Question

rs即使在servlet数据库中不存在表employee时也始终为true，只有在数据库中不存在表时才想创建表 SELECT count(*)FROM INFORMATION_SCHEMA.TABLES WHERE(TABLE_SCHEMA = 'servlet') AND (TABLE_NAME = 'employee')"会是什么在使用PreparedStement执行时将返回，并且当表不存在时结果存储在resultSet中，并且当表存在两个情况时

public class Table {
public static void main(String[] args)
{
Connection con=null;
PreparedStatement pstmt=null;
PreparedStatement pstmt1=null;
PreparedStatement pstmt2=null;
ResultSet rs=null;
String qry="SELECT count(*)FROM INFORMATION_SCHEMA.TABLES WHERE(TABLE_SCHEMA = 'servlet') AND (TABLE_NAME = 'employee')";
String qry1="CREATE TABLE servlet.Employee (" +
            " ID INT(6) NOT NULL AUTO_INCREMENT, " +
            " NAME VARCHAR(50) NOT NULL, " + 
            " Department varchar(20) NOT NULL, " + 
            " Salary DECIMAL(8,2) NOT NULL, " + 
            " PRIMARY KEY (ID))";
String qry2="insert into servlet.Employee values(?,?,?,?)";

Class.forName("com.mysql.jdbc.Driver");
con=DriverManager.getConnection("jdbc:mysql://localhost:3306?user=root&password=passworD");
pstmt=con.prepareStatement(qry);
rs=pstmt.executeQuery();
System.out.println(rs.next());
    if(true==!rs.next()){
    pstmt1=con.prepareStatement(qry1);
    pstmt1.executeUpdate();
    pstmt2=con.prepareStatement(qry2);
    pstmt2.setInt(1,id);
    pstmt2.setString(2, name);
    pstmt2.setString(3, dept);
    pstmt2.setDouble(4, salary);
    pstmt2.executeUpdate();
    }
    else{

    pstmt2=con.prepareStatement(qry2);
    pstmt2.setInt(1,id);
    pstmt2.setString(2, name);
    pstmt2.setString(3, dept);
    pstmt2.setDouble(4, salary);
    pstmt2.executeUpdate();
    }
 }

}

Answer 1

如果表格存在，则值为1，否则为0。所以记录总是存在的。正如文档所说：

ResultSet.next：将光标向前移动一行。如果光标现在位于行上，则返回true;如果光标位于最后一行之后，则返回false。

rs.next将始终返回true，因为它将光标定位在您的案例中的第一条记录上。您需要根据count(*)的值处理逻辑。你不需要else子句，因为你没有做不同的事情。

if(rs.next())
  int count = rs.getInt(1);
  if (count == 0) {
    pstmt1=con.prepareStatement(qry1);
    pstmt1.executeUpdate();
  }
  pstmt2=con.prepareStatement(qry2);
  pstmt2.setInt(1,id);
  pstmt2.setString(2, name);
  pstmt2.setString(3, dept);
  pstmt2.setDouble(4, salary);
  pstmt2.executeUpdate();
}

即使servlet数据库中不存在表employee，rs也始终为true

1 个答案: