Question

I have two tables with parent – child relationship:

PARENT table with id as index

    id  | name
----------------
    1   | Bob
    2   | Mike
    3   | Joe

CHILDREN table with id as index

   id   | parent | name
-------------------------
    1   | 1      | Ronald
    2   | 2      | Sammy
    3   | 3      | Jenny
    4   | 1      | Tara
    5   | 3      | Jack

On my website I want to display list of all parents with count of their children as follows:

    id  | name | Children Count
-------------------------------
    1   | Bob  |      2
    2   | Mike |      1
    3   | Joe  |      2

What is the efficient way to do this? Note that here I represented an analogous simplified version of my data structure, in my actual situation I have about 10000 records in my PARENT table and each parent has about 10 - 200 children.

In case you suggest to add a count column to PARENT table, do you know of any way such that the count will be automatically updated by MYSQL when I make a change in the CHILDREN table?

Answer 1

SELECT p.id,
       p.name,
       COUNT(c.id) AS children_count
FROM parent p
LEFT JOIN children c ON p.id = c.parent
GROUP BY p.id