Question

我正在尝试建立一个视图，该视图允许我跟踪两个连续month_ids的支付值之间的差异。但是，当缺少一个数字时，那是因为它是第一个条目，因此支付的金额为0。目前，由于[,default]参数尚未实现，因此我使用下面的数字表示前一个数字在MariaDB中。

CASE WHEN (
    NOT(policy_agent_month.policy_agent_month_id IS NOT NULL        
    AND LAG(days_paid, 1) OVER (PARTITION BY claim_id ORDER BY month_id ) IS NULL)) THEN        
         LAG(days_paid, 1) OVER ( PARTITION BY claim_id ORDER BY month_id)        
    ELSE 
         0        
    END

我遇到的问题是，我大约有30个变量需要应用此函数，这使我的代码不可读且非常笨拙。有更好的解决方案吗？

Answer 1

您可以在MariaDB 10.2+中使用CTE（公用表表达式）来预先计算常用的表达式并将其命名以供以后使用：

with
x as ( -- first we compute the CTE that we name "x"
  select
    *,
    coalesce(
      LAG(days_paid, 1) OVER (PARTITION BY claim_id ORDER BY month_id), 
      123456
    ) as prev_month -- this expression gets the name "prev_month"
  from my_table -- or a simple/complex join here
)
select -- now the main query
  prev_month
from x
... -- rest of your query here where "prev_month" is computed.

在主查询中，prev_month具有滞后值，如果为空，则为默认值123456。

Answer 2

为什么使用WITH？

SELECT province, tot_pop,
       tot_pop - COALESCE(
              (LAG(tot_pop) OVER (ORDER BY tot_pop ASC)),
                        0) AS delta
    FROM provinces
    ORDER BY tot_pop asc;

+---------------------------+----------+---------+
| province                  | tot_pop  | delta   |
+---------------------------+----------+---------+
| Nunavut                   |    14585 |   14585 |
| Yukon                     |    21304 |    6719 |
| Northwest Territories     |    24571 |    3267 |
| Prince Edward Island      |    63071 |   38500 |
| Newfoundland and Labrador |   100761 |   37690 |
| New Brunswick             |   332715 |  231954 |
| Nova Scotia               |   471284 |  138569 |
| Saskatchewan              |   622467 |  151183 |
| Manitoba                  |   772672 |  150205 |
| Alberta                   |  2481213 | 1708541 |
| British Columbia          |  3287519 |  806306 |
| Quebec                    |  5321098 | 2033579 |
| Ontario                   | 10071458 | 4750360 |
+---------------------------+----------+---------+
13 rows in set (0.00 sec)

但是，它并不便宜（至少在MySQL 8.0中）；该表有13行，但

FLUSH STATUS;
SELECT ...
SHOW SESSION STATUS LIKE 'Handler%';
MySQL 8.0:
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Handler_read_rnd           | 89    |
| Handler_read_rnd_next      | 52    |
| Handler_write              | 26    |
  (and others)

MariaDB 10.3:
| Handler_read_rnd           | 77    |
| Handler_read_rnd_next      | 42    |
| Handler_tmp_write          | 13    |
| Handler_update             | 13    |

MariaDB中LAG函数的默认值

2 个答案: