mysql - 如何使用选择 AND

Question

我一直尝试在 SELECT 中使用 AND，但出现错误。

我编写代码的正确方式可能是什么？

代码是:

$sel=mysql_query("SELECT  * from student, subject, studentmark
 where student.username='$name' AND studentmark.student_id='$name' AND studentmark.YEAR='$ya' AND studentmark.Term='FIRST' AND studentmark.Term='SECOND' AND studentmark.Term='THIRD' AND subject.code=studentmark.code AND student.username=studentmark.student_id");

Answer 1

一个简单的规则:永远不要在FROM子句中使用逗号。 始终使用显式的JOIN语法。结果是这样的:

SELECT  *
from studentmark sm join
     student st 
     on sm.student_id = st.username join
     subject join
     on sm.code = su.code     
 where st.username='$name' AND sm.YEAR = '$ya' AND
       sm.Term in ('FIRST', 'SECOND', 'THIRD')

此外，学习使用表别名。它们使查询更容易编写和阅读。