Update a column value to the COUNT of rows for specific values in same table

Question

I am working on an export of data from MySQL to .csv where I need to include a row count based on a specific column in the table.

I have a table called e with the columns id , e1 and e2

|id|e1|e2|
| 1| A|  |
| 2| A|  |
| 3| A|  |
| 4| B|  |
| 5| C|  |
| 6| B|  |

The result I'm looking for is:

|id|e1|e2|
| 1| A| 3|
| 2| A| 3|
| 3| A| 3|
| 4| B| 2|
| 5| C| 1|
| 6| B| 2|

If I try something like this:

UPDATE `e` SET `e2` = (SELECT COUNT(`id`) FROM `e` WHERE `e1` = 'A') WHERE `e1` = 'A';

I get: You can't specify target table 'e' for update in FROM clause

How can I go about updating my data so that the value in e2 is the COUNT of the rows with the value from e1 ? Is this something I can't/shouldn't do in MySQL but should be doing in PHP?

(I feel like I'm missing something obvious....)

Answer 1

UPDATE e a, (SELECT e1,COUNT(*) cnt 
               FROM e 
           GROUP BY e1) b

   SET a.e2 = b.cnt
 WHERE a.e1 = b.e1;

Answer 2

with
AAA AS
(
    select e1,count(*) as cnt from e group by e1
)

update e
set e2 = AAA.cnt
from e
inner join AAA ON e.e1 = AAA.e1

Answer 3

Try Following Query :

update e
set e2 = a.cnt
from e inner join (select e1,count(*) as cnt from e as a group by e1) as a ON e.e1 = A.e1