Question

我有桌子学生和相遇。我想将此WHERE转换为LEFT JOIN。这可能吗？

WHERE stu.student_id NOT IN (
SELECT student_id
FROM encounter
WHERE 
AND deletion_date IS NULL
AND '2013-12-04'::date BETWEEN start_date AND Coalesce(end_date, '2013-12-04'::date)
)

Answer 1

SELECT
  ...
FROM
  student stu
  LEFT JOIN encounter enc ON enc.student_id = stu.student_id
    AND enc.deletion_date IS NULL
    AND '2013-12-04'::date BETWEEN start_date AND COALESCE(end_date, '2013-12-04'::date)
WHERE
  enc.student_id IS NULL

我个人会使用NOT EXISTS子查询

WHERE 
  NOT EXISTS (
    SELECT 1 
    FROM   encounter 
    WHERE  student_id = stu.student_id
           AND deletion_date IS NULL
           AND '2013-12-04'::date > start_date
           AND (end_date IS NULL OR '2013-12-04'::date < end_date)
  )

以及与(student_id, start_date, end_date, deletion_date)相遇的索引。

这样，子查询将相当于每个student记录的一个索引查找操作，这应该相当快。如果它比替代品快，你必须自己做基准。

Answer 2

这可以帮到你：

SELECT <columns you want to view> FROM student s
LEFT JOIN encounter e ON s.student_id != e.student_id
WHERE <provide your filtration here>

将WHERE条件转换为LEFT JOIN

2 个答案: