Postgres有条件选择吗？

Question

I'm not really sure how to describe this problem without an example, so here's an example:

I have three tables:

Table 1: Boxes

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

Table 2: Box_ProcessIDs

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

Table 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

I want the output columns to be: Box Name, Box Cost, Box Shipping Duration, Box Packing Duration, Box Invoice Duration

I know I can use like %pack% to select my packing fields, and I can figure out how to use Date_Part or somesuch to get my durations, but I'm at a loss as to how to get my shipping duration , packing duration , and invoice duration into separate fields.

Answer 1

So you want to do something like this pseudo-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'

(will need null checks, proper datetime handling, better names and so on)

Answer 2

First off, your design is flawed. time is no good for this purpose. It breaks as soon as you cross midnight. Use timestamp or timestamptz instead.

Next, since unquoted identifiers are cast to lower case automatically in PostgreSQL, your naming convention is no good either. I adapted it to something more useful.

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');

Your query could then look like this:

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
Use to_char() to format your interval any way you want.