Postgres 9.6

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CREATE TABLE alpha (
    letter character varying
);

INSERT INTO alpha VALUES
('A'),
('B'),
('Z'),
('a'),
('z')
5 rows affected

SELECT
    letter,
    ascii(upper(letter)) - 64
FROM
    alpha


letter
?column?


A
1

B
2

Z
26

a
1

z
26


SELECT 
    letter, 
    strpos('ABCDEFGHIJKLMNOPQRSTUVWXYZ', upper(letter))
FROM 
    alpha


letter
strpos


A
1

B
2

Z
26

a
1

z
26