id parent\_id

1 0

2 0

3 2

4 0

5 1

6 0

I need a query that will return parent rows (parent\_id=0) followed by its child rows

first parent

all children of first parent

second parent

all children of second parent

third parent

fourth parent

Expected result: ordered by id

id parent\_id

1 0 (first parent)

5 1 (all children of first parent)

2 0 second parent

3 2 (all children of second parent)

4 0 third parent

6 0 fourth parent

I can use union of parents followed by all childs
But that gives me parents first then the children.
I need parent and immediately its children.

Anyone can help?