MySQL 我怎样才能 SELECT MIN 从哪里得到结果

Question

I created a query to select the minimum OrderDate of 2020 or 2019 but I always get the result of 2019 when I use 'OR', I have tried to use the case too but always the same result..

so what I want is to select the minimum orderDate of 2020 but when there is no order in 2020 then select the orders of 2019.

SELECT o.EmployeeID , min(o.OrderDate) as minOrderDate
FROM Orders o
where  o.EmployeeID = 1 and (year(o.OrderDate) = year('2020') or year(o.OrderDate) = year('2019'))
group by o.EmployeeID;

Answer 1

How about using direct date comparisons for the filtering. Then use COALESCE() :

SELECT o.EmployeeID, 
       COALESCE(MIN(CASE WHEN o.OrderDate >= '2020-01-01' THEN o.OrderDate END),
                MIN(o.OrderDate)
               ) as minOrderDate
FROM Orders o 
WHERE o.EmployeeID = 1 AND
      o.OrderDate >= '2019-01-01' AND
      o.OrderDate < '2021-01-01' 
GROUP BY o.EmployeeID;

The COALESCE() first looks for the minimum date in 2020. If not present, it uses the minimum for 2019.

I changed the comparison logic for two reasons. First, the YEAR() function should operate on dates and '2020' is not a date. Second, direct comparisons are more efficient -- they help the optimizer use indexes and partitions for instance.

EDIT:

Actually, another interesting way to think about the problem is that you want the maximum of the minimum order dates for the two years. That suggests two levels of aggregation:

SELECT EmployeeId, MAX(min_orderDate) as min_orderDate
FROM (SELECT o.EmployeeID, MIN(o.OrderDate) as min_orderDate
      FROM Orders o 
      WHERE o.EmployeeID = 1 AND
            o.OrderDate >= '2019-01-01' AND
            o.OrderDate < '2021-01-01' 
      GROUP BY o.EmployeeID, YEAR(o.OrderDate)
     ) ey
GROUP BY EmployeeId;

I should note that the first version will be more efficient.

Answer 2

Here is a way to do this using analytic functions, along with union:

WITH cte AS (
    SELECT EmployeeID, MIN(OrderDate) AS minOrderDate, 1 AS priority
    FROM Orders
    WHERE YEAR(OrderDate) = 2020
    GROUP BY EmployeeID
    UNION ALL
    SELECT EmployeeID, MIN(OrderDate), 2
    FROM Orders
    WHERE YEAR(OrderDate) = 2019
    GROUP BY EmployeeID
),
cte2 AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY EmployeeID ORDER BY priority) rn
    FROM cte
)

SELECT EmployeeID, minOrderDate
FROM cte2
WHERE rn = 1
ORDER BY EmployeeID;

The logic here is that we find, for each employee, the earliest order dates for both 2019 and 2020. In the first CTE, this is done, and we also assign a priority to each year, with 2020 having the higher priority (1 vs. 2, with 1 coming first logically). Then, we assign a row number to each of the 1 or 2 employee records from 2019/2020, and retain the highest priority record. Note that for employees having both years, 2020 would be retained, while 2019 would be retained in the event that an employee has no 2020 data.