Update Count column in Postgresql

Question

I have a single table laid out as such:

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

I need to fill out the count column such that the result is the number of times the specific name shows up in the column name.

The result should be:

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

I can get the count of occurrences of unique names easily using:

SELECT COUNT(name)
FROM table
GROUP BY name

But that doesn't fit into an UPDATE statement due to it returning multiple rows.

I can also get it narrowed down to a single row by doing this:

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

But that doesn't allow me to fill out the entire column, just the 'John' rows.

Answer 1

you can do that with a common table expression:

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;

Another (slower) option would be to use a co-related sub-query:

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

But in general it is a bad idea to store values that can easily be calculated on the fly:

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