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PostgreSQL: Compare and change values in the same column based on conditions

January 9, 2022

I have data in the form:

Name   Category  my_value
Ana    A         42
Ana    B         33
Bob    A         33
Bob    B         33
Carla  A         42
Carla  B         33

I would like the following to happen for the same Name:

when A and B share the same value, A has no value associated to it (in col my_alue).
when A an B are different, A keeps its value and B has no value associated to it (in col my_value).

I’ve attempted:

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

Obviously wrong… not sure how I can also implement the condition to set B as null if the values are different. And how to implement a group by here to compare the categories for the same name…

Ideally, This is the output I’m after (changes in the same column as I have more categories for each name in it, i.e. C, D, E… – only need changes in A and B)

Name   Category  Value
Ana    A         42                      
Ana    B                                 
Bob    A                                 
Bob    B         33                                       
Carla  A         42    
Carla  B

>Solution :

Join to itself where the other row is the A/B compliment:

select
  t1.Name,
  t1.Category,
  case
    when t1.my_value = t2.my_value and t1.Category = 'A' then null
    when t1.my_value != t2.my_value and t1.Category = 'B' then null
    else t1.my_value
  end as my_value
from my_table t1
left join my_table t2 on t2.Name = t1.Name
  and t2.Category != t1.Category
  and t2.Category in ('A', 'B')
  and t1.Category in ('A', 'B')

See live demo.