Question

我有一个数据库，其中有58位客户（58行）和一列（国家/地区）。如何对一个国家仅出现一次的所有行进行分组，并使用intent.bar = <Ask Each Time>语句将其名称更改为“其他”

CASE

问题是，当我运行查询时，所有国家/地区都变成“其他”。我只希望曾经出现过的国家称为“其他”。

Answer 1

这是操作方法。

SELECT
CASE WHEN COUNT(country) = 1 THEN 'Other' ELSE country END
FROM customer
GROUP BY country;

Answer 2

使用子查询来计算一个国家/地区在表格中出现的次数：

SELECT 
  CASE 
    WHEN (SELECT COUNT(*) FROM customer WHERE country = c.country) = 1 THEN 'Other' 
    ELSE c.country 
  END country 
FROM customer c

或者通过表与子查询的联接返回所有国家/地区的所有计数器：

SELECT 
  CASE 
    WHEN g.counter = 1 THEN 'Other' 
    ELSE c.country 
  END country 
FROM customer c INNER JOIN (
  SELECT country, COUNT(*) counter 
  FROM customer 
  GROUP BY country
) g ON g.country = c.country

如果要根据此CASE语句的结果对表进行分组，请使用别名来区分表的列country：

SELECT 
  CASE 
    WHEN (SELECT COUNT(*) FROM customer WHERE country = c.country) = 1 THEN 'Other' 
    ELSE c.country 
  END _country 
FROM customer c
GROUP BY _country

Answer 3

要更新实际表：

WITH map AS
  (SELECT country, 
          CASE WHEN COUNT(country) = 1 THEN 'Other'
               ELSE country END mapped
   FROM customer
   GROUP BY country) 
UPDATE customer
SET country = (SELECT mapped FROM map WHERE map.country = customer.country);

如何使用CASE语句合并行

3 个答案: