SQL Server 2014

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CREATE TABLE elbat
             (sitecode varchar(1));
INSERT INTO elbat
            (sitecode)
            VALUES ('A'),
                   ('A'),
                   ('A'),
                   ('B'),
                   ('C'),
                   ('C'),
                   ('D'),
                   ('D');
8 rows affected

SELECT sitecode,
       dense_rank() OVER (ORDER BY sitecode) % 2 helper
       FROM elbat;


sitecode
helper


A
1

A
1

A
1

B
0

C
1

C
1

D
0

D
0