使用where子句左连接查询

Question

我有3个表: Group,Person和Workout.Group是一个外键Person,Person是一个外键Workout.我想选择某个组中所有人的列表,如果在该人的给定日期之间进行了锻炼,那么也获得持续时间,如果没有,则将其设为空.

不太确定如何编写SQL,我陷入了这样的困境:

SELECT Person.id, Person.firstName, Workout.duration 
FROM Person LEFT OUTER JOIN Workout 
ON Person.id = Workout.person_id 
WHERE Workout.start_date BETWEEN %s AND %s 
AND Person.group_id = %s

有任何想法吗？

Answer 1

你的where条款是撤消left join.当没有匹配时,start_date为NULL,这不能进行比较.

将条件移动到on子句修复了这个问题:

SELECT Person.id, Person.firstName, Workout.duration 
FROM Person LEFT OUTER JOIN Workout 
     ON Person.id = Workout.person_id and Workout.start_date BETWEEN %s AND %s 
where Person.group_id = %s