MySql update not recognizing temporary table - mysql

Question

I have the following query script that is resulting in an error:
SET #row_number = 0;
Drop Table If Exists testtable2;
Create Temporary Table testtable2
SELECT
*
FROM
(SELECT
row_names, (#row_number:=#row_number + 1) AS num, date_add(date_time, INTERVAL 1 MINUTE) AS date_time, meter, kw
FROM
testtable
WHERE
DAY(date_time) = 1
AND HOUR(date_time) = 2) AS testtable2
WHERE
MOD(testtable2.num, 2) = 0;
UPDATE testtable
SET
testtable.date_time = testtable2.date_time
WHERE
testtable.row_names = testtable2.row_names;
The error says Error Code: 1054. Unknown column 'testtable2.row_names' in 'where clause'
I created a temporary table that contains the column date_time but my update query fails to recognize that the column exists. I can run something like SELECT * FROM testtable2; and it returns showing that the column is indeed generated with the correct title. Why can my update not recognize this column?

Answer 1

This is your update statement:
UPDATE testtable
SET testtable.date_time = testtable2.date_time
WHERE testtable.row_names = testtable2.row_names;
What is testtable? You have not defined that. You created testtable2. If you do have testtable, then perhaps you want a join:
UPDATE testtable tt JOIN
testtable2 tt2
ON tt.row_names = tt2.row_names
SET tt.date_time = tt2.date_time;

Answer 2

You actually meant to use CREATE TABLE ... AS construct but in your case it's wrongly formed. It should be like below.
Create Temporary Table testtable2 AS
SELECT
*
FROM
(SELECT
row_names, (#row_number:=#row_number + 1) AS num,
date_add(date_time, INTERVAL 1 MINUTE) AS date_time,
meter,
kw
FROM
testtable
WHERE
DAY(date_time) = 1
AND HOUR(date_time) = 2) AS XXX <-- Here
WHERE
MOD(testtable2.num, 2) = 0;
Issue in your case is, your Temporary table and inline view alias are same. name them differently. See edited query.
You can directly JOIN with the inner query and can perform UPDATE like
UPDATE testtable
JOIN (
SELECT
*
FROM
(SELECT
row_names, (#row_number:=#row_number + 1) AS num, date_add(date_time, INTERVAL 1 MINUTE) AS date_time, meter, kw
FROM
testtable
WHERE
DAY(date_time) = 1
AND HOUR(date_time) = 2) AS testtable2
WHERE
MOD(testtable2.num, 2) = 0 ) xx ON testtable.row_names = xx.row_names
SET
testtable.date_time = xx.date_time;