MySQL 8.0

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CREATE TABLE MY_TABLE
(
  column1 INT NOT NULL,
  column2 VARCHAR(50) NULL
);
INSERT INTO MY_TABLE (column1, column2)
VALUES
  (1, '123abcde'),
  (2, '789xyz')
;

Records: 2  Duplicates: 0  Warnings: 0


SELECT
  t.column1,
  t.column2,
  c.custom_column
FROM
  MY_TABLE AS t
  CROSS JOIN LATERAL (SELECT SUBSTRING(column2, 4) AS custom_column) AS c
WHERE
  c.custom_column = 'xyz'
;


column1
column2
custom_column


2
789xyz
xyz