COUNT() FILTER和COUNT()over
时间: 2024-09-30 16:11:57 浏览: 19
`COUNT()` 是 SQL 中的一个基础聚合函数,用于计算一组值的数量,包括 NULL 值。而在 SQL 的窗口函数 (`OVER`) 部分,`COUNT()` 可以配合 `FILTER` 或者不配合,提供不同的功能。
`COUNT() OVER ()` 通常表示全局窗口函数,即在整个数据集上应用 COUNT,返回的是所有记录的总数。如果没有指定其他参数,它默认包括所有的行。
然而,`COUNT() OVER (PARTITION BY ...)` 则是在每个分区(通常是基于某个列)内计算满足特定条件的记录数。这里的 `FILTER` 子句可以用于筛选出特定条件的行再计数,例如:
```sql
SELECT id, status, COUNT(*) OVER (PARTITION BY status FILTER (WHERE status = 'active')) as active_count
FROM users;
```
这个例子会返回每个用户状态为 "active" 时的活跃用户数量。
简而言之,`COUNT() FILTER` 用于限制在计算时要考虑的行,使得结果更精确地反映特定条件下的数据量。
相关问题
val survived_df = df2.filter(col("Survived") === 1) val pclass_survived_count = survived_df.groupBy("Pclass").count() val pclass_survived_percent = pclass_survived_count.withColumn("percent", format_number(col("count") .divide(sum("count").over()) .multiply(100), 5)); pclass_survived_percent.show() 中sum("count").over()用其他方式替换
如果您想使用其他方式替换 `sum("count").over()`,可以考虑使用 `groupBy` 和 `join` 操作将每个组的 count 数量总和计算出来。具体做法如下:
```
import org.apache.spark.sql.functions.{sum, col}
val total_count = survived_df.select(sum(col("count")).as("total_count")).collect()(0)(0).asInstanceOf[Long]
val pclass_survived_percent = pclass_survived_count.join(broadcast(lit(total_count)).as("total_count")).withColumn("percent", format_number(col("count") / col("total_count") * 100, 5))
```
这里,我们首先使用 `select` 和 `collect` 操作计算出 survived_df 中所有行的 count 数量总和。然后,使用 `join` 操作将每个组的 count 数量总和与 `pclass_survived_count` DataFrame 进行连接,并将连接结果存储在 `pclass_survived_percent` DataFrame 中。最后,使用 `format_number` 函数将每个组的百分比保留小数点后 5 位,并存储在新的列 "percent" 中。
需要注意的是,这种做法的优点是不需要使用窗口函数,因此性能可能会更好。但是缺点是需要进行额外的 `groupBy` 和 `join` 操作,可能会导致代码复杂度和维护成本的增加。因此,在实际应用中,需要根据具体情况进行选择。
ssm管理系统文件中util文件里PageHelper类解释以下代码package com.util; import java.util.ArrayList; import java.util.List; import javax.servlet.http.HttpServletRequest; public class PageHelper { public static void getPage(List<?> list, String name, List<String> nameList, List<String> valueList, int pageSize, String number, HttpServletRequest request, String method) { StringBuffer buffer = new StringBuffer(); String name2 = name.substring(0, 1).toUpperCase() + name.substring(1); String path = ""; String action = "getAll" + name2 + ".action"; if (method != null) { action = "query" + name2 + "ByCond.action"; } List<Object> objList = new ArrayList<Object>(); if (nameList != null && valueList != null) { for (int i = 0; i < nameList.size(); i++) { path += "&" + nameList.get(i) + "=" + valueList.get(i); } } int pageNumber = list.size(); int maxPage = pageNumber; if (maxPage % pageSize == 0) { maxPage = maxPage / pageSize; } else { maxPage = maxPage / pageSize + 1; } if (number == null) { number = "0"; } int start = Integer.parseInt(number) * pageSize; int over = (Integer.parseInt(number) + 1) * pageSize; int count = pageNumber - over; if (count <= 0) { over = pageNumber; } for (int i = start; i < over; i++) { Object obj = list.get(i); objList.add(obj); } buffer.append(" 共为"); buffer.append(maxPage); buffer.append("页 共有"); buffer.append(pageNumber); buffer.append("条 当前为第"); buffer.append((Integer.parseInt(number) + 1)); buffer.append("页 "); if ((Integer.parseInt(number) + 1) == 1) { buffer.append("首页"); } else { buffer.append("<a href=\"" + name + "/" + action + "?number=0" + path + "\">首页</a>"); } buffer.append(" "); if ((Integer.parseInt(number) + 1) == 1) { buffer.append("上一页"); } else { buffer.append("<a href=\"" + name + "/" + action + "?number=" + (Integer.parseInt(number) - 1) + "" + path + "\">上一页</a>"); } buffer.append(" "); if (maxPage <= (Integer.parseInt(number) + 1)) { buffer.append("下一页"); } else { buffer.append("<a href=\"" + name + "/" + action + "?number=" + (Integer.parseInt(number) + 1) + "" + path + "\">下一页</a>"); } buffer.append(" "); if (maxPage <= (Integer.parseInt(number) + 1)) { buffer.append("尾页"); } else { buffer.append("<a href=\"" + name + "/" + action + "?number=" + (maxPage - 1) + "" + path + "\">尾页</a>"); } String html = buffer.toString(); request.setAttribute("html", html); request.setAttribute(name + "List", objList); }
The `PageHelper` class in the `util` package of the SSM management system is used to implement pagination functionality for displaying data in pages.
The `getPage` method takes the following parameters:
- `list`: the list of objects to be paginated
- `name`: the name of the entity being paginated
- `nameList`: a list of parameter names used for filtering the data
- `valueList`: a list of parameter values used for filtering the data
- `pageSize`: the number of objects to be displayed per page
- `number`: the current page number
- `request`: the HTTP servlet request object
- `method`: the name of the method used for filtering the data
The method first constructs the URL for the pagination links by appending the `action` parameter to the entity name. If a filter method is provided, it replaces the default `getAll` method with the filter method.
Next, it creates a new list to hold the paginated data and calculates the total number of pages based on the size of the original data list and the page size. It then adds the relevant parameters to the URL for each page and constructs the HTML code for the pagination links.
Finally, it sets the paginated data list and pagination HTML code as attributes of the request object to be passed back to the view for display.
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