在 SQL 中使用 JOIN 时如何使用引用多个 ID

Question

我尝试显示员工提供的评论数和他收到的评论数。我尝试做这样的事情，但在我的输出中 table (3) 我收到了不正确的数据。我相信这是由于错误的引用而发生的：

pr.reviewer_id = e.employee_id

但是，如果我这样引用它：

pr.employee_id = e.employee_id

那么 nr_of_reviews_receieved 是正确的，而 nr_of_reviews_posted 不是，如果我把它改回去，反之亦然。因此，我需要在一种情况下使用一个参考，在另一种情况下使用不同的参考，但都在一个查询中。

SELECT 
    e.employee_id, 
    CONCAT_WS(' ',employee_first_name, employee_last_name) AS full_name,
    COUNT(pr.reviewer_id) AS nr_of_reviews_posted,
    COUNT(pr.employee_id) AS nr_of_reviews_received
FROM 
    employee AS e
LEFT JOIN 
    performance_review AS pr ON pr.reviewer_id = e.employee_id
GROUP BY 
    employee_first_name, employee_last_name, e.employee_id

Answer 1

您必须对 table 进行两次采样，然后每次都加入 table 例如：

SELECT 
e.employee_id, 
CONCAT_WS(' ',employee_first_name, employee_last_name) AS full_name,
COUNT(pr.reviewer_id) AS nr_of_reviews_posted,
COUNT(pr2.employee_id) AS nr_of_reviews_received
FROM 
employee AS e
LEFT JOIN 
performance_review AS pr ON pr.reviewer_id = e.employee_id
LEFT JOIN 
performance_review AS pr2 ON pr2.employee_id = e.employee_id
GROUP BY 
Necessary columns ...

在 SQL 中使用 JOIN 时如何使用引用多个 ID

How to use refer to multiple id's when using JOINs in SQL

sql

sql-server