[Sqlalchemy-tickets] Issue #2974: Union queries fail if there are literal unlabeled columns (zzzeek/sqlalchemy)

[yajo <iss...@bi...>]

New issue 2974: Union queries fail if there are literal unlabeled columns
https://bitbucket.org/zzzeek/sqlalchemy/issue/2974/union-queries-fail-if-there-are-literal

yajo:

See this union query:

```
#!python

queries = list()

queries.append(sql.select(
    columns=[
        sql.expression.literal("column_1").label("label_1"),
        sql.expression.literal("column_2").label("label_2"),
    ],
    bind=conn.engine
))

queries.append(sql.select(
    columns=[
        sql.expression.literal("column_1"),
        sql.expression.literal("column_2"),
    ],
    bind=conn.engine
))

one = sql.expression.union(*queries)
```

It fails with this exception:

```
sqlalchemy.exc.ArgumentError: All selectables passed to CompoundSelect must have identical numbers of columns; select #1 has 2 columns, select #2 has 1
```

But both have 2 columns, it's just that the 2nd one is not labeled (because it is not needed in union queries, at least with MySQL).

**Workaround:** labeling all columns in 2nd query.

Notice that *real* columns, wich automatically seem to get a name, don't expose this weird behavior.