Oracle SQL Developer（4.0.0.12）

Question

First time posting here, hopes it goes well.

I try to make a query with Oracle SQL Developer, where it returns a customer_ID from a table and the time of the payment from another. I'm pretty sure that the problems lies within my logicflow (It was a long time I used SQL, and it was back in school so I'm a bit rusty in it). I wanted to list the IDs as DISTINCT and ORDER BY the dates ASCENDING, so only the first date would show up.

However the returned table contains the same ID's twice or even more in some cases. I even found the same ID and same DATE a few times while I was scrolling through it.

If you would like to know more please ask!

SELECT DISTINCT
    FIRM.customer.CUSTOMER_ID,
    FIRM.account_recharge.X__INSDATE FELTOLTES
FROM
    FIRM.customer
        INNER JOIN FIRM.account
        ON FIRM.customer.CUSTOMER_ID = FIRM.account.CUSTOMER
            INNER JOIN FIRM.account_recharge
            ON FIRM.account.ACCOUNT_ID = FIRM.account_recharge.ACCOUNT
WHERE
    FIRM.account_recharge.X__INSDATE BETWEEN TO_DATE('14-01-01', 'YY-MM-DD') AND TO_DATE('14-12-31', 'YY-MM-DD')
ORDER
    BY FELTOLTES

Answer 1

SELECT DISTINCT FIRM.customer.CUSTOMER_ID, FIRM.account_recharge.X__INSDATE FELTOLTES

Distinct is applied to both the columns together, which means you will get a distinct ROW for the set of values from the two columns. So, basically the distinct refers to all the columns in the select list.

It is equivalent to a select without distinct but a group by clause.

It means,

select distinct a, b....

is equivalent to,

select a, b...group by a, b

If you want the desired output, then CONCATENATE the columns. The distict will then work on the single concatenated resultset.

Answer 2

Your select works like this because a CUSTOMER_ID indeed has more than one X__INSDATE , therefore the records in the result will be distinct. If you need only the first date then don't use DISTINCT and ORDER BY but try to select for MIN(X__INSDATE) and use GROUP BY CUSTOMER_ID .