Question

如果我有两张桌子，雇员。我需要从工作中选择所有工作并为每项工作追加员工人数。

假设结果

job table
+-------------+-------------+
| id          | name        |
+-------------+-------------+
| 1           | Teacher     |
+-------------+-------------+
| 2           | Engineer    |
+-------------+-------------+
| 3           | Programmer  |
+-------------+-------------+

employee table
+-------------+-------------+-------------+
| id          | name        |   job_id    |
+-------------+-------------+-------------+
| 1           | employee N  |      1      |
+-------------+-------------+-------------+
| 2           | employee N  |      2      |
+-------------+-------------+-------------+
| 3           | employee N  |      3      |
+-------------+-------------+-------------+
| 4           | employee N  |      1      |
+-------------+-------------+-------------+
| 5           | employee N  |      3      |
+-------------+-------------+-------------+
| 6           | employee N  |      1      |
+-------------+-------------+-------------+

我需要从工作中选择所有工作并为每项工作追加员工人数。

假设结果

Result table
+-------------+-------------+--------------+
| id          | name        |employee count|
+-------------+-------------+--------------+
| 1           | Teacher     |      3       |
+-------------+-------------+--------------+
| 2           | Engineer    |      1       |
+-------------+-------------+--------------+
| 3           | Programmer  |      2       |
+-------------+-------------+--------------+

Answer 1

我喜欢使用左连接：）

SELECT job.id, job.name, count( * ) AS 'employee count'
FROM job
LEFT JOIN employee 
ON job.id = employee.job_id
GROUP BY job.id

Answer 2

您希望使用INNER JOIN加入表格，然后使用GROUP BY对作业ID进行分组。最后使用COUNT()来计算每个组的数量

SELECT job.id, job.name, count(*) AS employee_count
FROM job
INNER JOIN employee 
  ON job.id = employee.job_id
GROUP BY job.id

你可以在这里看到http://sqlfiddle.com/#!2/9d59c1/1

mysql从多个表中选择计数

2 个答案: