Replace string from column with NULL in sql

Question

I have a df:

Col1   col2 
1222    abc
---    bbb
111  nmnm

Whenever I detect --- in Col1 I want to convert that to NULL, how to do that?

Tried so far:

 select replace(Col1, '---', NULL) from df

But even after this the original table is unchanged

Answer 1

You need to update the table:

UPDATE df
SET Col1 = NULL
WHERE Col1 = '---';

As for ad-hoc query:

select replace(Col1, '---', NULL) from df
<->
select nullif(col1, '---') AS col1 from df;