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`PostgreSQL Arrays <http://www.postgresql.org/docs/current/static/arrays.html>`_
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Now we can find the center point by collecting all the stations for each route into a single multi-point, and calculating the centroid of that multi-point.
现在，我们可以通过将每条线路的所有站点收集为一个多点（multi-point），然后计算该多点的中心点（centroid）来找到中心点。
The center point of the collection of 'Q' train stops looks like this:
'Q'线列车站点集合的中心点如下：
So the northern most stop, the end point, appears to also be the stop furthest from the center. Let's calculate the furthest point for every route.
所以最北端的车站（终点站）似乎也是距离中心最远的车站。让我们计算每条线路距离中心最远的站点。
We've added two sub-queries this time:
这次我们添加了两个子查询：
**stops_distance** joins the centers points back to the stations table and calculates the distance between the stations and center for each route. The result is ordered such that the records come out in batches for each route, with the furthest station as the first record of the batch.
**stops_distance** 将中心点与车站表连接，并计算每条线路中车站与中心点之间的距离。结果按线路分批排序，每批的第一条记录是距离中心最远的车站。
**first_stops** filters the **stops_distance** output by only taking the first record for each distinct group. Because of the way we ordered **stops_distance** the first record is the furthest record, which means it's the station we want to use as our starting seed to build each subway route.
**first_stops** 通过仅获取每个不同分组的第一条记录来过滤**stops_distance** 的输出。由于 **stops_distance**已按特定方式排序，因第一条记录就是距离中心最远的记录，也就是我们希望用作起始点来构建每条地铁线路的站点。
Now we know every route, and we know (approximately) what station each route starts from: we're ready to generate the route lines!
现在我们已经确定了每条线路，并且大致知道了每条线路的起始站点：我们可以开始生成路线线条了！
But first, we need to turn our recursive CTE expression into a function we can call with parameters.
首先，我们需要将递归 CTE 表达式转换为一个可以通过参数调用的函数。
And now we are ready to go!
现在，我们已经准备就绪，可以开始了！
Here's what our final table looks like visualized in QGIS:
这是我们的最终表在 QGIS 中的可视化效果：
As usual, there are some problems with our simple understanding of the data:
和往常一样，我们对数据的简单理解存在一些问题：
there are actually two 'S' (short distance "shuttle") trains, one in Manhattan and one in the Rockaways, and we join them together because they are both called 'S';
系统中实际存在两条简称均为'S'的短途接驳线路：一条位于曼哈顿区，另一条位于洛克威区。尽管二者地理上不相连，但因共享线路标识符'S'，我们在查询中将其合并处理;
the '4' train (and a few others) splits at the end of one line into two terminuses, so the "follow one line" assumption breaks and the result has a funny hook on the end.
'4' 号线（以及其他几条线路）在某一端分成了两个终点站，因此“沿着一条线路前进”的假设被打破，导致结果在末端出现了一个奇怪的弯钩。
Hopefully this example has provided a taste of some of the complex data manipulations that are possible combining the advanced features of PostgreSQL and PostGIS.
希望这个示例能让大家体验到，结合 PostgreSQL 和 PostGIS 的高级特性，可以实现多种复杂的数据处理操作。
See Also
另请参阅
`PostgreSQL Arrays <http://www.postgresql.org/docs/current/static/arrays.html>`_
`PostgreSQL Arrays <http://www.postgresql.org/docs/current/static/arrays.html>`_
`PostgreSQL Array Functions <http://www.postgresql.org/docs/current/static/functions-array.html>`_
`PostgreSQL Array Functions <http://www.postgresql.org/docs/current/static/functions-array.html>`_
`PostgreSQL Recursive Common TABLE Expressions <http://www.postgresql.org/docs/current/static/queries-with.html>`_
`PostgreSQL Recursive Common TABLE Expressions <http://www.postgresql.org/docs/current/static/queries-with.html>`_
`PostGIS ST_MakeLine <http://postgis.net/docs/ST_MakeLine.html>`_
`PostGIS ST_MakeLine <http://postgis.net/docs/ST_MakeLine.html>`_

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User avatar xinbo

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  1. postgis-workshop
  2. advanced_geometry_construction
  3. Chinese (Simplified)
`PostgreSQL Arrays <http://www.postgresql.org/docs/current/static/arrays.html>`_
`PostgreSQL Arrays <http://www.postgresql.org/docs/current/static/arrays.html>`_
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  1. postgis-workshop
  2. advanced_geometry_construction
  3. Chinese (Simplified)
`PostgreSQL Arrays <http://www.postgresql.org/docs/current/static/arrays.html>`_
 
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Source string location
../../en/advanced_geometry_construction.rst:343
String age
11 months ago
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11 months ago
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Translation file
postgis-intro/sources/locale/zh_Hans/LC_MESSAGES/advanced_geometry_construction.po, string 55