How to create a new column like a self-join in SQL?

Assuming the blanks in T1[Staff Name] are nulls, you can use the following measure:

Started Staff = 
CALCULATE(
    FIRSTNONBLANK( T1[Staff Name], T1[Staff Name] ),
    ALL( T1[Action] )
)

In a table where you set filter of T1[Action] = End:

Hi OceanExplorer ,
Thank you MarkLaf for the helpful response!

I tried to recreate it on my local with the sample data.So that I used below measure to create a calculated column:

Started Staff =

VAR CurrentID = T1[ID]

VAR StaffName =

    CALCULATE(

        LASTNONBLANK(T1[Staff Name], 1),

        FILTER(

            T1,

            T1[ID] = CurrentID &&

            T1[Action] IN {"Start", "Transfer"}

        )

    )

RETURN

    IF(T1[Action] = "End", StaffName)

Here is the screenshot and pbix for your reference:

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Thank you for being a valued member in Microsoft Fabric Community Forum!

Regards,
Pallavi.