Cara menjumlahkan dua kolom bersyarat di PostgreSQL 9.3

Anda dapat melakukannya dengan SubQuery, jika versi PostgreSQL Anda tidak (belum) mengizinkan fungsi jendela):

WITH t (month, marketid, totalsold, totalshipped, lefttoship_thismonth) AS
(VALUES
    ('01-01-2015'::date,   1, 100, 50,  50),
    ('01-01-2015'::date,   2,  10,  3,   7),
    ('01-01-2015'::date,   3,   0,  0,   0),
    ('01-02-2015'::date,   1,   0, 50, -50),
    ('01-02-2015'::date,   2,  20,  0,  20),
    ('01-02-2015'::date,   3,   0,  0,   0) 
)

SELECT
    month, 
    marketid, 
    totalsold, 
    totalshipped, 
    lefttoship_thismonth, 
    (SELECT sum(lefttoship_thismonth) 
       FROM t t2 
      WHERE t2.marketid  = t1.marketid AND
            t2.month    <= t1.month
    ) AS total_left
FROM 
    t t1
ORDER BY
    month, marketid ;

akan memberi Anda hasil berikut:

|------------+----------+-----------+--------------+----------------------+------------|
|   month    | marketid | totalsold | totalshipped | lefttoship_thismonth | total_left |
|------------+----------+-----------+--------------+----------------------+------------|
| 2015-01-01 |    1     |    100    |      50      |          50          |     50     |
|------------+----------+-----------+--------------+----------------------+------------|
| 2015-01-01 |    2     |    10     |      3       |          7           |     7      |
|------------+----------+-----------+--------------+----------------------+------------|
| 2015-01-01 |    3     |     0     |      0       |          0           |     0      |
|------------+----------+-----------+--------------+----------------------+------------|
| 2015-01-02 |    1     |     0     |      50      |         -50          |     0      |
|------------+----------+-----------+--------------+----------------------+------------|
| 2015-01-02 |    2     |    20     |      0       |          20          |     27     |
|------------+----------+-----------+--------------+----------------------+------------|
| 2015-01-02 |    3     |     0     |      0       |          0           |     0      |
|------------+----------+-----------+--------------+----------------------+------------|

Jika Anda dapat menggunakan fungsi Window (yang lebih efisien), Anda dapat melakukan hal berikut:

SELECT
    month, 
    marketid, 
    totalsold, 
    totalshipped, 
    lefttoship_thismonth, 
    ( sum(lefttoship_thismonth) 
           OVER (PARTITION BY marketid ORDER BY month ROWS UNBOUNDED PRECEDING)
    ) AS total_left
FROM 
    t t1
ORDER BY
    month, marketid ;

Jika month . Anda kolom adalah varchar (bukan ide yang baik), Anda dapat mentransmisikannya hingga saat ini, atau menggunakan to_date fungsi.