SQL：如何更改查询以获得以下结果？

Question

For each customer-product pair, I need to calculate average sale of that product in each state.

Actual table ("sales" table):

cust  | prod  | state | quant
Bloom | Bread |  NY   | 1
Bloom | Butter|  NJ   | 2
.
.
.

My query:

SELECT cust, prod, state, AVG(quant)
from sales
group by cust, prod, state;

Result:

cust  | prod  | state | avg(quant)
Bloom | Bread |  NY   | 1
Bloom | Butter|  NJ   | 2

The result I want:

cust  | prod  |  NY  | NJ
Bloom | Bread |  1   | 2

Answer 1

This is a pivot.

SELECT
    cust,
    prod,
    AVG(CASE WHEN state = 'NY' THEN quant END) AS 'NY',
    AVG(CASE WHEN state = 'NJ' THEN quant END) AS 'NJ'
FROM sales
GROUP BY cust, prod

If you can't hard-code the states like that, see MySQL pivot row into dynamic number of columns for how to implement a dynamic pivot in a stored procedure.