为什么这个 SQL 脚本可以这样工作?

Why does this SQL script work as it does?(为什么这个 SQL 脚本可以这样工作?)

问题描述

我正在从 SQL Server 2005 的表中提取电子邮件地址记录，并希望构建一个字符串以用作带有 sp_send_dbmail 的 @recipients 列表.该表有一个名为 EmailAddress 的字段，表中有 10 条记录.

I'm pulling email address records from a table in SQL Server 2005, and want to build a single string to use as the @recipients list with sp_send_dbmail. The table has a field called EmailAddress and there are 10 records in the table.

我正在这样做:

DECLARE @email VARCHAR(MAX)
SELECT
    @email = ISNULL(@email + '; ', '') + EmailAddress
FROM
    accounts

现在@email 有一个包含 10 个来自帐户表的电子邮件地址的半分隔列表.

Now @email has a semi-delimited list of 10 email address from the accounts table.

我的问题是为什么/如何工作?为什么@email 没有表中只有最后一个电子邮件地址?

My questions is why/how does this work? Why doesn't @email only have the last email address in the table?

推荐答案

因为对于每一行，您将 @email 的当前值与 EmailAddress 中的下一个结果连接起来.字符串连接就像调用函数一样，它必须按顺序计算每一行的结果.

Because for each row you concatentate the current value of @email with the next result in EmailAddress. String concatenation is just like calling a function, in that it must evaluate the result for each row in sequence.

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