我尝试从该SQL查询中编写等效的Django查询代码,但我遇到了麻烦。欢迎任何帮助。我收到了一场比赛id
,从这个比赛中我想做一些统计:nb_race
=给出比赛之前一匹马的比赛数量,best_chrono
=给出比赛之前一匹马的最佳时间。
SELECT *, (SELECT count(run.id)
FROM runner run
INNER JOIN race
ON run.race_id = race.id
WHERE run.horse_id = r.horse_id
AND race.datetime_start < rc.datetime_start
) AS nb_race,
(SELECT min(run.chrono)
FROM runner run
INNER JOIN race
ON run.race_id = race.id
WHERE run.horse_id = r.horse_id
AND race.datetime_start < rc.datetime_start
) AS best_time
FROM runner r, race rc
WHERE r.race_id = rc.id
AND rc.id = 7890
Django模型:
class Horse(models.Model):
id = AutoField(primary_key=True)
name = models.CharField(max_length=255, blank=True, null=True, default=None)
class Race(models.Model):
id = AutoField(primary_key=True)
datetime_start = models.DateTimeField(blank=True, null=True, default=None)
name = models.CharField(max_length=255, blank=True, null=True, default=None)
class Runner(models.Model):
id = AutoField(primary_key=True)
horse = models.ForeignKey(Horse, on_delete=models.PROTECT)
race = models.ForeignKey(Race, on_delete=models.PROTECT)
chrono = models.DecimalField(max_digits=10, decimal_places=2, blank=True, null=True, default=None)
子查询表达式可用于将其他查询集编译为依赖于主查询集的子查询,并将它们作为一个SQL一起执行。
from django.db.models import OuterRef, Subquery, Count, Min, F
# prepare a repeated expression about previous runners, but don't execute it yet
prev_run = (
Runner.objects
.filter(
horse=OuterRef('horse'),
race__datetime_start__lt=OuterRef('race__datetime_start'))
.values('horse')
)
queryset = (
Runner.objects
.values('id', 'horse_id', 'race_id', 'chrono', 'race__name', 'race__datetime_start')
.annotate(
nb_race=Subquery(prev_run.annotate(nb_race=Count('id')).values('nb_race')),
best_time=Subquery(prev_run.annotate(best_time=Min('chrono')).values('best_time'))
)
)
链接的文档中介绍了此处使用的一些技巧:
.values(...)
为一个字段:仅汇总值.annotate()
用于子查询(不是.aggregate()
)。这会增加一个GROUP BY race.horse_id
,但不是问题WHERE race.horse_id = ...
,因为在现代数据库后端中,SQL优化器最终也会忽略“ group by”。它被编译为与示例中的SQL等效的查询。检查SQL:
>>> print(str(queryset.query))
SELECT ...,
(SELECT COUNT(U0.id)
FROM runner U0 INNER JOIN race U1 ON (U0.race_id = U1.id)
WHERE (U0.horse_id = runner.horse_id AND U1.datetime_start < race.datetime_start)
GROUP BY U0.horse_id
) AS nb_race,
...
FROM runner INNER JOIN race ON (runner.race_id = race.id)
边际差异是子查询使用一些内部别名,例如U0和U1。
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