[英]How to subtract second row from first row and generate a output column in mysql
我有一种情况,请考虑以下查询:
select
A.cakename, A.status, B.ordertime
from
Cake as A
inner join
Orders as B on A.cakeid = B.cakeid
该查询的输出
cakename, status, ordertime
Apple Pie, available, 2014-03-20 22:34:46
Apple Pie, available, 2014-10-10 04:00:00
Apple Pie, available, 2015-03-20 22:34:46
Brownie, available, 2014-03-20 22:35:07
Brownie, available, 2015-05-20 22:35:07
Brownie, available, 2014-03-26 22:36:07
Brownie, available, 2015-05-20 14:35:48
Brownie, available, 2015-05-20 22:35:07
Brownie, available, 2015-05-10 22:35:07
现在,我想减去ordertime row n+1 - ordertime row n
并创建一个单独的列来存储结果。 现在您可以忽略最后一行,因为将没有下一行。
例如: 2014-10-10 04:00:00
04: 2014-03-20 22:34:46
: 2014-03-20 22:34:46
式将用于第一行,其结果将存储在新列中,以difference of consecutive ordertime
。 谁能给我一些解决这种情况的指示?
如果您可以帮助创建一个auto increment column
,这会很棒,因为那样我就可以自我加入并有所作为。
您可以添加一个额外的计算字段:
select
A.cakename, A.status, B.ordertime,
(SELECT MAX(ordertime) FROM Orders AS C WHERE C.ordertime < B.ordertime) AS prev
from
Cake as A
inner join
Orders as B on A.cakeid = B.cakeid
ORDER BY ordertime;
但是您最好在ordertime
上有一个索引:
CREATE INDEX ord_ndx ON Orders(ordertime, cakeid);
实际上,您可以直接计算与上一次的差值 (假设它不是NULL;您必须为此提供默认值IF)。
但是,根据您的体系结构,通过添加一列( previous_order
)可能会更好。 插入订单后,您便知道需要什么。 使用TRIGGER
参见以下示例:
CREATE TABLE test ( itemid integer, ordertime timestamp, previous datetime );
CREATE INDEX test_ndx ON test (itemid, ordertime);
CREATE TRIGGER test_set_prev
BEFORE INSERT ON test
FOR EACH ROW
SET NEW.previous = (
SELECT MAX(ordertime) FROM test WHERE itemid = NEW.itemid
);
现在您的表格会自动工作 。 时间戳列会自动更新,因此会把previous
列:
insert into test (itemid) values (2);
select sleep(2);
insert into test (itemid) values (2);
+--------+---------------------+---------------------+
| itemid | ordertime | previous |
+--------+---------------------+---------------------+
| 2 | 2015-10-21 09:30:44 | NULL |
| 2 | 2015-10-21 09:30:46 | 2015-10-21 09:30:44 |
+--------+---------------------+---------------------+
它还适用于多个插入:
insert into test (itemid) values (3), (2), (3);
SELECT t.*,
IF(@prev IS NULL ,0 , TIMESTAMPDIFF(SECOND,t.ordertime,@prev)) diff,
@prev = t.ordertime
FROM (
SELECT
A.cakename, A.status, B.ordertime
FROM Cake as A
INNER JOIN Orders as B
ON A.cakeid = B.cakeid
ORDER BY ordertime
) t
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