将一个表中的计数值设置为另一个表

Question

I am trying to count matching values from customer column on table 'Customers' and update their values on the Count Column in table 'Summary'. I also want to Check if the Date is <= Todays Date.

Table " Customers ":

ID	Customer	Date
1	John	2022-01-01
2	John	2022-01-01
3	Mary	2022-01-01
4	Mary	2022-01-01

.......+2000 More Customers

Table " Summary ":

ID	Customer	Count	DateInput
1	John	2	2021-01-01
2	Mary	2	2021-01-01

.........+100 More Customers

I can update one row at a time like this:

update Summary
set Count = (SELECT COUNT(*) 
             FROM Customers 
             WHERE Customer = "John" AND Date <=CURRENT_DATE()) 
WHERE Customer = "John";

Is there a way to use the above query to update the count column for John, mary, etc, etc without doing Multiple individual requests?

Answer 1

Given that your count values will change, you should consider creating a view instead of updating a table:

CREATE VIEW summary AS 
SELECT ID, Customer, COALESCE(COUNT(CASE WHEN Date <= CURRENT_DATE() THEN 1 END), 0) AS cnt
FROM Customers
GROUP BY ID, Customer

If you really want to have a table and update it every time, you need such UPDATE statement:

WITH cte AS (
    SELECT ID, Customer, COUNT(*) AS count
    FROM Customers 
    WHERE Date <= CURRENT_DATE()
    GROUP BY ID, Customer
)
UPDATE Summary
INNER JOIN cte ON Summary.ID = cte.ID AND Summary.Customer = cte.Customer
SET Summary.count = cte.count

Answer 2

Is this something you are looking for?

UPDATE
  Summary s
  INNER JOIN Customers c ON s.Customer = c.Customer
SET
  s.Count = (
    SELECT
      COUNT(*)
    FROM
      Customers c2
    WHERE
      c2.Customer = s.Customer
      AND c2.Date <= CURRENT_DATE()
  )

If you are going to test the query, please test it on a small dataset before applying it to the entire table since it may not achieve the results you are expecting.

Answer 3

You can do it as follows:

UPDATE Summary s
INNER JOIN (
  SELECT Customer, count(1) as _count
  FROM Customers
  where Date <=CURRENT_DATE()
  group by Customer
) as c on s.Customer = c.Customer
set s.Count = c._count ;

I have used inner join to join a list of customers and their counts. and the relation is Customer.