Postgres有条件的选择？

Question

我真的不知道该怎么形容这个问题没有一个例子，所以这里有一个例子：

我有三个表：

表1：盒

BoxID | Box Name | Box Cost 
1 | Bob's Box | $20 
2 | Matt's Box | $21 
3 | Jacob's Box | $22 
4 | Beth's Box | $23 
5 | Rachel's Box | $24 

表2：Box_ProcessIDs

BoxID | ParentProcessID 
1 | 123 
2 | 456 
3 | 789 
4 | 012 
5 | 234 

表3：Box_Processes

ParentProcess | ChildProcess | Start Time | End Time | ProcessName 
123 | AAA | 1:00 | 1:05 | Invoiced 
123 | AAB | 1:30 | 1:35 | Packed 
123 | BBB | 2:00 | 2:05 | Shipped 

456 | CDD | 3:15 | 3:20 | Invoiced 
456 | DDD | 3:25 | 3:30 | Packaging_Complete 
456 | CCD | 3:35 | 3:40 | Shipped 

789 | EEE | 4:15 | 4:20 | Invoiced 
789 | EEF | 4:25 | 4:30 | Done_Packing 
789 | EFF | 4:35 | 4:40 | Shipped 

我希望输出列是： 箱名称，箱成本，箱装运期限，装盒时间，盒发票时间

我知道我可以使用类似％包％选择我的包装领域，我可以计算出如何使用Date_Part或某些来获得我的持续时间，但我不知道如何将shipping duration,packing duration和invoice duration分成不同的字段。

Answer 1

所以你想要做这样的伪sql？

select b.box_name, b.box_cost, 
bp2.end_time - bp2.start_time as box_shipping_duration, 
bp3.end_time - bp3.start_time as box_packing_duration, 
bp1.end_time - bp1.start_time as box_invoice_duration 
from boxes b 

join box_processids bpid on b.boxid = bpid.boxid 

left outer join box_processes bp1 on bpid.parentprocessid = bp1.parentprocess 
and processname = 'Invoiced' 

left outer join box_processes bp2 on bpid.parentprocessid = bp2.parentprocess 
and processname = 'Shipped' 

left outer join box_processes bp3 on bpid.parentprocessid = bp3.parentprocess 
and processname = 'Packed' 

（需要null检查，妥善处理日期时间，更好的名称等）

Answer 2

首先，你的设计是有缺陷的。 time对此没有好处。你一过午夜就会中断。改为使用timestamp or timestamptz。

接下来，由于在PostgreSQL中未加引号的标识符会自动转换为小写，所以您的命名约定也不好。我将它改编成更有用的东西。然后

CREATE TABLE boxes (box_id int PRIMARY KEY, box_name text, box_cost numeric); 
INSERT INTO boxes VALUES 
    (1, 'Bob''s Box', 20) 
,(2, 'Matt''s Box', 21) 
,(3, 'Jacob''s Box', 22) 
,(4, 'Beth''s Box', 23) 
,(5, 'Rachel''s Box', 24); 

CREATE TABLE box_processids (
    box_id int 
,parentprocess_id int 
,PRIMARY KEY(box_id, parentprocess_id) 
); 

INSERT INTO box_processids VALUES 
    (1, 123) 
,(2, 456) 
,(3, 789) 
,(4, 012) 
,(5, 234); 

CREATE TABLE box_processes (
    parentprocess_id int 
,childprocess_id char(3) 
,start_time timestamp 
,end_time timestamp 
,processname text 
,PRIMARY KEY(parentprocess_id, childprocess_id) 
); 

INSERT INTO box_processes VALUES 
    (123, 'AAA', '2013-2-10 1:00', '2013-2-10 1:05', 'Invoiced') 
,(123, 'AAB', '2013-2-10 1:30', '2013-2-10 1:35', 'Packed') 
,(123, 'BBB', '2013-2-10 2:00', '2013-2-10 2:05', 'Shipped') 

,(456, 'CDD', '2013-2-10 3:15', '2013-2-10 3:20', 'Invoiced') 
,(456, 'DDD', '2013-2-10 3:25', '2013-2-10 3:30', 'Packaging_Complete') 
,(456, 'CCD', '2013-2-10 3:35', '2013-2-10 3:40', 'Shipped') 

,(789, 'EEE', '2013-2-10 4:15', '2013-2-10 4:20', 'Invoiced') 
,(789, 'EEF', '2013-2-10 4:25', '2013-2-10 4:30', 'Done_Packing') 
,(789, 'EFF', '2013-2-10 4:35', '2013-2-10 4:40', 'Shipped'); 

您的查询看起来是这样的：

SELECT b.box_name 
     ,b.box_cost 
     ,(i.end_time - i.start_time) AS box_invoice_duration 
     ,(p.end_time - p.start_time) AS box_packing_duration 
     ,(s.end_time - s.start_time) AS box_shipping_duration 
FROM boxes    b 
LEFT JOIN box_processids bp USING (box_id) 
LEFT JOIN box_processes i ON i.parentprocess_id = bp.parentprocess_id 
          AND i.processname = 'Invoiced' 
LEFT JOIN box_processes p ON i.parentprocess_id = bp.parentprocess_id 
          AND p.processname = 'Packed' 
LEFT JOIN box_processes s ON i.parentprocess_id = bp.parentprocess_id 
          AND s.processname = 'Shipped' 

->sqlfiddle
使用to_char()格式化您interval你想要的任何方式。