前文
以为很早就搞完了，但老王说还没完，需求没有满足——查询学生信息的时候，惩罚信息、奖励信息、学籍变更信息没有显示！为了解决这个问题，也费尽了周折。但最后还是解决了，这里码一下，做个纪念。
正文
student表：studentID、class、sex、name、department、native_place、birthday；
class表：code、name、monitor
department表：code、name
reward表：ID、levels、studentID、rec_time、description
reward_levels：code、description
punishment：ID、levels、studentID、rec_time、description
punish_leves: code、description
student
刚开始学生信息的显示只是将上面图片的class、department的编号用名称（name）表示，建立三表查询。
String sql = "select student.studentID ,student.name,sex,class.name,department.name,birthday,native_place from student ,class,department where student.class = class.Id AND student.department = department.Id";
后来需要加上其他信息（如上），就遇到了几个问题：
其他表可能没有存在相应的记录，所以查询失败，以至于学生信息都没查询到。
多条记录（如奖励多条）会显示多条学生记录。
走了一段弯路之后，回过头来，想到既然多个表一起查询会失败，要不就将学生信息查询和其它表分开来查询，然后根据学号再到其他表中去找，有记录就显示，没记录就显示无。
于是整个查询代码如下：
String sql = "select student.studentID ,student.name,sex,class.name,department.name,birthday,native_place from student ,class,department where student.class = class.Id AND student.department = department.Id";
dbProcess.connect();
ResultSet rs = dbProcess.executeQuery(sql);
studentvector.clear();
try { while(rs.next()) {
Vector<String> stu = new Vector<String>();
stu.add(rs.getString(1));
stu.add(rs.getString(2));
stu.add(rs.getString(3));
stu.add(rs.getString(4));
stu.add(rs.getString(5));
stu.add(rs.getString(6));
stu.add(rs.getString(7)); String sqlforpunish = "select punish_levels.description from punishment,punish_levels where punish_levels.code = punishment.levels and punishment.studentID = "+ "'" + rs.getString(1)+"'"; String sqlforreward = "select reward_levels.description from reward,reward_levels where reward.levels = reward_levels.code and reward.studentID = "+ "'" + rs.getString(1)+"'"; String sqlchange = "select change_code.description from change_code ,changement where changement.change = change_code.code and changement.studentID = "+"'" + rs.getString(1)+"'";
ResultSet rsforpunish = dbProcess.executeQuery(sqlforpunish);
ResultSet rsforreward = dbProcess.executeQuery(sqlforreward);
ResultSet rsforchange = dbProcess.executeQuery(sqlchange); String punishstrs = " "; while(rsforpunish.next()) {
punishstrs+= rsforpunish.getString(1) + " ";
} String rewardstrs = " "; while(rsforreward.next()) {
rewardstrs+=rsforreward.getString(1)+ " ";
} String changestrs = " "; while(rsforchange.next()) {
changestrs+=rsforchange.getString(1)+" ";
}
punishstrs=punishstrs.equals(" ")?"无":punishstrs;
rewardstrs=rewardstrs.equals(" ")?"无":rewardstrs;
changestrs=changestrs.equals(" ")?"无":changestrs;
stu.add(punishstrs);
stu.add(rewardstrs);
stu.add(changestrs);
studentvector.add(stu);最后的一点小处理，真的不能再爱了！！！
结果
作者：蒋子默
链接：https://www.jianshu.com/p/39985614f22f
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