PostgreSQL: using case to compare values between columns

Question

I would like to compare two columns, A and B.

If Col A and Col B share the same value, I would like Col A to have a no value set. If Col A and Col B do not share the same value, I would like Col A to keep the original value it has.

Below is what I attempted. However, it returns "true" for all entries in the case column - Col A (corrected).

select *,
    case when 'Col A' = 'Col B' 
        then 'Col A' = null 
        else 'Col A' = 'Col A'
        end as "Col A (corrected)"
from my_table

´'Col A'` isn't a column, it's a string literal - and it will never be the same as the string literal 'Col B'. — jarlh
– jarlh, Commented Jan 9, 2022 at 12:59

Sergey · Accepted Answer · 2022-01-09 12:44:58Z

2

select
  case
   when col_a=col_b then null
   else col_a
 end as col_a_corrected
from my_table

answered Jan 9, 2022 at 12:44

Sergey

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Comments

jarlh · Accepted Answer · 2022-01-09 13:03:14Z

-1

Simply do

select *,
       NULLIF(col_a, col_b)
from my_table

The NULLIF returns null if col_a = col_b, otherwise col_a is returned.

https://www.postgresql.org/docs/current/functions-conditional.html#FUNCTIONS-NULLIF

answered Jan 9, 2022 at 13:03

jarlh

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