Update one table using the data from another table - sql

Question

I am dealing with 2 tables.
Users:
+----+----------+-------------------------+
| id | user_id | datetime |
+----+----------+-------------------------+
| 1 | 95678367 | 2015-07-03 02:02:29.863 |
| 2 | 72876424 | 2015-07-07 01:04:14.436 |
| 3 | 74293582 | 2015-07-11 10:02:45.523 |
+----+----------+-------------------------+
UserActivation:
+-----+----------+-------------------------+
| id | user_id | datetime |
+-----+----------+-------------------------+
| 1 | 95678367 | 2015-07-03 02:02:29.863 |
| 2 | 09235892 | 2015-07-03 02:02:29.863 |
| 3 | 90328574 | 2015-07-03 02:02:29.863 |
| 4 | 24714287 | 2015-07-03 02:02:29.863 |
| 5 | 02743723 | 2015-07-03 02:02:29.863 |
| 6 | 72876424 | 2015-07-07 01:04:14.436 |
| 7 | 09385732 | 2015-07-07 01:04:14.436 |
| 8 | 74576234 | 2015-07-07 01:04:14.436 |
| 9 | 75439273 | 2015-07-07 01:04:14.436 |
| 10 | 74293582 | 2015-07-11 10:02:45.523 |
| 11 | 94562872 | 2015-07-11 10:02:45.523 |
| 12 | 80367456 | 2015-07-11 10:02:45.523 |
| 13 | 76537924 | 2015-07-11 10:02:45.523 |
+-----+----------+-------------------------+
I am using SQL server 2012. I want to update the timings of table UserActivation. What is going on here that first code insert data in Users table when a user registers. After sometime he activates his account, that data is saving in UserActivation. UserActivation contains numerous columns, but I am showing only those which I am using. The problem is we added datetime column afterwards and till that time hundreds of data is there. What I am trying to do is to update the datetime of UserActivation like follows:
In User table, user_id: 95678367 is first. second is 72876424. I want to update datetime of UserActivation table of rows having id 1 to 5, because id 6 contains the user_id 72876424. So I want to update datetime of rows 1 to 5 in such a way that they comes in 3 seconds increment of datetime from User table.
User table first row has user_id 95678367 and datetime 2015-07-03 02:02:29.863, so update datetime of rows 1-5 of UserActivation(till second user_id of User encounters) as
row1 -> **2015-07-03 02:02:31.863**
row2 -> **2015-07-03 02:02:34.863**
row3 -> **2015-07-03 02:02:37.863**
row4 -> **2015-07-03 02:02:40.863**
row5 -> **2015-07-03 02:02:43.863**
After that if we strikes second id from Users table. take that datetime from Users table 2015-07-07 01:04:14.436
And start update datetime of UserActivation table with increments of three for rows 6-9 as 10th row cotains 3rd user_id of Users table.
Note: I am trying to write a script so that can I loop through both tables and check one by one user_id of both table and update accordingly, but I am not expert in sql server scripts. Showing how to loop through a SELECT result and update in loop will also help.

Answer 1

One way to calculate the "rolling" timestamp is to use a cross apply. See this:
WITH cte AS (
SELECT ua.id, ua.[datetime], CASE WHEN u.id IS NULL THEN -1 ELSE 0 END AS gapN
FROM UserActivation ua LEFT JOIN Users u
ON ua.user_id = u.user_id
)
SELECT co.id, co.datetime, DATEADD(second, 3*(co.id-cap.maxID), co.[datetime]) newDatetime
FROM cte co CROSS APPLY (
SELECT MAX(id) maxID
FROM cte ca
WHERE ca.id <= co.id AND ca.gapN = 0
) cap
You can see that "live" on SQLFiddle
In order to UPDATE the table, you need to replace the SELECT clause with:
/* SAME AS ABOVE UNTIL THE SELECT LINE */
UPDATE co
SET co.datetime = DATEADD(second, 3 * (co.id - cap.maxID), co.[datetime])
FROM UserActivation co CROSS APPLY (
/* SAME AS ABOVE AFTER THE FROM LINE */

Answer 2

Try this
update ua
set ua.datetime = u.datetime
from users u left join useractivation ua on u.user_id = ua.user_id
;with cte
as
(
select id,user_id,datetime from useractivation where id=1
union all
select ua.id,ua.user_id,
case when ua.datetime is null then DATEADD(ss,3,c.datetime) else ua.datetime end as datetime
from cte c inner join useractivation ua on c.id+1 = ua.id
)
update ua
set ua.datetime = c.datetime
from cte c inner join useractivation ua on c.id = ua.id