SQL error: Invalid identifier

Answer 1

I don't know which DBMS you're using, but some will get upset that you are using .avg (because avg is reserved) and require you to escape it.

Try changing it from avg to average, just as a test:

SELECT comp_table.average
FROM (SELECT column_1, avg(column_2) as average
      FROM table_1, GROUP BY column_1) comp_table

Answer 2

What database server are you using? AVG is a built-in function in all the ones I know of, so you would need to escape it correctly - which depends on database server. In MS SQL Server it's [avg]

Answer 3

When the column name is not enclosed in "double quotes", the name is normalized to uppercase; therefore, you were asking for column "AVG", whereas the column name is actually "avg":

select comp_table."avg"
from (select column_1,avg(column_2) as "avg"
  from table_1, group by column_1) comp_table