ORACLE Query to check if one field has changed to another specific - oracle

Question

We recently noticed that some workes have the maximum rate (13) when they barely have any bought items, so we need a query that checks if any worker rate changes to the maximum so we can check if its legit.
So I have these two tables:
Table1-> Workers (DNI,RATE,TICKET,PRICE,...)
Table2-> Sells (DNI,NAME,DATE)
edit: DATE and RATE are both Strings, Date follows this structure: YYYYMMDD
Sells table have a special discount depending on the rate of the client.
I want to check if any of those wokers rate changes from something that is not 13 --> to 13
The only way this can happen if is the worker buys a lot, or if someone changes it manually so they get a better discount (NOT ALLOWED).
So, I want to check if yesterday any worker had a 6 rate and today its 13.
SELECT RATE,DNI FROM SELLS WHERE RATE='13' AND DNI IN
(
SELECT DNI FROM WORKERS
)
AND DATE = to_char(sysdate-1, 'yyyymmdd')
Is there any way to UNION those so the matched remain?
SELECT RATE,DNI FROM SELLS WHERE RATE <> '13' AND DNI IN
(
SELECT DNI FROM WORKERS
)
AND DATE to_char(sysdate, 'yyyymmdd')
Or is there any better way?
Edit of what tried:
Real query:
select
coalesce(t.dni, y.dni) as dni,
t.Tarifa as today_rate,
y.Tarifa as yesterday_rate
from (
SELECT Tarifa, DNI FROM pws_ventas_materiales,pws_ventas_cabecera
WHERE Tarifa='13'
AND DNI IN (SELECT DNI FROM trabajadores_sirgo)
AND pws_ventas_materiales.fecha = to_char(sysdate-1, 'yyyymmdd')
and pws_ventas_materiales.ticket = pws_ventas_cabecera.ticket
and pws_ventas_cabecera.fecha = pws_ventas_materiales.fecha
) y
full join (
SELECT Tarifa, DNI FROM pws_ventas_materiales ,pws_ventas_cabecera
WHERE Tarifa <> '13' and Tarifa is not null
AND DNI IN (SELECT DNI FROM trabajadores_sirgo)
AND pws_ventas_materiales.fecha = to_char(sysdate, 'yyyymmdd')
and pws_ventas_materiales.ticket = pws_ventas_cabecera.ticket
and pws_ventas_cabecera.fecha = pws_ventas_materiales.fecha
) t on t.dni = y.dni
;
Table Updates:
pws_ventas_materiales=Sells
trabajadores_sirgo=Workers
pws_ventas_cabecera --> There was a mistake on the Sells table, that table does not contain a DNI column, this table does.
Sorry for that mistake
Also, rate=Tarifa and date=fecha + I added a pws_ventas_materiales.ticket = pws_ventas_cabecera.ticket to make sure that we are talking about the same sell between those two tables.

Answer 1

You can join both queries to compare. In this case I would think that an full outer join is more appropriate. For example:
select
coalesce(t.dni, y.dni) as dni,
t.rate as today_rate,
y.rate as yesterday_rate
from (
SELECT RATE, DNI FROM SELLS
WHERE RATE='13'
AND DNI IN (SELECT DNI FROM WORKERS)
AND DATE = to_char(sysdate-1, 'yyyymmdd')
) y
full join (
SELECT RATE, DNI FROM SELLS
WHERE RATE <> '13'
AND DNI IN (SELECT DNI FROM WORKERS)
AND DATE to_char(sysdate, 'yyyymmdd')
) t on t.dni = y.dni
-- WHERE today_rate is not null -- extra filtering here
Also, the following query can compare all dates, not just today and yesterday:
select *
from (
select
dni, date, rate,
lag(date) over(partition by dni order by date) as prev_date,
lag(rate) over(partition by dni order by date) as prev_rate
from workers
) x
where prev_rate <> '13' and rate = '13'