SQL Server 2019

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


i


1

2

3


-- (T-SQL)
IF NOT EXISTS(SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='TAB' AND COLUMN_NAME = 'COL')
BEGIN
   ALTER TABLE tab ADD col NVARCHAR(10);
END;
 SELECT * FROM tab;


i
col


1
null

2
null

3
null

i	col
1	null
2	null
3	null