MYSQL-内部联接3个表

Question

1. rule_of_exam表字段是id_rule，title_exam，id_class，id_subject，date_posting，id_teacher，信息，状态，时间
2. score_multichoice表字段是id_score，id_rule，true，false，空，score，id_student
3. score_essay表字段是id_scoressay，id_rule，id_student，score_essay

我加入了他们的查询

"Select * 
from rule_of_exam 
INNER JOIN score_multichoice ON rule_of_exam.id_rule = score_multichoice.id_rule 
WHERE id_student='$id_student'"

当我加入2张桌子时，它起作用了，但是当我加入3张桌子时，它没有效果。

"Select * from rule_of_exam 
INNER JOIN score_multichoice ON rule_of_exam.id_rule = score_multichoice.id_rule
INNER JOIN score_essay ON rule_of_exam.id_rule = score_essay.id_rule
WHERE id_student='$id_student'"

$id_student来自会话学生。

我期望输出

title_exam | score_essay | score

但它显示

警告：为foreach（）提供了无效的参数

Answer 1

问题是id_student出现在多个表中： score_multichoice和score_essay 。 因此查询不知道选择哪个。 像这样更改查询：

SELECT DISTINCT * FROM rule_of_exam 
INNER JOIN score_multichoice ON rule_of_exam.id_rule = score_multichoice.id_rule
INNER JOIN score_essay ON rule_of_exam.id_rule = score_essay.id_rule
WHERE score_essay.id_student = '$id_student'"

换句话说：告诉MySQL要使用哪个表，它可以工作。 确保您的PHP代码可以处理零个结果行。

如果在两个表中都没有id_student可能会更好。 始终规范化数据库 。