如何从创建虚拟列的表中获取不同值的“类型” - MySQL

Question

我有这些表。

tb_employee：

ID_EMP   NAME_EMP 
1    Employee 1 
2    Employee 2 
3    Employee 3 
4    Employee 4 

tb_requirements：

ID_REQ     DESCRIPTION_REQ    TYPE_REQ 
1      Requirement 1     1 
2      Requirement 2     1 
3      Requirement 3     1 
4      Requirement 4     2 
5      Requirement 5     2 
6      Requirement 6     2 
7      Requirement 7     2 

tb_detail：

ID_DET      ID_EMP      ID_REQ 
1        1        1 
2        1        2 
3        1        4 
4        2        1 
5        2        6 
6        3        4 
7        3        7 

我需要ŧ Ø做出SELECT QUERY计算每个员工有多少requirements得到和类型，像这样：

ID_EMP    NAME_EMP   TYPE_REQ1(virtual column)  TYPE_REQ2 (virt. c.) 
1     Employee 1    2       4 
2     Employee 2    1       1 
3     Employee 3    0       2 
4     Employee 4    0       0 

我真的不知道该怎么做。

Answer 1

试试这个

SELECT 
e.ID_EMP 
,e.NAME_EMP 
,(CASE WHEN SUM(r.TYPE_REQ=1) IS NULL THEN 0 ELSE SUM(r.TYPE_REQ=1) END) TYPE_REQ1 
,(CASE WHEN SUM(r.TYPE_REQ=2) IS NULL THEN 0 ELSE SUM(r.TYPE_REQ=2) END) TYPE_REQ2 
FROM 
tb_employee e 
LEFT JOIN tb_detail d ON (e.ID_EMP=d.ID_EMP) 
LEFT JOIN tb_requirements r ON (d.ID_REQ=r.ID_REQ) 
GROUP BY e.ID_EMP