xudong963 edited a comment on issue #1082:
URL:
https://github.com/apache/arrow-datafusion/issues/1082#issuecomment-942442780
> perhaps since `NULL = NULL` is not true, then semi join with equality
isn't correct. 🤔
Yes.
```sql
postgres=# select * from bar;
a | b
---+---
1 | 2
| 3
3 | 4
(3 rows)
postgres=# select * from foo;
a | b
---+---
1 | 2
| 3
(2 rows)
-- intersect
select a from foo intersect select a from bar;
/**
a
---
1
(2 rows)
**/
-- equivalent transformation ? (x) since NULL = NULL is not true
select distinct a from foo where a in (select a from bar);
/**
a
---
1
(1 row)
**/
```
psql doesn't seem to have direct `SEMI JOIN`. It uses `IN` to do the same
thing as `SEMI JOIN`. So how to use `is distinct from` in `select distinct a
from foo where a in (select a from bar);`?
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