用于窗口函数的窗口定义可以包含一个帧子句。帧是当前分区的一个子集，而帧子句指定如何定义该子集。The definition of a window used with a window function can include a frame clause. A frame is a subset of the current partition and the frame clause specifies how to define the subset.

帧是相对于当前行确定的，这使得帧可以根据当前行在其分区中的位置在分区内移动。示例：Frames are determined with respect to the current row, which enables a frame to move within a partition depending on the location of the current row within its partition. Examples:

下面的查询演示了使用移动帧在每组按时间排序的level值中计算运行总和，以及从当前行及其前后紧邻的行计算滚动平均值的方法：The following query demonstrates the use of moving frames to compute running totals within each group of time-ordered level values, as well as rolling averages computed from the current row and the rows that immediately precede and follow it:

mysql> SELECT
         time, subject, val,
         SUM(val) OVER (PARTITION BY subject ORDER BY time
                        ROWS UNBOUNDED PRECEDING)
           AS running_total,
         AVG(val) OVER (PARTITION BY subject ORDER BY time
                        ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
           AS running_average
       FROM observations;
+----------+---------+------+---------------+-----------------+
| time     | subject | val  | running_total | running_average |
+----------+---------+------+---------------+-----------------+
| 07:00:00 | st113   |   10 |            10 |          9.5000 |
| 07:15:00 | st113   |    9 |            19 |         14.6667 |
| 07:30:00 | st113   |   25 |            44 |         18.0000 |
| 07:45:00 | st113   |   20 |            64 |         22.5000 |
| 07:00:00 | xh458   |    0 |             0 |          5.0000 |
| 07:15:00 | xh458   |   10 |            10 |          5.0000 |
| 07:30:00 | xh458   |    5 |            15 |         15.0000 |
| 07:45:00 | xh458   |   30 |            45 |         20.0000 |
| 08:00:00 | xh458   |   25 |            70 |         27.5000 |
+----------+---------+------+---------------+-----------------+

对于running_average列，第一行之前或最后一行之后没有帧行。在这些情况下，AVG()会计算可用行的平均值。For the running_average column, there is no frame row preceding the first one or following the last. In these cases, AVG() computes the average of the rows that are available.

作为窗口函数使用的聚合函数在当前行帧中的行上操作，非聚合窗口函数也是如此：Aggregate functions used as window functions operate on rows in the current row frame, as do these nonaggregate window functions:

FIRST_VALUE()
LAST_VALUE()
NTH_VALUE()

标准 SQL 指定，对整个分区操作的窗口函数不应有帧子句。MySQL 允许这些函数带帧子句，但会忽略它。即使指定了帧，这些函数也会使用整个分区：Standard SQL specifies that window functions that operate on the entire partition should have no frame clause. MySQL permits a frame clause for such functions but ignores it. These functions use the entire partition even if a frame is specified:

CUME_DIST()
DENSE_RANK()
LAG()
LEAD()
NTILE()
PERCENT_RANK()
RANK()
ROW_NUMBER()

如果给出了帧子句，其语法如下：The frame clause, if given, has this syntax:

frame_clause:
    frame_units frame_extent

frame_units:
    {ROWS | RANGE}

在没有帧子句的情况下，默认帧取决于是否存在 ORDER BY 子句，如本节后面所述。In the absence of a frame clause, the default frame depends on whether an ORDER BY clause is present, as described later in this section.

frame_units 值表示当前行与帧行之间的关系类型：The frame_units value indicates the type of relationship between the current row and frame rows:

frame_extent 值表示帧的起点和终点。你可以只指定帧的起点（此时当前行为隐含的终点），或者使用 BETWEEN 来指定帧的两个端点：The frame_extent value indicates the start and end points of the frame. You can specify just the start of the frame (in which case the current row is implicitly the end) or use BETWEEN to specify both frame endpoints:

frame_extent:
    {frame_start | frame_between}

frame_between:
    BETWEEN frame_start AND frame_end

frame_start, frame_end: {
    CURRENT ROW
  | UNBOUNDED PRECEDING
  | UNBOUNDED FOLLOWING
  | expr PRECEDING
  | expr FOLLOWING
}

使用 BETWEEN 语法时，frame_start 不得晚于 frame_end。With BETWEEN syntax, frame_start must not occur later than frame_end.

允许的 frame_start 和 frame_end 值具有以下含义：The permitted frame_start and frame_end values have these meanings:

下面的查询演示了 FIRST_VALUE()、LAST_VALUE() 以及两个 NTH_VALUE() 的用例：The following query demonstrates FIRST_VALUE(), LAST_VALUE(), and two instances of NTH_VALUE():

mysql> SELECT
         time, subject, val,
         FIRST_VALUE(val)  OVER w AS 'first',
         LAST_VALUE(val)   OVER w AS 'last',
         NTH_VALUE(val, 2) OVER w AS 'second',
         NTH_VALUE(val, 4) OVER w AS 'fourth'
       FROM observations
       WINDOW w AS (PARTITION BY subject ORDER BY time
                    ROWS UNBOUNDED PRECEDING);
+----------+---------+------+-------+------+--------+--------+
| time     | subject | val  | first | last | second | fourth |
+----------+---------+------+-------+------+--------+--------+
| 07:00:00 | st113   |   10 |    10 |   10 |   NULL |   NULL |
| 07:15:00 | st113   |    9 |    10 |    9 |      9 |   NULL |
| 07:30:00 | st113   |   25 |    10 |   25 |      9 |   NULL |
| 07:45:00 | st113   |   20 |    10 |   20 |      9 |     20 |
| 07:00:00 | xh458   |    0 |     0 |    0 |   NULL |   NULL |
| 07:15:00 | xh458   |   10 |     0 |   10 |     10 |   NULL |
| 07:30:00 | xh458   |    5 |     0 |    5 |     10 |   NULL |
| 07:45:00 | xh458   |   30 |     0 |   30 |     10 |     30 |
| 08:00:00 | xh458   |   25 |     0 |   25 |     10 |     30 |
+----------+---------+------+-------+------+--------+--------+

每个函数使用当前框架中的行，根据所示的窗口定义，该框架从第一个分区行延伸到当前行。对于 NTH_VALUE() 调用，当前框架并不总是包含请求的行；在这种情况下，返回值为 NULL。Each function uses the rows in the current frame, which, per the window definition shown, extends from the first partition row to the current row. For the NTH_VALUE() calls, the current frame does not always include the requested row; in such cases, the return value is NULL.

如果没有指定 frame 子句，默认的 frame 取决于是否存在 ORDER BY 子句：In the absence of a frame clause, the default frame depends on whether an ORDER BY clause is present:

由于默认帧会根据是否存在 ORDER BY 而有所不同，因此在查询中添加 ORDER BY 以获得确定性结果可能会改变结果。（例如，由 SUM() 生成的值可能会发生变化。）为了获得相同的结果，但按 ORDER BY 排序，请提供一个明确的帧规范，无论是否存在 ORDER BY 都使用该规范。Because the default frame differs depending on presence or absence of ORDER BY, adding ORDER BY to a query to get deterministic results may change the results. (For example, the values produced by SUM() might change.) To obtain the same results but ordered per ORDER BY, provide an explicit frame specification to be used regardless of whether ORDER BY is present.

当当前行值为 NULL 时，帧规范的含义可能不明显。在这种假设下，这些示例说明了各种帧规范的应用方式：The meaning of a frame specification can be nonobvious when the current row value is NULL. Assuming that to be the case, these examples illustrate how various frame specifications apply: