java - 即使servlet 数据库中不存在表employee，rs 也始终为true

Question

即使servlet数据库中不存在表employee，rs也始终为true，仅当数据库中不存在表时才想创建表 以及 SELECT count(*)FROM INFORMATION_SCHEMA.TABLES WHERE(TABLE_SCHEMA = 'servlet') AND (TABLE_NAME = 'employee')" 当使用PreparedStement执行时将返回，并且当表不存在和表存在两种情况时结果存储在resultSet中

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

如果表存在，则值将为1，否则将为0。所以记录总是存在的。正如文档所述:

ResultSet.next: Moves the cursor forward one row. Returns true if the cursor is now positioned on a row and false if the cursor is positioned after the last row.

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(*)的值来处理逻辑。您不需要 else 子句，因为您没有做不同的事情。

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();
}