CASE statement when criterias are in different rows and columns - mysql

Question

DB Fiddle
CREATE TABLE sales (
campaign VARCHAR(255),
event_type VARCHAR(255),
event_date VARCHAR(255),
quantity VARCHAR(255)
);
INSERT INTO sales
(campaign, event_type, event_date, quantity)
VALUES
("C001", "buy", "2019-05-08", "500"),
("C001", "sale", "2019-04-20", "400"),
("C002", "buy", "2019-06-07", "800"),
("C002", "sale", "2019-06-15", "900"),
("C003", "buy", "2020-02-23", "700"),
("C003", "sale", "2020-03-17", "300"),
("C004", "buy", "2020-04-05", "200"),
("C004", "sale", "2020-03-17", "600");
The table displays the event_types (buy, sell) and the corresponing quantities per campaign.
Now, I want to run a query which checks the following:
a) event_date for event_type buy > event_date for event_type sell
b) quantity for event_type buy > quantity for event_type sell
The result should look like this:
campaign Check_Date Check_Quantity
C001 Error OK
C002 OK Error
C004 OK Error
I think I need to go with the CASE statement somehow like this:
SELECT
campaign,
(CASE WHEN event_date of buy > event_date of sale THEN "Error" ELSE "OK") AS Check_Date,
(CASE WHEN quantity of buy > quantity of sale THEN "Error" ELSE "OK") AS Check_Quantity
quantity
FROM sales;

Answer 1

You want to compare values across rows, so this suggests aggregation. You can then implement the logic with case expressions:
select
campaign,
case when
max(case when event_type = 'buy' then event_date end)
> max(case when event_type = 'sale' then event_date end)
then 'OK'
else 'Error'
end check_date,
case when
max(case when event_type = 'buy' then quantity end)
> max(case when event_type = 'sale' then quantity end)
then 'OK'
else 'Error'
end check_date
from sales
group by campaign;
In your DB Fiddle, this produces:
campaign | check_date | check_date
:------- | :--------- | :---------
C001 | OK | OK
C002 | Error | Error
C003 | Error | OK
C004 | OK | Error

Answer 2

Split the table in 2 queries, then join and use CASE expressions:
select
b.campaign,
case when b.event_date > s.event_date then 'Error' else 'OK' end Check_Date,
case when b.quantity > s.quantity then 'Error' else 'OK' end Check_Quantity
from (select * from sales where event_type = 'buy') b
inner join (select * from sales where event_type = 'sale') s
on s.campaign = b.campaign
See the demo.
Results:
> campaign | Check_Date | Check_Quantity
> :------- | :--------- | :-------------
> C001 | Error | Error
> C002 | OK | OK
> C003 | OK | Error
> C004 | Error | OK

Answer 3

You can use sub-query & do aggregation :
select t.campaign,
(case when buy_dt > sale_dt then 'OK' else 'Error' end) as Check_date,
(case when buy_qty > sale_qty then 'OK' else 'Error' end) as Check_qty
from (select s.campaign,
max(case when s.event_type = 'buy' then s.event_date end) as buy_dt,
max(case when s.event_type = 'sale' then s.event_date end) as sale_dt,
max(case when s.event_type = 'buy' then s.quantity end) as buy_qty,
max(case when s.event_type = 'sale' then s.quantity end) as sale_qty
from sales s
group by s.campaign
) t;

Answer 4

SELECT t1.campaign,
CASE WHEN t1.event_date > t2.event_date
THEN 'OK'
ELSE 'Error'
END Check_Date,
CASE WHEN t1.quantity > t2.quantity
THEN 'OK'
ELSE 'Error'
END Check_Quantity
FROM sales t1
JOIN sales t2 USING ( campaign )
WHERE t1.event_type = 'buy'
AND t2.event_type = 'sale';
fiddle
For the result to be meaningful you MUST add unique index by (campaign, event_type) and restrict values list safe for event_type field (make it ENUM?).
Also you have not specified what must be returned for a campaign when only one record for it exists - my query will not return such campaign at all.