AMBIGUOUS COLUMN DEFINITION

Question

I'M GETTING "AMBIGUOUS COLUMN DEFINITION" WHICH IS CLEARLY WRONG SO HOW DO I FIX IT ?

Answer 1

user8093006 wrote:
I'M GETTING "AMBIGUOUS COLUMN DEFINITION" WHICH IS CLEARLY WRONG SO HOW DO I FIX IT ?
To explain what is happening. It is called name resolution.
You use object names in code - where the object you refer to by name can be a function, procedure, variable, parameter, constant, global, etc. In SQL code, this also includes columns, tables, views, and so on.
You get name collisions. This is when 2 objects have the same name. In SQL this often happens (and should happen!). The DEPT table has a column (primary key) called DEPT_ID. The EMP table indicates which department the employee works for, via its column (foreign key) DEPT_ID.
So the name reference DEPT_ID is used multiple times by multiple tables.
When you use DEPT_ID in a SQL statement, the SQL engine needs to resolve that name and determine to which column in which table you are referring to. This name resolution has a scope. The scope says which tables need to be considered (checked) for having a DEPT_ID column. The scope is determined (in this case) by the tables your SQL is selecting from.
Now if your scope includes both EMP and DEPT tables, there is a name collision - and the SQL engine does not know whether your DEPT_ID name refers to the column in the EMP table, or the column in the DEPT table.
And this is the error you are seeing. The SQL engine is saying that your name reference DEPT_ID is ambiguous, as it exists as different objects within that SQL's resolution scope.
The fix is explicit scope. Where you do not use DEPT_ID and have the SQL engine figure out in which table it is (within the current scope) - instead you tell that to the SQL engine and indicate which table it is in.
There are 2 method to use explicit scope in SQL. You can do that via using the name of the owner of the object - in this case, the name of the table that owns that column:
select
    dept.dept_id,
    dept.department_name,
    emp.surname
from dept, emp
where dept.dept_id = emp.dept_id
Or you can use aliasing (less typing and a bit more flexible ito code maintenance):
select
    d.dept_id,
    d.department_name,
    e.surname
from dept d, emp e
where d.dept_id = e.dept_id
Explicit scope is usually a good thing. Where implicit scope is needed, a feature such as synonyms should be considered (it allows explicit scope to be used, while enabling one to easily change the object that the name resolves to).