Question

我遇到了SQL查询。假设我们有一个员工，一个任务表以及他们之间的多对多关联。表格如下所示：

employees
id|name
1 | John
2 | Peter
3 | Mike

tasks
id | name
1 | Support
2 | Programming
3 | Call customers
4 | Write Newsletters
5 | Write Invoices

employees_tasks
employee_id | task_id
1 | 1
1 | 2
2 | 3
2 | 4
2 | 5
3 | 2

现在我想让所有以“编程”为己任的员工。正确的查询是：

SELECT employees.id, employees.name
FROM employees 
    INNER JOIN employees_tasks ON employees.id = employees_tasks.employee_id
    INNER JOIN tasks ON employees_tasks.task_id = tasks.id
WHERE
    tasks.name LIKE 'Programming'

到目前为止一切都很好......但现在我想要所有员工，他们的任务是“编程”和“支持”。这个查询给了我NULL：

SELECT employees.id, employees.name
FROM employees 
    INNER JOIN employees_tasks ON employees.id = employees_tasks.employee_id
    INNER JOIN tasks ON employees_tasks.task_id = tasks.id
WHERE
    tasks.name LIKE 'Programming' AND tasks.name LIKE 'Support'

我通过此查询收到三条记录

SELECT employees.id, employees.name
FROM employees 
    INNER JOIN employees_tasks ON employees.id = employees_tasks.employee_id
    INNER JOIN tasks ON employees_tasks.task_id = tasks.id
WHERE
    tasks.name IN ('Programming', 'Support')

2x约翰和1x迈克。但这不是我想要的。我希望所有拥有“编程”和“支持”任务的员工 - 而不是那些只有其中一项任务的员工。

还有另一种选择。我用子查询使用ALL。我们走了：

SELECT employees.id, employees.name
FROM employees 
    INNER JOIN employees_tasks ON employees.id = employees_tasks.employee_id
    INNER JOIN tasks ON employees_tasks.task_id = tasks.id
WHERE
    tasks.name = ALL
    (SELECT DISTINCT name
    FROM tasks
    WHERE name LIKE 'Programming' OR name LIKE 'Support')

但我收到此查询为NULL，尽管有一名员工，他们同时拥有两项任务：John！

如何实现此类查询？

最诚挚的问候基督教

Answer 1

您需要第二次将employees_tasks加入您的查询：

SELECT employees.id, employees.name
FROM employees
  INNER JOIN employees_tasks AS et1 ON employees.id = et1.employee_id
  INNER JOIN employees_tasks AS et2 ON employees.id = et2.employee_id
  INNER JOIN tasks AS t1 ON et1.task_id = t1.id AND t1.name = 'Programming'
  INNER JOIN tasks AS t2 ON et2.task_id = t2.id AND t2.name = 'Support'

<强>更新

或者，如果您仅针对感兴趣的任务过滤结果，则GROUP BY员工可以只返回具有所需任务数的人：

SELECT   employees.id, employees.name
FROM     employees
  INNER JOIN employees_tasks ON employees_tasks.employee_id = employees.id
  INNER JOIN tasks           ON employees_tasks.task_id     = tasks.id
WHERE    tasks.name IN ('Programming', 'Support')
GROUP BY employees.id, employees.name
HAVING   COUNT(DISTINCT tasks.id) = 2

具有多个条件的SQL SELECT查询

1 个答案: