Question

我有2个表（学生类和用户）

表studentclass

.method private hidebysig static char  BitwiseOr(char c1,
                                                   char c2,
                                                   char c3,
                                                   char c4) cil managed
  {
    // 
    .maxstack  2
    .locals init (char V_0)
    IL_0000:  nop
    IL_0001:  ldarg.0
    IL_0002:  ldarg.1
    IL_0003:  or
    IL_0004:  ldarg.2
    IL_0005:  or
    IL_0006:  ldarg.3
    IL_0007:  or
    IL_0008:  call       char [mscorlib]System.Convert::ToChar(int32)
    IL_000d:  stloc.0
    IL_000e:  br.s       IL_0010

    IL_0010:  ldloc.0
    IL_0011:  ret
  } // end of method Program::BitwiseOr

表用户

columns (id, class_name, user_schoolid_faculty, user_schoolid_student)

我的第一个问题是

columns (id, firstname, lastname, schoolid, position)

user_schoolid_faculty is a foreign key from schoolid

user_schoolid_student is a foreign key from schoolid

我想使用schoolid查看用户的名字和姓氏的数据

 SELECT * FROM studentclass WHERE class_name = '$class_name'

它会引发我的错误，而不是唯一的表/别名＆＃39;用户＆＃39;

Answer 1

对于同一个表user，您应该使用不同的别名，当您使用JOIN时，您应该使用ON来合并两个表，而不是WHERE，试试这个;）

SELECT studentclass.class_name, user1.firstname, user2.lastname
FROM studentclass
INNER JOIN user user1
ON studentclass.user_schoolid_faculty = user1.schoolid
AND user1.position = 'Faculty'
LEFT JOIN user user2
ON studentclass.user_schoolid_student = user2.schoolid
AND user2.position = 'Student'

还有一个不清楚的地方，为什么在此查询中使用一个INNER JOIN和一个LEFT JOIN？

Answer 2

您必须为表使用别名：

SELECT studentclass.class_name, u1.firstname, u1.lastname
FROM studentclass
 INNER JOIN user u1
 ON studentclass.user_schoolid_faculty = u1.schoolid
 WHERE user.position = 'Faculty'
 LEFT JOIN user u2
 ON studentclass.user_schoolid_student = u2.schoolid
 WHERE u1.position = 'Student'

Answer 3

对于SQL Server，使用方括号作为保留关键字user 还要为表添加别名。

SELECT SC.class_name, U1.firstname, U2.lastname
FROM studentclass SC
INNER JOIN [user] U1 ON SC.user_schoolid_faculty = U1.schoolid
LEFT JOIN [user] U2  ON SC.user_schoolid_student = U2.schoolid AND U2.position = 'Student'
WHERE U1.position = 'Faculty'

对于MySQL：

SELECT SC.class_name, U1.firstname, U2.lastname
FROM studentclass SC
INNER JOIN `user` U1 ON SC.user_schoolid_faculty = U1.schoolid
LEFT JOIN `user` U2  ON SC.user_schoolid_student = U2.schoolid AND U2.position = 'Student'
WHERE U1.position = 'Faculty'

MySql - 连接2个表，其中第二个表连接两次

3 个答案: