Combine several rows with null values into one

问题

I have a little problem with my select statement. I need to select the latest record for each type of phones from my table, so I try this code:

CREATE TABLE contacts(
tel_number VARCHAR2(14),
tel_type NUMBER,
row_id_con NUMBER,
record_id NUMBER
);

CREATE TABLE orders(
order_id VARCHAR2(9),
row_id_ord NUMBER
);

INSERT INTO contacts VALUES('444-444-444', 1, 1, 1);
INSERT INTO contacts VALUES('22-22-22', 2, 1, 2);
INSERT INTO contacts VALUES('555-555-555', 1, 1, 3);

INSERT INTO orders VALUES('111111111', 1);

SELECT ord.order_id, 
       DECODE(con.tel_type, 1, con.tel_number) AS number1,
       DECODE(con.tel_type, 2, con.tel_number) AS number2
FROM  (SELECT MAX(c.record_id) as max_row_id
       FROM contacts c, orders o
       WHERE c.row_id_con = o.row_id_ord
       and o.order_id = '111111111'
       GROUP BY c.tel_type) c, contacts con, orders ord
WHERE  con.row_id_con = ord.row_id_ord 
       and con.record_id = c.max_row_id
GROUP BY ord.order_id, con.tel_type, con.tel_number;

The output now is like that:

order_id   number1      number2
111111111  null         22-22-22
111111111  555-555-555  null

But I need something like that:

order_id   number1      number2
111111111  555-555-555  22-22-22

How can I do that?

回答1:

On more solution:

SELECT o.order_id,
       max( case c.tel_type when 1 then tel_number end ) 
           KEEP ( DENSE_RANK LAST ORDER BY row_id_con NULLS FIRST ) as number1,
       max( case c.tel_type when 2 then tel_number end ) 
           KEEP ( DENSE_RANK LAST ORDER BY row_id_con NULLS FIRST ) as number2
FROM orders o
LEFT JOIN contacts c ON c.row_id_con = o.row_id_ord
GROUP BY o.order_id
;

回答2:

Wrap it with rownum < 2 and add order by to get largest record_id:

select * from (
SELECT ord.order_id, 
       DECODE(con.tel_type, 1, con.tel_number) AS number1,
       DECODE(con.tel_type, 2, con.tel_number) AS number2
FROM  (SELECT MAX(c.record_id) as max_row_id
       FROM contacts c, orders o
       WHERE c.row_id_con = o.row_id_ord
       and o.order_id = '111111111'
       GROUP BY c.tel_type) c, contacts con, orders ord
WHERE  con.row_id_con = ord.row_id_ord 
       and con.record_id = c.max_row_id
GROUP BY ord.order_id, con.tel_type, con.tel_number
order by c.record_id desc
)
where rownum < 2;

You can change the order by clause to get other order than c.record_id desc

回答3:

You want to use row_number(). Something like this:

SELECT o.order_id,
       MAX(CASE WHEN tel_type = 1 AND seqnum = 1 THEN tel_number END) as number1,
       MAX(CASE WHEN tel_type = 2 AND seqnum = 1 THEN tel_number END) as number2
FROM  (SELECT o.order_id, c.*,
              ROW_NUMBER() OVER (PARTITION BY o.order_id, c.tel_type
                                 ORDER BY record_id DESC
                                ) as seqnum
       FROM contacts c JOIN
            orders o
            ON c.row_id_con = o.row_id_ord AND
               o.order_id = '111111111'
      ) o
GROUP BY order_id;

Notes:

Never use commas in the FROM clause. Always use proper, explicit JOIN syntax.
CASE is preferable to DECODE() because it is ANSI standard SQL, so it is part of the standard language.
You can use window functions to greatly simplify the query and improve performance.
You can remove the condition on order_id in the subquery to get rows for all orders.
You want the GROUP BY to only contain the columns that define each row in your desired result set.

来源：https://stackoverflow.com/questions/45400335/combine-several-rows-with-null-values-into-one

标签

Oracle

oracle-sqldeveloper