结合两个MySQL查询以为每个列显示一个唯一值和一个AVG值

Question

Is there a way to combine two MySQL queries to display one unique value and an AVG value for each column? Such as in this query:

SELECT `price`, `cost`, `shipping` 
FROM `orders` 
WHERE `year` = 2019 AND `product_id` = 5 AND `order_id` = 77 
LIMIT 1

WITH

SELECT AVG(`price`), AVG(`cost`), AVG(`shipping`) 
FROM `orders` 
WHERE `year` = 2019 AND `product_id` = 5

I've been playing with unions and joins but I'm not finding a way to return this data in the same query. (I can make two separate queries and put the resulting data side-by-side, but I'd prefer to do it with one query if possible.)

Any ideas?

Answer 1

Since both queries return just one record, you could just turn them to subqueries and CROSS JOIN them :

SELECT a.price, a.cost, a.shipping, avg.price, avg.cost, avg.shipping 
FROM 
    ( 
        SELECT `price`, `cost`, `shipping` 
        FROM `orders` 
        WHERE `year` = 2019 AND `product_id` = 5 AND `order_id` = 77 
        LIMIT 1 
    ) a
    CROSS JOIN ( 
        SELECT AVG(`price`) price, AVG(`cost`) cost, AVG(`shipping`) shipping 
        FROM `orders` 
        WHERE `year` = 2019 AND `product_id` = 5 
    ) avg

The purpose of the LIMIT 1 clause in the first subquery is unclear : since there is no ORDER BY , it is unpredictable which record will be returned if more than one matches.

Here is an alternative approach using conditional aggregation (if several record exist with order id 77 , the maximum value of each column will be displayed) :

SELECT
    MAX(CASE WHEN `order_id` = 77 THEN `price` END) price,
    MAX(CASE WHEN `order_id` = 77 THEN `cost` END) cost,
    MAX(CASE WHEN `order_id` = 77 THEN `shipping` END) shipping,
    AVG(`price`) avg_price, 
    AVG(`cost`) avg_cost,
    AVG(`shipping`) avg_shipping
FROM `orders`
WHERE `year` = 2019 AND `product_id` = 5