Add fix routine for L025

This query has two problems:

$ echo 'select * from (select 1 as a) subquery' | sqlfluff lint -
== [stdin] FAIL
L:   1 | P:  31 | L011 | Implicit aliasing of table not allowed. Use explicit
                       | `AS` clause.
L:   1 | P:  31 | L025 | Alias 'subquery' is never used in SELECT statement.

It gets fixed as such

$ echo 'select * from (select 1 as a) subquery' | sqlfluff fix - --rules L011,L010,L025                                
select * from (select 1 as a) as subquery

But if you lint the fixed sql, L025 remains!

$ echo 'select * from (select 1 as a) subquery' | sqlfluff fix - --rules L011,L010,L025 | sqlfluff lint -
== [stdin] FAIL
L:   1 | P:  34 | L025 | Alias 'subquery' is never used in SELECT statement.

The conjunction of these two errors shows that the “real” fix would be this query

$ echo 'select * from (select 1 as a)' | sqlfluff lint -
# no violations!

Should fluff remove the implicit alias if it is never used?

[barrywhart]

As it turns out, the example query and fix is invalid syntax in every database I tested (MySQL, Postgres, T-SQL). Basically, “derived queries” require an alias. See issue #3545. For that issue, I’m adding an exception to the rule, so derived table aliases will never be removed, whether they are used or not.

[nolanbconaway]

Great! If you’re interested some time, I’d be happy to schedule an hour to pair program on an issue (Zoom screen share or something like that). Maybe that could be step 1 towards the “new contributor” docs you suggested. If you’re interested, ping me on Slack and we’ll set something up.

That’s really nice of you! I’ll take you up on it when i get some more bandwidth