMass
            from t_dmv_car_info ci
        join stat
where ci.id = ''

总结

根据sql去捋清楚思路不难，但是要我自己写还是很困难的，多看多练才行
select sql的本质就是查出满足条件的数据，所有要先明确需要哪些数据，从哪个表中找，需要与什么关联。