MySQL - 選擇包含值的列並創建“虛擬列”

Question

我有3張桌子。

tb_employees：

+---------------+---------------+
|  id_employee  |   employee    |
+---------------+---------------+
|     1         | Robert Tomson |
|     2         | Jhonatan Weg  |
|     3         |   Eva Uhte    |
+---------------+---------------+

tb_requirements：

+---------------+-----------------+
|  id_requirem  |    requirem     |
+---------------+-----------------+
|     11        |      Photo      |
|     12        | Criminal Record |
|     13        |  Shooting Test  |
+---------------+-----------------+

tb_details：

+---------------+-----------------+---------------+
|  id_details   |  id_employee    |  id_requirem  |
+---------------+-----------------+---------------+
|     21        |       1         |       11      |
|     22        |       1         |       12      |
|     23        |       1         |       13      |
|     24        |       2         |       12      |
|     25        |       2         |       13      |
|     26        |       3         |       11      |
|     27        |       3         |       13      |
+---------------+-----------------+---------------+

我要做的是SELECT of requirem （描述）和一個特定的員工（例如，對於WHERE id_employee = '2' ），並添加一個“虛擬列”，顯示員工是否有該要求。

像這樣： WHERE id_employee = '2'

+----------------+-----------------+
|    requirem    | virtual_column  |
+----------------+-----------------+
|     Photo      |       0         |
|Criminal Record |       1         |
| Shooting Test  |       1         |
+---------------+------------------+

WHERE id_employee = '3'

+----------------+-----------------+
|    requirem    | virtual_column  |
+----------------+-----------------+
|     Photo      |       1         |
|Criminal Record |       0         |
| Shooting Test  |       1         |
+---------------+------------------+

其中0 =員工沒有該要求，1 =員工具有該要求：來自tb_detail。

我真的不知道如何做到這一點。

你可以幫我嗎？

謝謝你的回答。

Answer 1

實現目標的關鍵是外部OUTER JOIN

SELECT r.requirem, (d.id_requirem IS NOT NULL) virtual_column
  FROM tb_requirements r LEFT JOIN tb_details d
    ON r.id_requirem = d.id_requirem
   AND d.id_employee = 2

id_employee = 2輸出：

|        REQUIREM | VIRTUAL_COLUMN |
|-----------------|----------------|
|           Photo |              0 |
| Criminal Record |              1 |
|   Shooting Test |              1 |

id_employee = 3輸出：

|        REQUIREM | VIRTUAL_COLUMN |
|-----------------|----------------|
|           Photo |              1 |
| Criminal Record |              0 |
|   Shooting Test |              1 |

這是SQLFiddle演示

進一步閱讀：

• SQL連接的可視化解釋