Retrieve one of two possible values from 1 table in INFORMIX
This answer assumes that your table Ex1 has columns:


key_col — the key column.
flag_col — the flag column, containing L or P (with a check constraint).
data_col — a proxy for the unidentified set of columns containing the other data.
Primary key on key_col and flag_col.


A view like this should work (I didn't say it would work fast):

CREATE VIEW Relevant_Ex1(key_col, flag_col, data_col) AS
    SELECT A.Key_Col, A.Flag_Col, A.Data_Col
      FROM Ex1 AS A
      JOIN (SELECT B.Key_Col, MIN(Flag_Col) AS Flag_Col
              FROM Ex1 AS B
             GROUP BY B.Key_Col
           ) AS C
        ON A.Key_Col = C.Key_Col AND A.Flag_Col = C.Flag_Col;


This uses the fact that the flag column contains either L or P and L sorts before P.  So, if there are two rows for a given key, the value L will be selected, else the value P will be selected.