Postgres 15

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select version();

version

PostgreSQL 15.0 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10), 64-bit

SELECT 1

CREATE TABLE x
(
  y CHAR(1)
);

CREATE TABLE

INSERT INTO x VALUES ('x'); 

INSERT 0 1

INSERT INTO x VALUES('y');

INSERT 0 1

SELECT
  y AS the_char
FROM 
  x
ORDER BY the_char;

the_char

x

y

SELECT 2

SELECT
  y AS the_char
FROM 
  x
ORDER BY ASCII(the_char);

ERROR:  column "the_char" does not exist
LINE 5: ORDER BY ASCII(the_char);
                       ^