JDBC MySQL 多表关联查询查询

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public static void main(String[] args) throws Exception{        Class.forName("com.mysql.jdbc.Driver");        Connection conn = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mydb","root","");        String sql ="select * from info";　　　　　//关联查询时,可以直接join on.但是效率不高　　　　　//String sql ="select info.Code,info.Name,info.Sex,nation.Name sb ,info.Birthday from info join nation on info.Nation=nation.Code "; 　　　　　         Statement state = conn.createStatement();        ResultSet rs =  state.executeQuery(sql);        while(rs.next()){//判断是否还有下一行　　　　　　　　　　            System.out.print(rs.getString(1)+"\t");            System.out.print(rs.getString(2)+"\t");            System.out.print(rs.getBoolean(3)?"男\t":"女\t"); //?:简单判断            System.out.print(minzu(rs.getString(4))+"\t");            System.out.println(bianhuan(rs.getDate(5)));        }        conn.close();    }    //关联查询时,也能写个方法再查一遍另一个表，然后赋给原来的列    private static String minzu(String m)throws Exception {        String mz= "";//定义空字符串        Class.forName("com.mysql.jdbc.Driver");        Connection conn = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mydb","root","");        Statement sta = conn.createStatement();        String sql = "select * from nation where code = '"+m+"'";        ResultSet rs = sta.executeQuery(sql);        if(rs.next() == true){　//有对应的sql语句的时候，才执行            mz = rs.getString(2);//另一个表的的列赋值给mz        }        conn.close();        return mz;   //返回mz    }    //日期时间转换    public static String bianhuan(Date d){        SimpleDateFormat f = new SimpleDateFormat("yyyy年mm月dd日");        return  f.format(d);    }

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