Question

是否有一个很好的例子说明如何使用Apache Derby实现分页，以便每次选择新页面时都不返回整个结果集？

如果查询可以按用户指定的列进行排序，也会有所帮助。

Answer 1

这些是在实施分页时有用的derby关键字：OFFSET and FETCH

使用示例：

//all these values are supplied by the GUI
int rowsPerPage = 10;
int pageNumber = 3;
String columnToSortBy = "first_name"; //this value should never be directly edited by the user. Otherwise it could be used for sql injection

StringBuilder statement = new StringBuilder();
statement.append("select * from users\n");
statement.append("order by " + columnToSortBy + "\n");
statement.append("offset ? rows fetch first ? rows only");

try (PreparedStatement ps = conn.prepareStatement(statement.toString())) {

   ps.setInt(1, pageNumber * rowsPerPage);
   ps.setInt(2, rowsPerPage);

   ResultSet rs = ps.executeQuery();
   printResultSet(rs);
}

更详细的例子：

//all these values are supplied by the GUI
int rowsPerPage = 10;
int pageNumber = 3;
String columnToSortBy = "first_name"; //this value should never be directly edited by the user. Otherwise it could be used for sql injection

String selectClause = "select * from users\n";
String whereClause = "where first_name like ?";
String orderClause = "order by " + columnToSortBy + "\n";
String limitClause = "offset ? rows fetch first ? rows only";

//build the sql statement
String statement = "";
statement += selectClause;
statement += whereClause;

//get the total rows
int totalRows = 0;
try (PreparedStatement ps = conn.prepareStatement(statement)) {
   ps.setString(1, "Sam%");

   ResultSet rs = ps.executeQuery();
   while (rs.next()) {
        totalRows++;
   }
}
System.out.println(totalRows + " total rows\n");        

//change the statement to use pagination
statement += orderClause;
statement += limitClause;

//get the 3rd page
try (PreparedStatement ps = conn.prepareStatement(statement)) {

   ps.setString(1, "Sam%");
   ps.setInt(2, pageNumber * rowsPerPage);
   ps.setInt(3, rowsPerPage);

   ResultSet rs = ps.executeQuery();
   printResultSet(rs);
}

System.out.println("\nNext page\n");

//get the 4th page
try (PreparedStatement ps = conn.prepareStatement(statement)) {

   ps.setString(1, "Sam%");
   ps.setInt(2, (pageNumber + 1) * rowsPerPage);
   ps.setInt(3, rowsPerPage);

   ResultSet rs = ps.executeQuery();
   printResultSet(rs);
}

使用Apache Derby进行分页

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