oracle 对多个字段排序规则
    ## Sorting by Multiple Fields in Oracle.
    To sort query results by multiple fields in Oracle, you can use the `ORDER BY` clause. The `ORDER BY` clause allows you to specify multiple fields, and for each field, you can specify whether to sort the results in ascending (ASC) or descending (DESC) order.
    The syntax for the `ORDER BY` clause is as follows:
    ORDER BY field1 [ASC | DESC], field2 [ASC | DESC], ...
    For example, the following query sorts the results of the `employees` table by the `last_name` field in ascending order and the `first_name` field in descending order:
    SELECT.
    FROM employees.
    ORDER BY last_name ASC, first_name DESC;
    You can also use the `ORDER BY` clause to sort the results by multiple fields in a single expression. For example, the following query sorts the results of the `employees` table by the `salary` field in descending order and the `last_name` field in ascending order:
    SELECT.
    FROM employees.
    ORDER BY salary DESC, last_name ASC;
    If you do not specify the `ASC` or `DESC` keyword for a field, the results will be sorted in ascending order by default.
    ## 多字段排序规则。
    要在 Oracle 中按多个字段对查询结果排序，可以使用 `ORDER BY` 子句。`ORDER BY` 子句允许指定多个字段，并且对于每个字段，可以指定按升序 (ASC) 或降序 (DESC) 对结果进行排序。
    `ORDER BY` 子句的语法如下：
    ORDER BY field1 [ASC | DESC], field2 [ASC | DESC], ...
    例如，以下查询按升序对 `employees` 表的结果按 `last_name` 字段排序，按降序对 `first_name` 字段排序：
    SELECT.
    FROM employees.
    ORDER BY last_name ASC, first_name DESC;
    还可以使用 `ORDER BY` 子句通过单个表达式按多个字段对结果进行排序。例如，以下查询按降序对 `employees` 表的结果按 `salary` 字段排序，按升序对 `last_name` 字段排序：
    SELECT.
    FROM employees.
sort of order    ORDER BY salary DESC, last_name ASC;
    如果未为字段指定 `ASC` 或 `DESC` 关键字，则结果将按默认方式按升序进行排序。

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