无法提供代码来回答此查询。

Question

SQL> 从显示的_on 中选择 *；

    ID NAME

---

     2 History of Art
     4 History of Art
     1 Berserk Charity
     5 Berserk Charity

SQL> 从 Art_Object 中选择 *；

    ID YEAR                      TITLE                     DES                       A_NAME

---

     1 1890                      Old Man                   An old man in the dark    van Gogh
     2 1894                      Cat                       White cat in black canvas van Gogh
     3 1853                      Monalisa                  Smiling woman             Leonardo
     4 1888                      The meeting               Two men talking           Picaso
     5 2017                      The crimson stone         Group of characters       Omar

SQL>

他们问我的查询是（显示每个艺术家现在正在展出的艺术品数量。）正在展出意味着它们在“displayed_on”表中。

Answer 1

就JOIN这两张表，group by和count：

SELECT
  o1.A_Name,
  COUNT(COALESCE(o2.ID, 0)) AS TotalNOfObjects
FROM Art_Object as o1
LEFT JOIN displayed_on AS o2 ON o1.ID = o2.ID
GROUP BY o1.A_Name;

Results：

|   A_Name | TotalNOfObjects |
|----------|-----------------|
| Leonardo |               1 |
|     Omar |               1 |
|   Picaso |               1 |
| van Gogh |               2 |

Answer 2

答案是：

select count(ID), A_NAME 
from Art_Object 
where ID in (select ID from displayed_on) group by (A_NAME);