sql 查询从 db2 中多个表的多个列中获取最大日期

Question

我有多个具有 date_modified 列的表，我需要使用所有这些表创建一个视图，但 date_modified 应该是这些表中 date_modified 的最大值。

table1 
    id vendor_number fiscal_year date_created date_modified
    1    124           2021        2021/11/01   2021/11/02
    2    231           2021        2021/11/01   2021/11/03 
    
    table2 
    id fiscal_year discount_amt date_created date_modified
    1  2021          10.30      2021/11/01   2021/11/03
    2  2021          15.23      2021/11/01   2021/11/02
    
    table3
    id vendor_number date_created date_modified
    1  124           2021/11/01   2021/11/04
    2  231           2021/11/01   2021/11/01
    
    Required Output :
    
    id|fiscal_year|discount_amt|vendor_number|date_created|date_modified
    1 | 2021      |  10.30     | 124         | 2021/11/01 | 2021/11/04
    2 | 2021      |  15.23     | 231         | 2021/11/01 | 2021/11/03

查看 SQL：

CREATE VIEW view_data
AS
  SELECT T1.id, T1.fiscal_year, T2.discount_amt, T3.vendor_number, T1.date_created, max(multiple date_modified columns..from multiple tables..)
    FROM   table1 AS T1
             LEFT JOIN table2 AS T2
                    ON T1.fiscal_year = T2.fiscal_year
             LEFT JOIN T3 v
                    ON T1.vendor_number = T3.vendor_number;

  

Answer 1

假设id列是每个表的键，您可以加入它们（可能使用完全外部连接），然后使用GREATEST()获取最新日期。

例如：

create view v as 
select
  coalesce(a.id, b.id, c.id) as id,
  b.fiscal_year,
  b.discount_amt,
  a.date_created,
  greatest(a.date_modified, b.date_modified, c.date_modified) as date_modified
from table1 a
full join table2 b on b.id = a.id
full join table3 c on c.id = b.id or c.id = a.id