Question

我有一张表格如下：

id  |  name  |  count
1   |  John  |
2   |  Jim   |
3   |  John  |
4   |  Tim   |

我需要填写count列，以便结果是特定名称显示在name列中的次数。

结果应为：

id  |  name  |  count
1   |  John  |  2
2   |  Jim   |  1
3   |  John  |  2
4   |  Tim   |  1

我可以使用以下方法轻松获取唯一名称的出现次数：

SELECT COUNT(name)
FROM table
GROUP BY name

但由于它返回多行，因此不适合UPDATE语句。

我还可以通过这样做将其缩小到一行：

SELECT COUNT(name)
FROM table
WHERE name = 'John'
GROUP BY name

但这不允许我填写整个列，只填写'John'行。

Answer 1

您可以使用公用表表达式执行此操作：

with counted as (
   select name, count(*) as name_count
   from the_table
   group by name
) 
update the_table
  set "count" = c.name_count
from counted c
where c.name = the_table.name;

另一个（较慢）选项是使用共同相关的子查询：

update the_table
  set "count" = (select count(*) 
                 from the_table t2 
                 where t2.name = the_table.name);

但总的来说，存储可以轻松计算的值是一个坏主意：

select id,
       name, 
       count(*) over (partition by name) as name_count
from the_table;

Answer 2

另一种方法：使用派生表

UPDATE tb
SET count = t.count
FROM (
    SELECT count(NAME)
        ,NAME
    FROM tb
    GROUP BY 2
    ) t
WHERE t.NAME = tb.NAME

Postgresql中的Update Count列

2 个答案: