Limit query result - mysql

Question

Given the schema and data below,
CREATE TABLE `Payroll` (
`payId` int,
`payName` varchar(20),
`groupId` int,
`startDate` date ,
`endDate` date,
`paymentDate` date
);
insert into Payroll values
(20,'June A',2,'2022-06-01','2022-06-30','2022-06-30'),
(21,'July A',2,'2022-07-01','2022-07-31','2022-07-31'),
(17,'April A',1,'2022-04-01','2022-04-30','2022-04-30'),
(18,'May A',1,'2022-05-01','2022-05-31','2022-05-31'),
(19,'July B',1,'2022-07-01','2022-07-31','2022-07-31')
;
CREATE TABLE `PayrollItems` (
`payId` int NOT NULL,
`employeeId` int ,
`payCategory` varchar(45) ,
`value` decimal(15,4) NOT NULL
);
insert into PayrollItems values
(20,12,'salary',200),
(20,12,'housing',500),
(20,13,'salary',400),
(20,14,'salary',1300),
(21,12,'salary',200),
(21,12,'housing',500),
(21,13,'salary',400),
(21,14,'salary',1300),
(18,13,'salary',400),
(18,13,'housing',1300),
(19,14,'salary',500),
(19,14,'housing',1200),
(17,14,'salary',700),
(17,14,'housing',1000)
;
How should i enhance the below query so that i get the previous payid row only, previous pay id would be a payid that has the same group id but a payment date dated prior, having said that i would want the one payid dated prior.
SELECT distinct
a.payId,a.payName, b.*
FROM
(SELECT
Payroll.payId,
employeeId,
payName,
groupId,
paymentDate,
SUM(value * (payCategory = 'housing')) housing,
SUM(value * (payCategory = 'salary')) salary
FROM
PayrollItems
JOIN Payroll ON (Payroll.payid = PayrollItems.payId)
GROUP BY Payroll.payId , employeeId , groupId , paymentDate,payName) a
JOIN
(SELECT
Payroll.payId,
employeeId,
payName,
groupId,
max(paymentDate) paymentDate,
SUM(value * (payCategory = 'housing')) housing,
SUM(value * (payCategory = 'salary')) salary
FROM
PayrollItems
JOIN Payroll ON (Payroll.payid = PayrollItems.payId)
GROUP BY Payroll.payId , employeeId , groupId , paymentDate,payName) b ON b.groupId = a.groupId
AND b.paymentDate < a.paymentDate
order by a.payId,b.payId,b.employeeId
;
In the result shown below i do not need the row marked in red to appear
dbfiddle

Answer 1

I think you need to learn how to use MySQL 8.0 window functions.
mysql> select payId, lag(payId) over (partition by groupId order by payId) as prevPayId
from Payroll;
+-------+-----------+
| payId | prevPayId |
+-------+-----------+
| 17 | NULL |
| 18 | 17 |
| 19 | 18 |
| 20 | NULL |
| 21 | 20 |
+-------+-----------+