Question

I am trying to add count, sum, and average values from one table to another, but I end up querying the same data for each value. I’m using PostgreSQL. I’m turning this over to the experts to learn how to make this update statement more efficient. Here it is:

update "table1" set 
"col1" = (SELECT COUNT(*) FROM "table2" WHERE "table2Id" = "table1"."table1Id"), 
"col2" = (SELECT AVG("someCol") FROM "table2" WHERE "table2Id" = "table1"."table1Id"),
"col3" = (SELECT SUM("someCol") FROM "table2" WHERE "table2Id" = "table1"."table1Id");

I should be able to run a subquery like this once and access the returned values for the update, correct?

SELECT COUNT(*), AVG("someCol"), SUM("someCol") FROM "table2" WHERE "table2Id" = "table1"."table1Id";

Any help is much appreciated.

Answer 1

Try a subquery:

UPDATE table1 
SET col1 = YourCount, col2 = YourAverage, col3 = YourSum
FROM table1 t1
INNER JOIN (
    SELECT table2Id, COUNT(*) AS YourCount, AVG(someCol1) YourAverage, 
        SUM(someCol2) YourSum
    FROM table2
    GROUP BY table2Id
) t2 ON t1.table1Id = t2.table2Id