java怎么连接数据库executeUpdate()和executeQuery()

Update

//没有返回值
public void update(int count){
conn=DBUtil.getConn();
String sql="update counter set count=?";
try {					
			PreparedStatement ps = conn.prepareStatement(sql);
			//传进去的
			ps.setInt(1,count);
			ps.executeUpdate();		
		} catch (SQLException e) {
			e.printStackTrace();
		}finally{
			DBUtil.closeConn();
		}   
}

Insert

//没有返回值,参数是个字符串部门名称就ok了,因为id的话是自增
	public void insert(String departmentname) {
		conn = ConnectionFactory.getConnection();
		String sql = "insert into department (departmentname) values(?)";
		try {
			PreparedStatement pstmt = conn.prepareStatement(sql);
			pstmt.setString(1, departmentname);
			pstmt.executeUpdate();
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} finally {
			ConnectionFactory.closeConnection();
		}
	}

 //因为employeeid自增,所以不用设置
public void insert(Employee employee){
		  conn=ConnectionFactory.getConnection();
		  String sql="insert into employee"
				  +
					"(employeename,username,password,phone,email,departmentid,status,role)" +
					" values(?,?,?,?,?,?,?,?)";
		  try {		
			PreparedStatement pstmt = conn.prepareStatement(sql);
			pstmt.setString(1,employee.getEmployeename());
			pstmt.setString(2,employee.getUsername());
			pstmt.setString(3,employee.getPassword() );
			pstmt.setString(4,employee.getPhone() );
			pstmt.setString(5,employee.getEmail());
			pstmt.setInt(6,employee.getDepartmentid());			
			//注册成功后，默认为正在审核，status为0
			pstmt.setString(7,"0");
			//注册时，默认为员工角色，role值为2
			pstmt.setString(8,"2");
			pstmt.executeUpdate();	
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}finally{
			ConnectionFactory.closeConnection();
		}	  
	  }

Delete

//删除不用返回值	
public void delete(int departmentid) {
		conn = ConnectionFactory.getConnection();
		String sql = "delete from department where departmentid=?;";
		try {
			PreparedStatement pstmt = conn.prepareStatement(sql);
			pstmt.setInt(1, departmentid);
			pstmt.executeUpdate();
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} finally {
			ConnectionFactory.closeConnection();
		}
	}

select

//返回int类型
public int select(){
	int count=0;
	conn=DBUtil.getConn();
	String sql = "select * from counter";
	try{
		PreparedStatement ps = conn.PreparedStatement(sql);
		ResultSet rs =ps.excuteQuery();
		if(rs.next()){
			count=rs.getInt("visitcount");
		}
	}catch{
 
	}finally{
		DBUtil.closeConn();
	}
	return count;
}

//返回部门集合
	public List selectAll() {
		conn = ConnectionFactory.getConnection();
		// 新建一个集合departmentsList
		List departmentsList = new ArrayList();
		try {
			Statement st = null;
			String sql = "select * from department";
			st = conn.createStatement();
			ResultSet rs = st.executeQuery(sql);
			Department department;
			while (rs.next()) {
				// 新建一个department来接收数据库的信息
				department = new Department();
				department.setDepartmentid(rs.getInt("departmentid"));
				department.setDepartmentname(rs.getString("departmentname"));
				departmentsList.add(department);
			}
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} finally {
			ConnectionFactory.closeConnection();
		}
		// 返回集合
		return departmentsList;
	} 
 
//返回员工
  public List selectAllEmployee(){
			 conn=ConnectionFactory.getConnection();
			 List employeeslist=new ArrayList();
			 Employee employee=null;	
			 try {
				PreparedStatement st=null;
				//只查询已注册且未审批 且 角色是员工的
				String sql="select * from employee where role='2' and status='0'";
		 		st = conn.prepareStatement(sql);
				ResultSet rs =st.executeQuery(sql);
				while(rs.next()){
					employee=new Employee();
					employee.setEmployeeid(rs.getInt("employeeid"));
					employee.setEmployeename(rs.getString("employeename"));
					employee.setUsername(rs.getString("username"));
					employee.setPhone(rs.getString("phone"));
					employee.setEmail(rs.getString("email"));
					employee.setStatus(rs.getString("status"));
					employee.setDepartmentid(rs.getInt("departmentid"));
					employee.setPassword(rs.getString("password"));
					employee.setRole(rs.getString("role"));
					employeeslist.add(employee);
				}
			 } catch (SQLException e) {
				    e.printStackTrace();
			}finally{
				//最后总要关闭连接
				ConnectionFactory.closeConnection();
			}
			 return employeeslist;
		 }

public Employee selectByNamePwd(String username, String pwd) {
		Employee employee = null;
		try {
			//创建PreparedStatement对象
			PreparedStatement st = null;
			//查询语句
			String sql = "select * from employee where username='" + username + "' and  password='" + pwd + "'";
			st = conn.prepareStatement(sql);
			ResultSet rs = st.executeQuery(sql);
			//判断结果集有无记录,如果有:则把内容取出来,变成一个employee对象,并且返回它
			if (rs.next() == true) {				
				employee = new Employee();				
				employee.setEmployeeid(rs.getInt("employeeid"));
				employee.setEmployeename(rs.getString("employeename"));
				employee.setUsername(rs.getString("username"));
				employee.setPhone(rs.getString("phone"));
				employee.setEmail(rs.getString("email"));
				employee.setStatus(rs.getString("status"));
				employee.setDepartmentid(rs.getInt("status"));
				employee.setPassword(rs.getString("password"));
				employee.setRole(rs.getString("role"));
			}
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} finally {
			ConnectionFactory.closeConnection();
		}
		return employee;
	}

 public Employee selectByUsername(String username){
		 conn=ConnectionFactory.getConnection();
		 Employee employee=null;	
		 try {
			 PreparedStatement st=null;
			String sql="select * from employee where username='"+username+"'";
	 		st = conn.prepareStatement(sql);
			ResultSet rs =st.executeQuery(sql);
			if(rs.next()==true){
				employee=new Employee();
				employee.setEmployeeid(rs.getInt("employeeid"));
				employee.setEmployeename(rs.getString("employeename"));
				employee.setUsername(rs.getString("username"));
				employee.setPhone(rs.getString("phone"));
				employee.setEmail(rs.getString("email"));
				employee.setStatus(rs.getString("status"));
				employee.setDepartmentid(rs.getInt("status"));
				employee.setPassword(rs.getString("password"));
				employee.setRole(rs.getString("role"));
			}
		 } catch (SQLException e) {
			    e.printStackTrace();
		}finally{
			ConnectionFactory.closeConnection();
		}
		 return employee;
	 }

需要注意的点

1.字符串的拼接必须在双引号的基础上被单引号套住

上面有个小陷阱

如果加了

会正常执行，如果没有加，会因为字段不是字符串而报错.

结果集为空

2.在Bean类,默认的构造方法还与参数顺序有关

也就是说public Employee(String user,int id, String pwd){}

和 public Employee(int id,String user,String pwd){}  是不一样的构造方法

测试main方法里,插入的数据的类型顺序决定了调用哪个构造方法.

3.构造方法的方法名就是类名....

4.system.out.println 里打印加不加toString的区别

看起来没有区别(这个不敢肯定)

5.sql语句里,双引号的里面套双引号,会有歧义

会报错

应该在里面放单引号