快速将列的副本添加到 MySQL 表
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Quickly add a copy of a column to a MySQL table
提问by Robbie
I need a fast way duplicate a DATETIME column in a table and give it a new name.
我需要一种快速的方法来复制表中的 DATETIME 列并为其指定一个新名称。
I have a column named myDate in my table called myResults, I need a query to make a new column in the table called newDate which has the exact same data as the myDate column.
我在名为 myResults 的表中有一个名为 myDate 的列，我需要一个查询来在名为 newDate 的表中创建一个新列，该列与 myDate 列具有完全相同的数据。
Is there a faster way to do this than by doing the obvious 2 step approach of make a new column, and then copying all the data (it's a large table and I'm looking for the fastest approach)?
有没有比通过明显的两步方法创建一个新列，然后复制所有数据（它是一个大表，我正在寻找最快的方法）更快的方法来做到这一点？
Obvious solution:
明显的解决方案：
ALTER TABLE `myResults` ADD `newDate` DATETIME;
UPDATE `myResults` SET `newDate` = `myDate`;
采纳答案by Daniel Vassallo
The obvious solution is the only solution, unfortunately.
不幸的是，显而易见的解决方案是唯一的解决方案。
However note that in general you shouldn't be copying a column in relational databases.
但是请注意，通常您不应复制关系数据库中的列。
回答by Basharat Ali
UPDATE `table_name` SET `new_column` = `existing_column` WHERE `id`=`id`
回答by Evan Carroll
Why would your workload ever demand a new datetime column, that duplicates another columns data? This sounds like horrable practice? How about telling us what you're trying to achieve? You can pull a second column with the same data in a few different ways, without actually duplicating the data:
为什么您的工作负载需要一个新的日期时间列来复制另一列数据？这听起来像可怕的做法？告诉我们你想要达到的目标如何？您可以通过几种不同的方式提取具有相同数据的第二列，而无需实际复制数据：
SELECT date1 AS date_old, date1 AS date_new FROM table;
-or-, you can create a view
-或者-，你可以创建一个视图
CREATE VIEW virtual_table AS
SELECT date1 AS date_old, date1 AS date_new FROM table
;
SELECT * FROM virtual_table;
回答by Matt
If you just need a default in there, you can either choose what the default is statically or use a function call.
如果您只需要那里的默认值，您可以静态选择默认值或使用函数调用。
ALTER TABLE `myResults` ADD `newDate` DATETIME DEFAULT '2010-01-01';
or
或者
ALTER TABLE `myResults` ADD `newDate` DATETIME DEFAULT current_timestamp;
