The problem arise when there are no data for books in specific library. Consider a following working scenario.
Table library
--------------------------------
| id | name | owner |
--------------------------------
| 1 | ABC | A |
| 2 | DEF | D |
| 3 | GHI | G |
--------------------------------
Table books
--------------------------------
| id | title | library |
--------------------------------
| a | xxx | 1 |
| b | yyy | 1 |
| c | zzz | 2 |
--------------------------------
Now when I do query like below:
SELECT library.name, array_agg(b.title) AS book_list FROM library,
(SELECT title FROM books WHERE books.library = :library_no) as b
WHERE library.id = :library_no GROUP BY library.id
The query generates output for library 1 & 2, but not for library 3. Why and how to solve this issue? (Generate an empty list on no library books)
Required Output:
----------------------
| name | book_list |
----------------------
| GHI | {} | # or {null}
-----------------------
I've even tried coalesce
as below:
SELECT library.name, coalesce(array_agg(b.title), ARRAY[]::VARCHAR[]) AS book_list FROM library,
(SELECT title FROM books WHERE books.library = :library_no) as b
WHERE library.id = :library_no GROUP BY library.id
Postgres version: 12
Best Answer
A
LEFT JOIN
can solve it, like Laurenz provided.But I suggest an ARRAY constructor in a
LATERAL
subquery instead:This way, you don't need to aggregate in the outer query level and don't need to
GROUP BY
there.You also don't need
COALESCE
, since the ARRAY constructor over an empty result already produces an empty array ({}
).And it should be faster for a small selection in
library
- obviously the query gets the result for a single given library.Aside, you only need the variable
:library_no
in a single place like demonstrated.About
LATERAL
joins:About the ARRAY constructor:
Basic about joining tables in the manual.