Select query result inside WHERE clause - mysql

Question

Hello I am trying to make a WHERE clause where the condition is the id of the previous selection, example:
SELECT
,P1.caseid
,(SELECT SUM(P1.amount) FROM table_s P1 WHERE P1.status = 4 AND P1.caseid = 20)
as variable
FROM table_s P1 GROUP BY P1.caseid";
let's say each iteration the P1.caseid have value of
20,
45,
20,
How I can insert this value to be the condition of the WHERE clause here: WHERE P1.status = 4 AND P1.caseid = 20
Instead of P1.caseid to be = to 20 it have to be equal to the actual caseid inside the database for each row.
So for each row it will be:
WHERE P1.caseid = 20
WHERE P1.caseid = 45
WHERE P1.caseid = 35
In this case the number is eqaul to the caseid inside the DB.
TABLE NAME: table_s
id | caseid | amount | status
-- | ------------------------
1 | 20 | 10 | 4
2 | 45 | 10 | 4
3 | 20 | 10 | 4
DB is as follows, the result should be:
1 ROW = caseid: 20 amount: 20 status 4
2 ROW = caseid: 45 amount: 10 status 4
Or
$variable = 20
$variable = 10

Answer 1

I think I've worked out what you're asking...
The important note here is to use different aliases for your table in the outer and inner queries. Otherwise you have a serious scope problem. (If two instances of the same entity have the same name, how can MySQL ever know which one you're referring to? It will choose the one in the nearest scope. So, instead, call one of them, for example, lookup.)
SELECT
P1.*,
(
SELECT SUM(lookup.amount)
FROM table_s lookup
WHERE lookup.status = 4
AND lookup.caseid = P1.caseid
)
correlated_sub_query_total_by_caseid
FROM
table_s P1
But that itself can be re-written without the correlated sub-query...
SELECT
P1.*,
SUM(CASE WHEN status = 3 THEN amount END) AS status_3_total,
SUM(CASE WHEN status = 4 THEN amount END) AS status_4_total
FROM
table_s P1
INNER JOIN
table_s lookup
ON lookup.caseid = P1.caseid
GROUP BY
P1.primary_key
That said, you added another comment that seems to contract your question...
the idea is to select the sum of the amount for each caseid and display it. as caseid - sum
For that you just need an aggregation...
SELECT
caseid,
SUM(amount)
FROM
table_s
GROUP BY
caseid
And if you only want to aggregate where the status is 3 or 4...
SELECT
caseid,
SUM(CASE WHEN status = 3 THEN amount ELSE 0 END) status_3_total
SUM(CASE WHEN status = 4 THEN amount ELSE 0 END) status_4_total
FROM
table_s
GROUP BY
caseid