How to select parent row only if has at least one child?

Question

I have a simple one-to-many relationship. I would like to select rows from the parent only when they have at least one child. So, if there are no children, then the parent row is not returned in the result set.

Eg.

Parent:
+--+---------+
|id|   text  |
+--+---------+
| 1|  Blah   |
| 2|  Blah2  |
| 3|  Blah3  |
+--+---------+
Children
+--+------+-------+
|id|parent| other |
+--+------+-------+
| 1|   1  | blah  |
| 2|   1  | blah2 |
| 3|   2  | blah3 |
+--+------+-------+

I want the results to be:

+----+------+
|p.id|p.text|
+----+------+
|  1 | Blah |
|  2 | Blah2|
+----+------+

Solution 1

You can do this using an EXISTS, like this:

SELECT *
FROM Parent p
WHERE EXISTS (SELECT 1
              FROM Chilren c
              WHERE c.Parent = p.id)

Or using a IN like this:

SELECT *
FROM Parent p
WHERE p.id IN (SELECT c.Parent
               FROM Chilren c)

Solution 2

An inner join only returns rows that match both tables:

select distinct p.*
from Parent p
inner join Children c on c.parent = p.id

Solution 3

Select p.id, p.text
from Parent p
inner join Children c on p.id = c.parent 
group by p.id, p.text

Solution 4

SELECT p.*
FROM Parent p
WHERE EXISTS (SELECT 'X' FROM Children c WHERE c.parent = p.id);

View more solutions

Comments

[Hogan]

I have not tested but I'm guessing this is slower than a join.

[Nick Craver]

@Hogan - The exists it the fastest approach. The optimizer will do the same execution plan for a null checked outer join...which will be as fast, but the exists is never slower.

[Hogan]

Neat. Thanks @Nick. This is true for mysql, ms sql and oracle?

[Nick Craver]

@Hogan - The exists being faster, yes that holds true...how smart the optimizer is in making an equivalent left join have the same execution plan, I'm not sure about MS SQL.