判断查询结果是否为空

• boolean next()
  throws 

  Moves the cursor forward one row from its current position. A  ResultSet cursor is initially positioned before the first row; the first call to the method  next makes the first row the current row; the second call makes the second row the current row, and so on.

  When a call to the next method returns false, the cursor is positioned after the last row. Any invocation of a ResultSet method which requires a current row will result in a SQLException being thrown. If the result set type is TYPE_FORWARD_ONLY, it is vendor specified whether their JDBC driver implementation will return false or throw an SQLException on a subsequent call to next.

  If an input stream is open for the current row, a call to the method next will implicitly close it. A ResultSet object's warning chain is cleared when a new row is read.

  Returns:

  true if the new current row is valid;  false if there are no more rows

  Throws:

  SQLException - if a database access error occurs or this method is called on a closed result set

  
  

Statement statement = conn.createStatement();
ResultSet res = statement.executeQuery(selectSql);
if (!res.next()) {
//res is null
} else {
// res is not null
}

ResultSet res = ...使用某种方法获取查询结果
int nRow = 0;
while(res.next()) {
++nRow;
}
res.beforeFirst();
// 其他代码不变

ResultSet res = ...使用某种方法获取查询结果
res.last();
final int nRow = res.getRow();
res.beforeFirst();
// 其他代码不变

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