How to update one table from another table based on some priority that depends on where clause?

问题

I have table A with product_id,cost,year,quarter,... etc columns. I have another table B with product_id,base_cost,current_year,p_year,p_quarter,p_order columns.

I want to write an update query to update A from B. My conditions are -

WHERE A.product_id=B.product_id
and A.year=B.current_year
and (A.year=B.p_year and A.quarter>B.p_quarter) or A.year>B.p_year
and A.cost=0;

But the problem is, with these conditions if i have more than one rows in B then i only want to update from the row of B according to priority.

Giving logic and examples for better understanding-

basically p_ stands for priority. i want to update A's cost from B according to the p_order. Secondly, i cannot consider current and future quarters for updation.

Example 1-

If A has one row as:

product_id  cost   year  quarter
102         0      2019  1
102         0      2019  2
102         0      2019  3
102         0      2019  4

And B has two rows corresponding to the where clause:

product_id  cost  current_year p_year  quarter p_order
102         3.5   2019         2019     3      1
102         1.8   2019         2019     1      2
102         0.5   2019         2019     2      3

Then updated A should be:

product_id  cost   year  quarter
102         0      2019  1
102         1.8    2019  2
102         1.8    2019  3
102         3.5    2019  4

Example 2-

If A has one row as:

product_id  cost   year  quarter
102         0      2019  1
102         0      2019  2
102         0      2019  3
102         0      2019  4

And B has two rows corresponding to the where clause:

product_id  cost  current_year p_year  quarter p_order
102         3.5   2019         2019     3      1
102         6.3   2019         2018     4      2
102         0.5   2019         2019     2      3

Then updated A should be:

product_id  cost   year  quarter
102         6.3    2019  1
102         6.3    2019  2
102         6.3    2019  3
102         3.5    2019  4

回答1:

Treat this as a comment as it should. However, for some small abount of clarity I think the additional answer formatting overrides.
Your description does not match the accompanying data. In example 1 you say table A has 1 row, then proceed to show 4 rows and table B has 2 rows then proceed to show 3 rows.
Also describe in detail how your mapping occurs, taking example 2 How does (where ==> reads as Maps to)

B(p_year,quarter,p_order) ==> A(year,quarter)
B(2019,3,1) ==> A(2019,4)
B(2018,4,2) ==> A(2019,1) and A(2019,2) and A(2019,3)
B(2019,2,3) ==> none

来源：https://stackoverflow.com/questions/61253120/how-to-update-one-table-from-another-table-based-on-some-priority-that-depends-o

标签

postgresql

join

sql-update

greatest-n-per-group