markdown sql，count，distinct

Posted 2021-05-05

## Hana not support count(distinct xx) over (partition by) syntax

I have a table show as: 
```
sqlite> select * from user;
pid         uname       email     
----------  ----------  ----------
1           u1          E         
1           u2          e         
2           u21         e21       
3           u31         e21       
4           u41         e41       
4           u42         E41       
5           u52         E41       
Run Time: real 0.000 user 0.000127 sys 0.000026

```
query this table and find out records that 
1. lower email is duplicated per pid
2. two user belong to same person with same email is allowed

if you are using Oracle, then you can do some query as: 
```sql
SELECT
    *
FROM
    (
        SELECT
            *,
            COUNT(DISTINCT pid) OVER(
                PARTITION BY lower(email)
            ) AS cnt
        FROM
            user
    )
WHERE
    cnt > 1;
```
but many other db is not support such kind of syntax, the 'count(distinct xxx)'.
so here is two solution for you:
```sql
--solution 01
SELECT
    *
FROM
    (
        SELECT
            *,
            ( DENSE_RANK() OVER(
                PARTITION BY lower(email)
                ORDER BY
                    pid
            ) + DENSE_RANK() OVER(
                PARTITION BY lower(email)
                ORDER BY
                    pid DESC
            ) - 1 ) AS cnt
        FROM
            user
    )
WHERE
    cnt > 1;
```
Refer to [StackOverflow](https://stackoverflow.com/questions/11202878/partition-function-count-over-possible-using-distinct)

```sql
--solution 02
SELECT
    *,
    MAX(cnt) OVER(
        PARTITION BY lower(email)
    ) AS distinct_cnt
FROM
    (
        SELECT
            *,
            DENSE_RANK() OVER(
                PARTITION BY lower(email)
                ORDER BY
                    pid
            ) AS cnt
        FROM
            user
    )
WHERE
    distinct_cnt > 1;
```
Refer to [SQL Server Post](https://social.msdn.microsoft.com/Forums/sqlserver/en-US/8868da5f-873f-4584-8831-4fcc7a620e74/count-distinct-over-partition-syntax-error?forum=transactsql)