Question

我的select语句有点问题。我需要从我的表中为每种类型的手机选择最新记录，所以我尝试这段代码：

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;

现在的输出是这样的：

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

但我需要这样的东西：

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

我该怎么做？

Answer 1

用rownum包裹它＆lt; 2并添加order by以获得最大的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;

您可以更改order by子句以获取除c.record_id desc

之外的其他订单

Answer 2

您想使用row_number()。像这样：

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;

注意：

从不在FROM子句中使用逗号。始终使用正确的JOIN语法。
CASE优于DECODE()，因为它是ANSI标准SQL，因此它是标准语言的一部分。
您可以使用窗口函数来大大简化查询并提高性能。
您可以删除子查询中order_id上的条件，以获取所有订单的行。
您希望GROUP BY仅包含定义所需结果集中每一行的列。

Answer 3

更多解决方案：

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
;

将具有空值的多个行合并为一个

3 个答案: