当特定图书馆中没有书籍数据时，就会出现问题。考虑以下工作场景。
桌子 library
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| id | name | owner |
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| 1 | ABC | A |
| 2 | DEF | D |
| 3 | GHI | G |
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桌子 books
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| id | title | library |
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| a | xxx | 1 |
| b | yyy | 1 |
| c | zzz | 2 |
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现在，当我进行如下查询时：
SELECT library.name, array_agg(b.title) AS book_list FROM library,
(SELECT title FROM books WHERE …