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;

	}

原文地址：https://www.cnblogs.com/Athelens/p/12111012.html