mysql - 具有当前月份条件的空条件

Question

我正在尝试运行一个 mysql 选择查询，我想在其中获取当月未支付费用的学生。如果我一个一个地使用它们，请检查我的情况，但它们同时不起作用

"SELECT A.studentid,A.stu_first_name,A.stu_last_name,
 A.stu_reg_no,A.phone,
 B.submit_on,
 C.class_name FROM students as A 
 LEFT JOIN student_fee as B on A.studentid = B.student_id 
 inner join class as C on 
 A.class_id=C.classid 
 WHERE MONTH(B.submit_on) = MONTH(NOW()) 
 AND B.student_id IS NULL"

Answer 1

您需要将 B.submit_on 的条件从 WHERE 子句移动到表 B< 上的 LEFT JOIN/。否则， B 中没有匹配项的记录将被 WHERE 子句过滤掉，我认为这与您想要的相反(这实际上将您的 LEFT JOIN 到 INNER JOIN)。

SELECT 
    A.studentid,
    A.stu_first_name,
    A.stu_last_name,
    A.stu_reg_no,
    A.phone,
    B.submit_on,
    C.class_name
FROM 
    students as A 
    LEFT JOIN student_fee as B on A.studentid = B.student_id AND MONTH(B.submit_on) = MONTH(NOW()) 
    INNER JOIN class as C ON A.class_id = C.classid 
WHERE B.student_id IS NULL"