Oracle SQL - Scanning attribute Changes - sql

Question

I have the following employee table
EMPID RECORD_DATE DEPARTMENT
123456 2020-01-01 HR
123456 2020-02-01 HR
123456 2020-03-01 FINANCE
123456 2020-04-01 FINANCE
987654 2020-01-01 HR
987654 2020-02-01 HR
987654 2020-03-01 HR
987654 2020-04-01 LEGAL
Using Oracle PL/SQL, I need to build an expression to ascertain a list of employee movement, specifically that have moved from HR to any other (non-HR) department.
Expected result:
EMPID MOVEMENT_DATE DEPT_BEFORE DEPT_AFTER
123456 2020-03-01 HR FINANCE
987654 2020-04-01 HR LEGAL
I know you can use the Lead or Lag function, but it's a little off for me:
SELECT
,EMP
,RECORD_DATE
,LAG(DEPARTMENT, 1, 0) OVER (PARTITION BY EMP ORDER BY RECORD_DATE) PREV
FROM EMP
Here are some values to work with:
CREATE TABLE #EMP
(
EMP VARCHAR(30) NOT NULL ,
RECORD_DATE DATE NOT NULL ,
DEPARTMENT VARCHAR(30) NOT NULL
);
INSERT INTO #EMP (EMP, DATE_WORKED, CITY)
VALUES
('123456','2020-01-01','HR'),
('123456','2020-02-01','HR'),
('123456','2020-03-01','FINANCE'),
('123456','2020-04-01','FINANCE'),
('987654','2020-01-01','HR'),
('987654','2020-02-01','HR'),
('987654','2020-03-01','HR'),
('987654','2020-04-01','LEGAL')

Answer 1

You could do it using LAG function:
WITH data AS(
SELECT 123456 EMPID, DATE '2020-01-01' RECORD_DATE, 'HR' DEPARTMENT FROM dual UNION ALL
SELECT 123456 EMPID, DATE '2020-02-01' RECORD_DATE, 'HR' DEPARTMENT FROM dual UNION ALL
SELECT 123456 EMPID, DATE '2020-03-01' RECORD_DATE, 'FINANCE' DEPARTMENT FROM dual UNION ALL
SELECT 123456 EMPID, DATE '2020-04-01' RECORD_DATE, 'FINANCE' DEPARTMENT FROM dual UNION ALL
SELECT 987654 EMPID, DATE '2020-01-01' RECORD_DATE, 'HR' DEPARTMENT FROM dual UNION ALL
SELECT 987654 EMPID, DATE '2020-02-01' RECORD_DATE, 'HR' DEPARTMENT FROM dual UNION ALL
SELECT 987654 EMPID, DATE '2020-03-01' RECORD_DATE, 'HR' DEPARTMENT FROM dual UNION ALL
SELECT 987654 EMPID, DATE '2020-04-01' RECORD_DATE, 'LEGAL' DEPARTMENT FROM dual
)
SELECT * FROM(
SELECT
EMPID,
RECORD_DATE MOVEMENT_DATE,
LAG(DEPARTMENT) OVER (PARTITION BY EMPID ORDER BY RECORD_DATE) DEPARTMENT_BEFORE,
DEPARTMENT DEPARTMENT_AFTER
FROM data
)
WHERE DEPARTMENT_BEFORE <> DEPARTMENT_AFTER;
EMPID MOVEMENT_DATE DEPARTMENT_BEFORE DEPARTMENT_AFTER
---------- --------------- ----------------- -----------------
123456 2020-03-01 HR FINANCE
987654 2020-04-01 HR LEGAL