我有3张桌子。
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 requirem（说明）和特定员工（例如WHERE id_employee = '2'）并添加“虚拟列”，以显示员工有这个要求。
喜欢这样：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 =员工没有该要求，1 =员工有此要求：来自tb_detail。
我真的不知道该怎么做。
你可以帮帮我吗？感谢您的回答。
答案 0 :(得分：3)
实现目标的关键是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的输出：
| REQUIREM | VIRTUAL_COLUMN | |-----------------|----------------| | Photo | 0 | | Criminal Record | 1 | | Shooting Test | 1 |
id_employee = 3的输出：
| REQUIREM | VIRTUAL_COLUMN | |-----------------|----------------| | Photo | 1 | | Criminal Record | 0 | | Shooting Test | 1 |
这是 SQLFiddle 演示
进一步阅读：