Basic of DAX query

// How to check null
ISBLANK(EmployeeDetails[DateofLeaving])

//if condition
IF(ISBLANK(EmployeeDetails[DateofLeaving]),TODAY(),EmployeeDetails[DateofLeaving])

//calculate finyear 

Finyear = 
var TillDate = IF(ISBLANK(EmployeeDetails[DateofLeaving]),TODAY(),EmployeeDetails[DateofLeaving])
VAR currentYear = IF(MONTH(TillDate) > 3, YEAR(TillDate), YEAR(TillDate)-1)
VAR check = IF(MONTH(TillDate) > 3, RIGHT(YEAR(TillDate) + 1, 2), RIGHT(YEAR(TillDate), 2))
RETURN
CONCATENATE(currentYear, check)

//Switch used

Salary Slab = 
SWITCH(
    TRUE(),
    PAY_SalaryDetails[GrossSalary] > 0 && PAY_SalaryDetails[GrossSalary] <= 10000, "0-10K",
    PAY_SalaryDetails[GrossSalary] > 10000 && PAY_SalaryDetails[GrossSalary] <= 20000, "10-20K",
    PAY_SalaryDetails[GrossSalary] > 20000 && PAY_SalaryDetails[GrossSalary] <= 30000, "20-30K",
    PAY_SalaryDetails[GrossSalary] > 30000 && PAY_SalaryDetails[GrossSalary] <= 40000, "30-40K",
    PAY_SalaryDetails[GrossSalary] > 40000 && PAY_SalaryDetails[GrossSalary] <= 50000, "40-50K",
    PAY_SalaryDetails[GrossSalary] > 50000 && PAY_SalaryDetails[GrossSalary] <= 60000, "50-60K",
    TRUE(), "60K"
)