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create table TAB(ID int)
declare
t NUMBER;
w NUMBER;
begin
-- checking if table exists
select count(*) into t from ALL_TABLES where TABLE_NAME='TAB';
-- checking if column does not exist
select count(*) into w
from ALL_TAB_COLUMNS
where TABLE_NAME='TAB' AND COLUMN_NAME = 'COLUMN_1';
if (t>0) AND (w=0) then
EXECUTE IMMEDIATE 'alter table TAB add COLUMN_1 varchar(20)';
end if;
end;
/
1 rows affected
select * from TAB