创建计数列查询

Question

我希望创建一个选择查询，该查询将有一列计算某人在某个月份接受培训的次数。我已经设法让它显示在那个特定月份参加培训课程的人的名单，但我也想向其他所有人展示他们参加的课程计数为 0。我该怎么做？

SELECT 
    carer.carer_firstname, carer.carer_lastname,   
    COUNT(carer_training_link.carer_id) AS sessions_attended_May
FROM 
    carer, carer_training_link 
WHERE 
    carer.carer_id = carer_training_link.carer_id 
    AND carer_training_link.training_date BETWEEN TO_DATE('01/MAY/2019', 'DD/MON/YYYY') 
                                              AND TO_DATE('01/JUN/2019', 'DD/MON/YYYY')
GROUP BY 
    carer.carer_firstname, carer.carer_lastname;

Answer 1

您正在使用只能称为古董的连接语法。

您想要一个外部加入，以便包括没有受过培训的护理人员。

日期范围最好与>= 和< 进行比较。

SELECT 
  c.carer_firstname, c.carer_lastname,   
  COUNT(cl.carer_id) AS sessions_attended_may
FROM carer c
LEFT OUTER JOIN carer_training_link cl
  ON cl.carer_id  = c.carer_id
  AND cl.training_date >= DATE '2019-05-01'
  AND cl.training_date <  DATE '2019-06-01'
GROUP BY c.carer_firstname, c.carer_lastname
ORDER BY c.carer_firstname, c.carer_lastname;