mysql - SQL 左内连接 (mysql)

Question

我一直在使用以下查询:

我正在使用两个表:(提到了其他一些但这个问题不需要)

assessment_criteria
+-------------------+--------------+------+-----+---------+----------------+
| Field             | Type         | Null | Key | Default | Extra          |
+-------------------+--------------+------+-----+---------+----------------+
| id                | mediumint(9) | NO   | PRI | NULL    | auto_increment |
| scheme_of_work_id | mediumint(9) | NO   |     | NULL    |                |
| level             | char(255)    | YES  |     | NULL    |                |
| criteria          | char(255)    | NO   |     | NULL    |                |
+-------------------+--------------+------+-----+---------+----------------+

criteria_completed
+------------------------+--------------+------+-----+---------+----------------+
| Field                  | Type         | Null | Key | Default | Extra          |
+------------------------+--------------+------+-----+---------+----------------+
| id                     | mediumint(9) | NO   | PRI | NULL    | auto_increment |
| student_ID             | mediumint(9) | NO   |     | NULL    |                |
| assessment_criteria_id | mediumint(9) | NO   |     | NULL    |                |
| date_marked            | date         | NO   |     | NULL    |                |
| notes                  | varchar(255) | YES  |     | NULL    |                |
| attainment             | varchar(15)  | YES  |     | NULL    |                |
| effort                 | varchar(15)  | YES  |     | NULL    |                |
| marked_by              | varchar(20)  | NO   |     | NULL    |                |
+------------------------+--------------+------+-----+---------+----------------+

我正在使用这样的查询来显示学生尚未完成的评估标准列表:

SELECT DISTINCT assessment_criteria.id, assessment_criteria.level, assessment_criteria.criteria FROM assessment_criteria, criteria_completed  
WHERE (assessment_criteria.scheme_of_work_id = '17') 
AND (assessment_criteria.id NOT IN (SELECT criteria_completed.assessment_criteria_id FROM criteria_completed WHERE (student_ID = '403'))) 
ORDER BY level;

此查询的运行速度变得异常缓慢，我一直在尝试使用 LEFT JOIN 使其更快。

SELECT DISTINCT a.id, a.level, a.criteria 
FROM assessment_criteria a
LEFT JOIN criteria_completed b
ON a.id = b.assessment_criteria_id
WHERE b.assessment_criteria_id IS NULL

但是当我尝试为项目和学生添加子句时，我没有成功；即。

SELECT DISTINCT a.id, a.level, a.criteria 
FROM assessment_criteria a
LEFT JOIN criteria_completed b
ON a.id = b.assessment_criteria_id
WHERE b.assessment_criteria_id IS NULL
AND (b.student_ID = '403')
AND (a.scheme_of_work_id = '17');

mysql 报告“空集”。我怀疑我没有正确引用这些外键？

Answer 1

(只是为了确认，您正在使用 b.assessment_criteria_id IS NULL 来检测失败的连接)

将表 b 上的过滤器应用于 WHERE 子句将过滤掉连接失败的任何记录，我认为这是问题的原因。

您可以尝试将 b 过滤器移动到 JOIN 条件中:

SELECT DISTINCT a.id, a.level, a.criteria 
FROM assessment_criteria a
LEFT JOIN criteria_completed b
  ON a.id = b.assessment_criteria_id
  AND (b.student_ID = 403)
WHERE b.assessment_criteria_id IS NULL
  AND (a.scheme_of_work_id = 17);

虽然我个人不喜欢filtering like this in a JOIN .替代方案是:

SELECT DISTINCT a.id, a.level, a.criteria 
FROM assessment_criteria a
LEFT JOIN criteria_completed b
  ON a.id = b.assessment_criteria_id
WHERE (a.scheme_of_work_id = 17)
   AND (b.assessment_criteria_id IS NULL OR b.student_ID = 403);