Question

I need to execute this query it’s like ( main cats and sub cats )

Here is my schema:

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+  id | Country     |  Parent  |
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+  1  |  India      |          |
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+  2  |  Usa        |          |
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+  3  | California  |   Usa    |
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+  4  | New York    |   Usa    |
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+  5  | New Delhi   |  India   |
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+  6  | France      |          |
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And I want to get the result like this:

+++++++++++++++++++++++++++++++++++++++++++++++++
+  id | Country     | Num |  Childs             |
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+  1  |  India      |  1  |  New Delhi          |
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+  2  | Usa         |  2  | California,New York |
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+  3  | France      |  0  |                     |
+++++++++++++++++++++++++++++++++++++++++++++++++

Answer 1

Try this:

SELECT t1.id, t1.Country, 
    (SELECT COUNT(id) FROM world t2
     WHERE t2.Parent = t1.Country) as Num,
    (SELECT GROUP_CONCAT(Country) FROM world t3
     WHERE t3.Parent = t1.Country) as Childs
FROM world t1
WHERE t1.Parent IS NULL

HOW IT WORKS

My query is made of a main part and two subqueries (the ones between parenthesis); you can think of a subquery as a normal query executed in a main query returing to this some field (column). Subqueries can interact with main query because they can use main fields in SELECT or WHERE.

Main part gets all base countries (the ones without parent)

   SELECT t1.id, t1.Country
   FROM world t1
   WHERE t1.Parent IS NULL

First subquery gets (for every item in main part) the count of countries whose parent is main item

SELECT COUNT(id) FROM world t2
WHERE t2.Parent = t1.Country

Note that in WHERE clause I ask to match subquery id with parent id, so I get only the count of countries related to main item.
Second subquery is similar to first one, but a different field is returned using GROUP_CONCAT function to get all subcountries separated with commas.