PROBLEM:

While dropping a column, got the below error.

SQL> ALTER TABLE dbaclass.EMPLIST DROP (empname);
ALTER TABLE dbaclass.EMPLIST DROP (empname)
                                   *
ERROR at line 1:
ORA-12991: column is referenced in a multi-column constraint

SOLUTION:

If the column is part of a multi-column constraint, then we have to drop all the columns of that constraint. Dropping a single column will throw an error.

1. Check whether the column is part of Multi-column constraint or not.

SQL>  select COLUMN_NAME,CONSTRAINT_NAME,POSITION from dba_cons_columns where table_name='EMPLIST';

COLUMN_NAME   CONSTRAINT_NAME   POSITION
------------- --------------- ----------
EMPNAME       EMP_PK                   1
EMPNUM        EMP_PK                   2

We can that the constraint EMP_PK has two columns.

2. So the only option is to drop both the columns. 

SQL> ALTER TABLE dbaclass.EMPLIST DROP (empname,empnum);

Table altered.