Postgres 12

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CREATE TABLE tab(i INT);
    INSERT INTO tab(i) VALUES(1),(2),(3);
    SELECT * FROM tab;

CREATE TABLE

INSERT 0 3

i

1

2

3

SELECT 3

ALTER TABLE IF EXISTS tab ADD COLUMN IF NOT EXISTS col VARCHAR(10);

ALTER TABLE

 SELECT * FROM tab;

i
col

1
null

2
null

3
null

SELECT 3

i	col
1	null
2	null
3	null