mysql - 将 Count 与 Count Distinct 和 Group By 一起使用

Question

我有两个表，一个用于 employees，其中包含 id、name 和 company 列，另一个表用于对于调查结果，其中包含employee_id、questions_id、answer作为一名员工获得多个结果。

|---------------------|
| id | name | company |
|---------------------|

|-----------------------------------------|
| id | employee_id | question_id | answer |
|-----------------------------------------|

我想选择每家公司的员工总数以及每家公司参与调查的参与者总数。

我尝试了以下查询，但执行时间太长:

SELECT employees.company as x, COUNT(DISTINCT (results.employee_id)) "Total Surveys", (SELECT COUNT(employees.id) FROM employees WHERE company = x) "Headcount"
FROM results
JOIN employees ON results.employee_id = employees.id
GROUP BY employees.company

结果

|--------------------------------|
|  x | Total Surveys | Headcount |
|--------------------------------|
| C1 |       15      |    3      |
| C2 |       10      |    5      |
|--------------------------------|

SQL Fiddle

有什么建议吗？

Answer 1

通过从employees到results的LEFT JOIN可以得到你想要的结果；那么您可以在不使用子查询的情况下计算这两个值:

SELECT e.company, 
       COUNT(DISTINCT r.employee_id) AS `Total Surveys`,
       COUNT(DISTINCT e.id) AS `HeadCount`
FROM employees e
LEFT JOIN results r ON r.employee_id = e.id
GROUP BY e.company

Demo on SQLFiddle