Query error with ambiguous column name in SQL

Question

I get an Ambiguous column name error with this query (InvoiceID). I can\'t figure out why. They all seem to be joined correctly so why doesn\'t the management studio know to

Answer 1

if you join 2 or more tables and they have similar names for their columns sql server wants you to qualify columns which they belong.

SELECT  ev.[ID]
    ,[Description]
    FROM   [Events] as ev 
    LEFT JOIN  [Units] as un ON ev.UnitID = un.UnitId  

if Events and Units tables has same column name (ID) SQL server wants you to use aliases.