Parent Child Relation in same Table . Select Query to get only parent records and Records not comes under parent child relation

Question

I have a table with Parent-Child Relation in the same Table. Some Sample Data

  id │ name                 │ parent │ is_active  
 ════╪══════════════════════╪════════╪═══════════ 
  1  │ Company A            │ null   │ true       
  2  │ Child of A           │ 1      │ true       
  3  │ 2Child of A          │ 1      │ true       
  4  │ 3Child of A          │ 1      │ true       
  5  │ 0 Single Company     │ null   │ true       
  6  │ 1 Single Company     │ null   │ true       
  7  │ 2 Single Company     │ null   │ true       
  8  │ Parent Company B     │ null   │ true       
  9  │ 0 Child Company of B │ 8      │ true       
  10 │ 1 Child Company of B │ 8      │ true       
  11 │ 2 Child Company of B │ 8      │ true       
  12 │ 3 Child Company of B │ 8      │ true   

I want to write two queries

1. get all the parent records (only parent records) Example: id 1,8

2. Get all the single records (only records does not come under parent-child) Example: 5,6,7

Please help to write select queries.

Answer 1

SELECT t1.*
FROM "table" t1
WHERE t1.parent IS NULL
  AND /* NOT */ EXISTS (SELECT 1
                        FROM "table" t2
                        WHERE t1.id = t2.parent)

EXISTS - for parent records.

NOT EXISTS - for single records.

Answer 2

If possible can you please elaborate your question, also write your Select queries for better understanding.

as you said "Parent-Child Relation in the same Table", do you mean to return all parent values as distinct?

Then,

1. get all the parent records (only parent records) Example: id 1,8

SELECT DISTINCT parent
FROM "table_name" t1
WHERE parent IS NOT NULL;

2. Get all the single records (only records does not come under parent-child) Example: 5,6,7

SELECT DISTINCT id
FROM "table_name" t1
WHERE t1.id= t1.parent IS NULL;