ORA-00918 also referred to as an ORA 918 is a common issue/error that occurs during the development of queries through joins of multiple tables and other issues listed below.  Most of the time an Ora-00918 occurs due to the fact that a column with the same name exists in both tables. Another not so well documented reason for this issue to occur is the initial definition of views using a "select *" followed by addition of columns to the underlying table.

Issue 1: ORA-00918 is a common error. Nothing to worry here. It happens when a column name used in a query exists in more than one table during a join operation.  So, this reference leads to the ambiguity causing oracle to throw an ORA-00918 error.

Issue 2: There are also reports of ORA-00918 error occuring when "full outer join" ANSI Syntax is used.  

Issue 3: When new columns are added to tables in a schema and the view on top of the table is created without specific definition, ie, create view as select * from table, ora-00918 is known to occur. This is not well documented by oracle, but its a best practice to create views using a full definition instead of select *.

For example, you are joining a table emp and table dept on deptid. Lets assume that the deptid column exists in both tables.  Look at the example below for more info on when an ORA-00918 error occurs and how to fix it.

ORA-00918 Error can occur when you write the following query.. the syntax creating the ORA-00918 is highlighted in red and the fix is highlighted in green.

select empid, deptid, name from emp e, dept d where e.deptid =d.deptid;

select empid, d.deptid, name from emp e, dept d where e.deptid = d.deptid;