Question

我尝试使用连接查询从不同的表中获取数据。但即使有数据，它也没有返回结果。

学生主表包含有关学生的详细信息。学生的卷号是此表中的主键。卷号将添加到所有结果表中（即first_term_results，second_term_results等）以供参考。

因此，为了获取学生的详细信息，他的第一和第二学期结果，我使用以下查询，但没有给出预期的结果。我正在尝试获取与roll_no匹配的详细信息。

SELECT a.*, b .*,c.*
FROM student_master AS a
INNER JOIN first_term_results AS b
INNER JOIN second_term_results AS c
ON a.roll_no = b.roll_no = c.roll_no
WHERE a.roll_no = '53'
AND b.roll_no='53'
AND c.roll_no = '53'

有人可以帮我纠正这个问题以获得我期待的结果吗？。如果我不清楚，也请告诉我。我可以解释。

提前致谢。

Answer 1

将您的查询修改为

SELECT a.*, b.*,c.*
FROM student_master AS a
INNER JOIN first_term_results AS b ON a.roll_no = b.roll_no
INNER JOIN second_term_results AS c ON a.roll_no = c.roll_no
WHERE a.roll_no = '53'

Answer 2

这应符合您的意愿：

SELECT a.*, b .*,c.*
FROM student_master AS a
LEFT JOIN first_term_results AS b ON(a.roll_no = b.roll_no)
LEFT JOIN second_term_results AS c ON( a.roll_no = c.roll_no)
WHERE a.roll_no = '53'

您是否检查过驱动程序是否返回错误？

Answer 3

在将查询格式化为可读的内容之后我得到了这个

SELECT a.*, b .*,c.* 
FROM   student_master AS a 
  INNER JOIN first_term_results AS b 
  INNER JOIN second_term_results AS c ON a.roll_no = b.roll_no = c.roll_no 
WHERE   a.roll_no = '53' 
AND     b.roll_no='53'
AND     c.roll_no = '53'

对我来说有点奇怪，我会选择这样的东西

SELECT a.*, b .*,c.* 
FROM   student_master AS a 
  INNER JOIN first_term_results AS b on a.roll_no = b.roll_no 
  INNER JOIN second_term_results AS c ON a.roll_no = c.roll_no 
WHERE   a.roll_no = '53'

MySQL-通过匹配引用从多个表中获取数据

3 个答案: