java怎么連接數據庫executeUpdate()和executeQuery()

這篇文章主要介紹了java怎么連接數據庫executeUpdate()和executeQuery()的相關知識，內容詳細易懂，操作簡單快捷，具有一定借鑒價值，相信大家閱讀完這篇java怎么連接數據庫executeUpdate()和executeQuery()文章都會有所收獲，下面我們一起來看看吧。

Update

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

Insert

//沒有返回值,參數是個字符串部門名稱就ok了,因為id的話是自增
	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();
		}
	}

 //因為employeeid自增,所以不用設置
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());			
			//注冊成功后，默認為正在審核，status為0
			pstmt.setString(7,"0");
			//注冊時，默認為員工角色，role值為2
			pstmt.setString(8,"2");
			pstmt.executeUpdate();	
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}finally{
			ConnectionFactory.closeConnection();
		}	  
	  }

Delete

//刪除不用返回值	
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類型
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;
}

//返回部門集合
	public List<Department> selectAll() {
		conn = ConnectionFactory.getConnection();
		// 新建一個集合departmentsList
		List<Department> departmentsList = new ArrayList<Department>();
		try {
			Statement st = null;
			String sql = "select * from department";
			st = conn.createStatement();
			ResultSet rs = st.executeQuery(sql);
			Department department;
			while (rs.next()) {
				// 新建一個department來接收數據庫的信息
				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;
	} 
 
//返回員工
  public List<Employee> selectAllEmployee(){
			 conn=ConnectionFactory.getConnection();
			 List<Employee> employeeslist=new ArrayList<Employee>();
			 Employee employee=null;	
			 try {
				PreparedStatement st=null;
				//只查詢已注冊且未審批 且 角色是員工的
				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{
				//最后總要關閉連接
				ConnectionFactory.closeConnection();
			}
			 return employeeslist;
		 }

public Employee selectByNamePwd(String username, String pwd) {
		Employee employee = null;
		try {
			//創建PreparedStatement對象
			PreparedStatement st = null;
			//查詢語句
			String sql = "select * from employee where username='" + username + "' and  password='" + pwd + "'";
			st = conn.prepareStatement(sql);
			ResultSet rs = st.executeQuery(sql);
			//判斷結果集有無記錄,如果有:則把內容取出來,變成一個employee對象,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;
	 }

需要注意的點

1.字符串的拼接必須在雙引號的基礎上被單引號套住

上面有個小陷阱

如果加了

會正常執行，如果沒有加，會因為字段不是字符串而報錯.

結果集為空

2.在Bean類,默認的構造方法還與參數順序有關

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

和 public Employee(int id,String user,String pwd){}  是不一樣的構造方法

測試main方法里,插入的數據的類型順序決定了調用哪個構造方法.

3.構造方法的方法名就是類名....

4.system.out.println 里打印加不加toString的區別

看起來沒有區別(這個不敢肯定)

5.sql語句里,雙引號的里面套雙引號,會有歧義

會報錯

應該在里面放單引號

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