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How to query only updated rows and merge with historical data

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Can I merge updated rows in a SQL table with historical data?

 

I have a SQL table in an event system. Data of interest are 10 years back in time. To decrease the load of the SQL DB, I would like to only read the modified data each day, and merge that data with a query that is update less frequently (e.g once per year). My data might look like below for some years ago:

  

id  open        last_mod    custom
== ========== ========== ======
1 2010-01-01 2010-01-01 a
2   2010-01-02  2010-01-02  a
3   2010-01-03  2010-01-03  a

 

 The next day, the data might look like below:

id  open        last_mod    custom
==  ==========  ==========  ======
1   2010-01-01  2010-01-01  a
2   2010-01-02  2017-04-04  b
3   2010-01-03  2010-01-03  a
4   2017-04-04  2017-04-04  a

I.E. now row-id 4 has been added and row-id 2 has changed from "a" to "b" in the custom column. This is detectable in the last_mod column as well

 

Is it possible to combine two queries where Q1 reads all the data, and Q2 reads the updated data?


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