sql多重查询的嵌套语句

前提：

假设现在有一个提交表单，里面有n个查询条件，用户可以填写几个或者不填写来提交。

思想“

判断每个条件用户输入是否为空，不为空则使用sql语句拼接

这是期末项目中用来查询图片的函数的代码：

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;
		
	}