Postgres 10

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create table parent(parent_id integer not null primary key, name text);

CREATE TABLE

create table child(child_id integer not null primary key, name text, parent_id integer references parent);

CREATE TABLE

insert into parent values (1, 'Bill');

INSERT 0 1

insert into parent values (2, 'Mary');

INSERT 0 1

insert into child values (1, 'Jennifer', 1)

INSERT 0 1

select name, exists (select 1 from child where parent_id = p.parent_id) as has_children
from parent p;

name
has_children

Bill
t

Mary
f

SELECT 2