postgresql - 更新 Postgresql 中的计数列

Question

我有一个这样布置的表:

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

我需要填写计数列，以便结果是特定名称在列 name 中出现的次数。

结果应该是:

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

但这不适合 UPDATE 语句，因为它返回多行。

我还可以通过这样做将它缩小到单行:

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

但这不允许我填写整个列，只能填写“John”行。

Answer 1

您可以使用公用表表达式来做到这一点:

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(较慢的)选项是使用相关的子查询:

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

但一般来说，存储可以轻松即时计算的值并不是一个好主意:

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