[GitHub] [arrow-datafusion] xudong963 edited a comment on issue #1082: Implement the rest of Set Operators

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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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