Question

我有以下查询。我需要一个表中的信息，但是没有将这两个表链接在一起的键，并且实际上没有一种方法可以从任何一个表中引用另一个表。

我的查询

SELECT COUNT(*) 
FROM courses as c 
WHERE c.StartYear = 2016 
AND  c.student_id IN (SELECT DISTINCT p.student_id 
                      FROM programs AS p 
                      WHERE p.StartYear = 2016 
                      AND p.program_code NOT LIKE "M%")

此查询大约需要5秒钟执行时间，太长了。有更好的方法吗？

服务器：MySQL 服务器版本：5.6.35

Answer 1

尝试存在

SELECT COUNT(*) 
  FROM courses as c 
 WHERE c.StartYear = 2016 
   and exists (SELECT 1
                 FROM programs AS p 
                WHERE p.StartYear = 2016 
                  AND p.program_code NOT LIKE "M%" 
                  and c.student_id = p.student_id)

Answer 2

exists应该比In子句更好，因此您可以尝试此查询。 Click here进行相关讨论。

SELECT COUNT(*) 
FROM courses as c 
WHERE c.StartYear = 2016 
AND Exists (SELECT 1
            FROM programs AS p 
            WHERE p.StartYear = 2016 
            AND student_id = c.student_id
            AND p.program_code NOT LIKE "M%")

Answer 3

这应该给出完全相同的结果，并且可能会更快，也可能不会更快，但是确实证明了您的查询确实暗示这两个表可以关联由student_id。

如果以下查询在逻辑上不正确，则示例查询也是如此，并且您应该提供一个虚拟示例来演示所需的逻辑：https://stackoverflow.com/help/mcve

SELECT
  COUNT(*)
FROM
  courses    c
INNER JOIN
(
  SELECT
    student_id,
    startYear
  FROM
    programs
  WHERE
    program_code NOT LIKE "M%"
  GROUP BY
    student_id,
    startYear
)
  p
    ON  p.student_id = c.student_id
    AND p.startYear  = c.startYear
WHERE
  c.StartYear = 2016

慢的SQL IN查询

3 个答案: