我不确定“group by”的目的是什么这里

发布于 2024-12-09 08:11:17 字数 222 浏览 0 评论 0原文

我很难理解这个查询在做什么：

SELECT branch_name, count(distinct customer_name)
FROM depositor, account
WHERE depositor.account_number = account.account_number
GROUP BY branch_name

GROUP BY 需要什么？

原文

I'm struggling to understand what this query is doing:

SELECT branch_name, count(distinct customer_name)
FROM depositor, account
WHERE depositor.account_number = account.account_number
GROUP BY branch_name

What's the need of GROUP BY?

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盗心人 2024-12-16 08:11:17

您必须使用 GROUP BY 才能以这种方式使用 COUNT 等聚合函数（使用聚合函数聚合与一个或多个值相对应的数据） em> 表内）。

该查询本质上是使用该列作为分组列来选择不同的branch_name，然后在组内对不同的customer_name进行计数。

如果没有 GROUP BY 子句（至少不是使用简单的查询规范 - 您可以使用其他方式，连接，子查询等...）。

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桃扇骨 2024-12-16 08:11:17

它为您提供每个分支机构的不同客户总数； GROUP BY 用于分组 COUNT 函数。
也可以写成：

SELECT branch_name, count(distinct customer_name)
FROM depositor INNER JOIN account
ON depositor.account_number = account.account_number
GROUP BY branch_name

It's giving you the total distinct customers for each branch; GROUP BY is used for grouping COUNT function.
It could be written also as:

SELECT branch_name, count(distinct customer_name)
FROM depositor INNER JOIN account
ON depositor.account_number = account.account_number
GROUP BY branch_name

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天冷不及心凉 2024-12-16 08:11:17

让我们暂时远离 SQL，看一下关系训练语言教程 D。

因为这两个关系（表）是在公共属性（列）名称 account_number 上连接的，所以我们可以使用自然连接：（

depositor JOIN account

因为结果是一个关系，根据定义它只有不同的元组（行）），我们不需要 DISTINCT 关键字。）

现在我们只需要使用 SUMMARIZE..BY 进行聚合：

SUMMARIZE (depositor JOIN account) 
   BY { branch_name } 
      ADD ( COUNT ( customer_name ) AS customer_tally )

回到 SQLland，GROUP BYbranch_name 的作用与 SUMMARIZE..BY {branch_name} 相同。由于 SQL 具有非常严格的结构，因此 branch_name 列必须在 SELECT 子句中重复。

Let's take a step away from SQL for a moment at look at the relational trainging language Tutorial D.

Because the two relations (tables) are joined on the common attribute (column) name account_number, we can use a natural join:

depositor JOIN account

(Because the result is a relation, which by definition has only distinct tuples (rows), we don't need a DISTINCT keyword.)

Now we just need to aggregate using SUMMARIZE..BY:

SUMMARIZE (depositor JOIN account) 
   BY { branch_name } 
      ADD ( COUNT ( customer_name ) AS customer_tally )

Back in SQLland, the GROUP BY branch_name is doing the same as SUMMARIZE..BY { branch_name }. Because SQL has a very rigid structure, the branch_name column must be repeated in the SELECT clause.

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