QUALIFY 子句 - Amazon Redshift
语法示例
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QUALIFY 子句

QUALITY 子句根据用户指定的搜索条件，筛选先前计算的窗口函数的结果。您可以使用此子句将筛选条件应用于窗口函数的结果，而无需使用子查询。

此子句与 HAVING 子句类似，后者应用条件以进一步从 WHERE 子句筛选行。QUALITY 和 HAVING 的区别在于，QUALITY 子句的筛选结果可以基于对数据运行窗口函数的结果。在一个查询中可以同时使用 QUALITY 和 HAVING 子句。

语法

QUALIFY condition
注意

如果您在 FROM 子句之后直接使用 QUALIFY 子句，则 FROM 关系名称必须在 QUALIFY 子句之前指定了别名。

示例

本节中的示例使用下面的示例数据。

create table store_sales (ss_sold_date date, ss_sold_time time, 
               ss_item text, ss_sales_price float);
insert into store_sales values ('2022-01-01', '09:00:00', 'Product 1', 100.0),
                               ('2022-01-01', '11:00:00', 'Product 2', 500.0),
                               ('2022-01-01', '15:00:00', 'Product 3', 20.0),
                               ('2022-01-01', '17:00:00', 'Product 4', 1000.0),
                               ('2022-01-01', '18:00:00', 'Product 5', 30.0),
                               ('2022-01-02', '10:00:00', 'Product 6', 5000.0),
                               ('2022-01-02', '16:00:00', 'Product 7', 5.0);

以下示例演示了如何查找每天 12:00 之后售出的两件最昂贵的商品。

SELECT *
FROM store_sales ss
WHERE ss_sold_time > time '12:00:00'
QUALIFY row_number()
OVER (PARTITION BY ss_sold_date ORDER BY ss_sales_price DESC) <= 2
               

 ss_sold_date | ss_sold_time |  ss_item  | ss_sales_price 
--------------+--------------+-----------+----------------
 2022-01-01   | 17:00:00     | Product 4 |           1000
 2022-01-01   | 18:00:00     | Product 5 |             30
 2022-01-02   | 16:00:00     | Product 7 |              5

然后，您可以找到每天售出的最后一件商品。

SELECT *
FROM store_sales ss
QUALIFY last_value(ss_item)
OVER (PARTITION BY ss_sold_date ORDER BY ss_sold_time ASC
      ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) = ss_item;
               
ss_sold_date | ss_sold_time |  ss_item  | ss_sales_price 
--------------+--------------+-----------+----------------
 2022-01-01   | 18:00:00     | Product 5 |             30
 2022-01-02   | 16:00:00     | Product 7 |              5

以下示例返回与前一个查询相同的记录，即每天售出的最后一件商品，但它没有使用 QUALIFY 子句。

SELECT * FROM (
  SELECT *,
  last_value(ss_item)
  OVER (PARTITION BY ss_sold_date ORDER BY ss_sold_time ASC
        ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) ss_last_item
  FROM store_sales ss
)
WHERE ss_last_item = ss_item;
               
 ss_sold_date | ss_sold_time |  ss_item  | ss_sales_price | ss_last_item 
--------------+--------------+-----------+----------------+--------------
 2022-01-02   | 16:00:00     | Product 7 |              5 | Product 7
 2022-01-01   | 18:00:00     | Product 5 |             30 | Product 5