Sub-select inside aggregate results in bad SQL

If you try something like

```haskell
sum_ (sub_select . from $ \x -> foo)
```

you will get the invalid SQL

```sql
SUM(SELECT foo FROM x)
```

In SQL, we might be able to solve this by writing

```sql
SELECT SUM(foo) FROM x
```

instead (I'm not sure if this always works), but if you're generating terms programmatically, it's possible to run into this case.