Unexpected output in Oracle SQL

Question

On executing the query

select * from employee
where salary <= previous_salary

I'm getting a wrong output as follows (it's returning the entire table). I'm doing this in Oracle Application Express Edition

ID  NAME    SALARY  DEPT    PREVIOUS_SALARY
1   nadeem  1000    10      1500
2   kutu    1000    10      1500
3   aisha   1500    20      500
4   aisha   1500    20      500

and on executing this query

select * from employee
where salary >= previous_salary

I get no data found.

Answer 1

You've probably used a CHAR or VARCHAR2 type for the SALARY and PREVIOUS_SALARY columns.

When compared as strings, the data should work as you claim it does.

If you change the columns containing numbers to a numeric type, then the numeric comparisons will work as you intended.