当我在 sqlite3 中使用 SUM(CASE WHEN .... THEN "column_name") 时，我得到了意想不到的结果

Question

I am trying to find the number of orders I got in the month of April. I have 3 orders but my query gets the result 0. What could be the problem?

Here's the table:

id | first   | middle | last    | product_name  | numberOut | Date
1 | Muhammad | Sameer | Khan    | Macbook       | 1         | 2020-04-01
2 | Chand    | Shah   | Khurram | Dell Optiplex | 1         | 2020-04-02
3 | Sultan   |        | Chohan  | HP EliteBook  | 1         | 2020-03-31
4 | Express  | Eva    | Plant   | Dell Optiplex | 1         | 2020-03-11
5 | Rana     | Faryad | Ali     | HP EliteBook  | 1         | 2020-04-02

And here's the query:

SELECT SUM(CASE WHEN strftime('%m', oDate) = '04' THEN 'id' END) FROM orders;

Answer 1

The problem with your code is this:

THEN 'id'

You are using the aggregate function SUM() and you sum over a string literal like 'id' which is implicitly converted to 0 (because it can't be converted to a number) so the result is 0 .
Even if you remove the single quotes you will not get the result that you want because you will get the sum of the id s.
But if you used:

THEN 1 ELSE 0

then you would get the correct result.
But with SQLite you can write it simpler:

SELECT SUM(strftime('%m', oDate) = '04') FROM orders;

without the CASE expression.
Or since you just want to count the orders then COUNT() will do it:

SELECT COUNT(*) FROM orders WHERE strftime('%m', oDate) = '04';

Edit.
If you want to count the orders for all the months then group by month:

SELECT strftime('%Y-%m', oDate) AS month, 
       COUNT(*) AS number_of_orders
FROM orders 
GROUP BY month;

Answer 2

If you want all Aprils, then you can just look at the month. I would recommend:

select count(*)
from orders o
where o.date >= '2020-04-01' and o.date < '2020-05-01';

Note that this does direct comparisons of date to a valid dates in the where clause.

Answer 3

There is a problem with your query. You do not need to do that aggregation operation.

SELECT COUNT(*) from table_name WHERE strftime('%m', Date) = '04';

Answer 4

SELECT SUM(CASE WHEN strftime('%m', oDate) = '04' THEN 1 ELSE 0 END) FROM orders; 

如果您需要使用 SUM

Answer 5

I would use explicit date comparisons rather than date functions - this makes the query SARGeable, ie it may benefit an existing index.

The most efficient approach, with a filter in the where clause:

select count(*) cnt
from orders 
where oDate >= '2020-04-01' and oDate < '2020-05-01'

Alternatively, if you want a result of 0 even when there are no orders in April you can do conditional aggregation, as you originally intended:

select sum(case when oDate >= '2020-04-01' and oDate < '2020-05-01' then 1 else 0 end) cnt
from orders