如何在 SQL 中创建虚拟行答案

【问题标题】：How to create dummy rows in SQL如何在 SQL 中创建虚拟行
【发布时间】：2021-07-10 09:12:23
【问题描述】：

我有下表：

loan_id	first_term	term	month_due_time	MOB	dpd	loan_principal	dpd30p
1	202006	202006	5/7/2020	3	99	600000	600000
1	202006	202007	5/8/2020	4	95	600000	600000

如果我想创建一些 MOB 跨度从 1 到 6 的虚拟行，我该怎么办？期望的输出是：

loan_id	first_term	term	month_due_time	MOB	dpd	loan_principal	dpd30p
1	202006	202006	5/7/2020	1
1	202006	202006	5/7/2020	2
1	202006	202006	5/7/2020	3	99	600000	600000
1	202006	202007	5/8/2020	4	95	600000	600000
1	202006	202006	5/7/2020	5
1	202006	202006	5/7/2020	6

【问题讨论】：

您使用的 RDBMS 是什么？
@EvaldasBuinauskas Presto
尝试 COALESCE。详情请见prestodb.io/docs/current/functions/conditional.html

标签： sql presto trino

【解决方案1】：

好吧，您可以使用values() 和cross join 生成行，left join 引入现有值，coalesce() 将值替换为默认值：

select lf.loan_id, lf.first_term,
       coalesce(t.term, 200006),
       coalesce(t.month_due_time, '2020-05-07'),
       v.mob,
       t.dpd, t.loan_principal, tdpd30p
from (select distinct loan_id, first_term
      from t
     ) lf cross join
     (values (1), (2), (3), (4), (5), (6)) v(mob) left join
     t
     on t.loan_id = lf.loan_id and t.first_term = lf.first_term and
        t.mob = v.mob

【讨论】：

【解决方案2】：

此解决方案与@Gordon Linoff 提供的解决方案非常相似，但使用了 Trino 提供的序列函数。

我们使用sequence(start, stop)生成范围，然后转换为列并执行RIGHT JOIN，然后使用coalesce填充默认值

WITH rev_table(loan_id, first_term, term, month_due_time, MOB, dpd, loan_principal, dpd30p)
         AS (
        SELECT *
        FROM (VALUES (1, 202006, 202006, date ('2020-07-05'), 3, 99, 600000, 600000),
                     (1, 202006, 202007, date ('2020-08-05'), 4, 95, 600000, 600000))),
     mob_table(MOB) AS (
         SELECT *
         from UNNEST(SEQUENCE(1, 6))
     )

SELECT coalesce(loan_id, 1) as loan_id,
       coalesce(first_term, 202006) as first_term,
       coalesce (term, 202006) as term,
       coalesce (month_due_time, date('2020-07-05')) as month_due_time,
       mt.MOB as MOB,
       dpd, loan_principal, dpd30p
FROM rev_table rt RIGHT JOIN mob_table mt on mt.MOB = rt.MOB;

【讨论】：