Quickly add a copy of a column to a MySQL table.

Question

I need a fast way duplicate a DATETIME column in a table and give it a new name.

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.

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)?

Obvious solution:

ALTER TABLE `myResults` ADD `newDate` DATETIME;  
UPDATE `myResults` SET `newDate` = `myDate`;

Answer 1

The obvious solution is the only solution, unfortunately.

However note that in general you shouldn't be copying a column in relational databases.

Answer 2

If you just need a default in there, you can either choose what the default is statically or use a function call.

ALTER TABLE `myResults` ADD `newDate` DATETIME DEFAULT '2010-01-01';

or

ALTER TABLE `myResults` ADD `newDate` DATETIME DEFAULT current_timestamp;

Answer 3

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:

SELECT date1 AS date_old, date1 AS date_new FROM table;

-or-, you can create a view

CREATE VIEW virtual_table AS
    SELECT date1 AS date_old, date1 AS date_new FROM table
;
SELECT * FROM virtual_table;