It works if you use
LPAD() so the string has 8 characters:
select t.*, DATE(TIMESTAMP_FORMAT(LPAD(decimalField, 8, '0') , 'MMDDYYYY')) from (SELECT 5122019 as decimalField FROM sysibm.sysdummy1 ) t
Here is a db<>fiddle.
You might want to be sure that the
DD component is zero-padded. Otherwise, you cannot reliably make the conversion, because 1112020 could be either November 1st or Jan 11th.
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