SQL & PL/SQL

Here my requirement is to get the angle between two x,y coordinates where they are from a poly line.
Pictorial representation is shown for better understanding

(x2,y2) ..................................... (x3,y3)
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(x1,y1)

I want to find angle between (x2,y2) and (x3,y3) or (x1,y1) and (x2,y2). Can you please help me in writing a query to find angle between two vertices of a poly line.

Thanks in Advance.