如何删除子查询？

Question

In PostgreSQL database I have such table with information about relationship between organizations. As you can see each organization has their own parent.

| organization_id | organization_name | organization_rang | parent_organization_id |
| ----------------|-------------------|-------------------|------------------------|
| 1               | Alphabet          | 1                 |                        |
| 2               | Google            | 2                 | 1                      |
| 3               | X Development     | 3                 | 2                      |
| 4               | Apple             | 1                 |                        |

I am trying to create sql query which will return new column. If organization has child I need to set true to that new column, otherwise I need to set false. Other word I need such result:

| organization_id | organization_name | organization_rang | parent_organization_id | has_child |
| ----------------|-------------------|-------------------|------------------------|-----------|
| 1               | Alphabet          | 1                 |                        | true      |
| 2               | Google            | 2                 | 1                      | true      |
| 3               | X Development     | 3                 | 2                      | false     |
| 4               | Apple             | 1                 |                        | false     |

I used next code which works, but in my opinion this is not the best solution to make a subquery on the same table. How can I improve the code and speed up the query in your opinion?

SELECT
    A.*,
    CASE
        WHEN EXISTS(
            SELECT *
            FROM HISTORY
            WHERE PARENT_ORGANIZATION_ID = A.ORGANIZATION_ID
        ) THEN TRUE
        ELSE FALSE
    END HAS_CHILD   
FROM HISTORY AS A

Answer 1

You can use conditional ( case..when ) statement:

SELECT A.*, 
       (CASE
           WHEN PARENT_ORGANIZATION_ID = A.ORGANIZATION_ID
           THEN 'true'
           ELSE 'false'
       END) AS HAS_CHILD
  FROM HISTORY AS A;

I quoted true and false because of the desired results are not boolean but of string type.

Answer 2

As @a_horse_with_no_name commented, there is no need for the case expression; EXISTS(...) already yields a boolean value:

---

SELECT a.*
    , EXISTS( SELECT * FROM history x
            WHERE x.parent_organization_id = a.organization_id
        ) AS has_child 
FROM history a
        ;

Answer 3

LEFT JOIN version:

SELECT
    A.*,
    B.PARENT_ORGANIZATION_ID is not null HAS_CHILD   
FROM
    HISTORY AS A
LEFT JOIN (select distinct PARENT_ORGANIZATION_ID from HISTORY) as B
    on B.PARENT_ORGANIZATION_ID = A.ORGANIZATION_ID