Postgres 14

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CREATE TABLE t (
  ID SERIAL,
  CharID varchar(10) GENERATED ALWAYS AS (
      CHR(65 + ID / 260000) ||
      CHR(65 + MOD(ID / 10000, 26)) ||
      lpad(MOD(id, 260000)::text, 4, '0')
    ) STORED
);

CREATE TABLE


INSERT INTO t (id)
VALUES (1),(2),(9999),(10000),(260000),(260001)

INSERT 0 6


SELECT *
FROM t


id
charid


1
AA0001

2
AA0002

9999
AA9999

10000
AB1000

260000
BA0000

260001
BA0001



SELECT 6