[sql] How to select the comparison of two columns as one column in Oracle

I cannot figure out how to add a column to my SELECT query indicating whether two columns contain the same data in Oracle.

I would like to write a query like:

select column1, column2, column1=column2 from table

and, if I have this table:

+---------+---------+
| column1 | column2 |
+---------+---------+
| value1  | value1  |
| value2  | value3  |
| value4  | value4  |
+---------+---------+

get a result like:

+---------+---------+-----------------+
| column1 | column2 | column1=column2 |
+---------+---------+-----------------+
| value1  | value1  | true            |
| value2  | value3  | false           |
| value4  | value4  | true            |
+---------+---------+-----------------+

What is the correct syntax to do this?

I stopped using DECODE several years ago because it is non-portable. Also, it is less flexible and less readable than a CASE/WHEN.

However, there is one neat "trick" you can do with decode because of how it deals with NULL. In decode, NULL is equal to NULL. That can be exploited to tell whether two columns are different as below.

select a, b, decode(a, b, 'true', 'false') as same
  from t;

     A       B  SAME
------  ------  -----
     1       1  true
     1       0  false
     1          false
  null    null  true  

select column1, coulumn2, case when colum1=column2 then 'true' else 'false' end from table;

HTH