sql多重查詢的嵌套語句

在處理提交表單時，用戶可能有一個、多個、或者沒有輸入，可通過sql語句的嵌套來查詢圖片

public List<Image> watchImageDao(Image img){
        Connection conn=null;
        PreparedStatement ps=null;
        ResultSet rs=null;
        List<Image> li=null;
        int index=-1;
        int flag=0;
        int count=0;
        String table_name="t_image";
        String sql=table_name;
        String[] paramater= {img.getCountry(),img.getName(),img.getPosition(),
                img.getResolution(),img.getLongitude(),img.getLatitude(),
                img.getAcquisition_time(),img.getScale()
                };
        boolean[] empty=new boolean[8];
        for(int i=0;i<8;i++)
            empty[i]=true;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/happy","root", "root");
            
            if(img.getCountry()!="") {
                sql="select* from "+table_name+" where country=?";
                empty[0]=false;
                flag=1;
                count++;
            }
            if(img.getName()!="") {
                if(flag==0) 
                 sql="select* from "+table_name +" where name=?";
                else
                    sql="select* from" + "(" + sql + ")subset_name" + " where name=?";
                empty[1]=false;
                flag=1;
                count++;
            }
            if(img.getPosition()!="") {    
                if(flag==0) 
                     sql="select* from "+table_name +" where position=?";
                else
                    sql="select* from" +"(" + sql + ")subset_position" + " where position=?";
                empty[2]=false;
                flag=1;
                count++;
            }
            if(img.getResolution()!="") {
                if(flag==0) 
                    sql="select* from "+table_name +" where resolution=?";
                else
                    sql="select* from" +"(" + sql + ")subset_resolution" + " where resolution=?";
                empty[3]=false;
                flag=1;
                count++;
            }
            if(img.getLongitude()!="") {
                if(flag==0) 
                    sql="select* from "+table_name +" where longitude=?";
                else
                    sql="select* from" +"(" + sql + ")subset_longitude" + " where longitude=?";
                empty[4]=false;
                flag=1;
                count++;
            }
            if(img.getLatitude()!="") {
                if(flag==0) 
                    sql="select* from "+table_name +" where latitude=?";
                else
                    sql="select* from" +"(" + sql + ")subset_latitude" + " where latitude=?";
                empty[5]=false;
                flag=1;
                count++;
            }            
            if(img.getAcquisition_time()!="") {
                if(flag==0) 
                    sql="select* from "+table_name +" where acquisition_time=?";
                else
                    sql="select* from" +"(" + sql + ")subset_acquisition_time" + " where acquisition_time=?";
                empty[6]=false;
                flag=1;
                count++;
            }    
            if(img.getScale()!="") {
                if(flag==0) 
                    sql="select* from "+table_name +" where scale=?";
                else
                    sql="select* from" +"(" + sql + ")subset_scale" + " where scale=?";
                empty[7]=false;
                flag=1;
                count++;
            }    
            if(count==0) {
                sql="SELECT* FROM "+table_name;
            }
            ps=conn.prepareStatement(sql);
            for(int i=0;i<count;i++) {
                for(int j=0;j<8;j++) {
                    if(!empty[j]) {
                        ps.setString(i+1, paramater[j]);
                        empty[j]=true;
                        break;
                    }
                }
            }
    
            rs=ps.executeQuery();
            li=new ArrayList<>();
            while(rs.next()) {
                Image i=new Image();
                i.setCountry(rs.getString("country"));
                i.setName(rs.getString("name"));
                i.setPosition(rs.getString("position"));
                i.setResolution(rs.getString("resolution"));
                i.setLongitude(rs.getString("longitude"));
                i.setLatitude(rs.getString("latitude"));
                i.setAcquisition_time(rs.getString("acquisition_time"));
                i.setScale(rs.getString("scale"));
                li.add(i);
            }
            } catch (Exception e) {
                e.printStackTrace();
            }finally{
                try {
                    rs.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
                try {
                    ps.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
                try {
                    conn.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
         
        return li;
        
    }