MySQL：查询UNION，有条件的WHERE JOIN

Question

假设我们有以下表格，employee 和 payroll_slip:

Table：员工

 id_employee | name_employee |
    ============|===============|
    1           | john          |
    2           | doe           |
    3           | alex          |

Table: payroll_slip

id_slip | id_employee | month_slip | year_slip |
========|=============|============|===========|
1       | 1           | 01         | 2016      |
2       | 2           | 01         | 2016      |
3       | 1           | 02         | 2016      |
4       | 2           | 02         | 2016      |
5       | 1           | 03         | 2016      |
6       | 3           | 03         | 2016      |

我们希望得到以下结果 month_slip = '03' AND year_slip = '2016'

id_employee | month_slip | year_slip | status_slip
============|============|===========|============
1           | 03         | 2016      | paid
2           | 03         | 2016      | unpaid
3           | 03         | 2016      | paid

我试过这个查询：

SELECT 
a.id_employee, 
payroll_slip.month_slip, 
payroll_slip.year_slip, 
IF(a.id_employee=payroll_slip.id_employee, 'paid', 'unpaid') AS status_slip
    FROM    (
                SELECT id_employee FROM employee
                UNION 
                SELECT id_employee FROM payroll_slip
            ) a
    LEFT OUTER JOIN payroll_slip ON a.id_employee = payroll_slip.id_employee
    LEFT OUTER JOIN employee ON a.id_employee = employee.id_employee
    WHERE payroll_slip.month_slip = '03' AND payroll_slip.year_slip = '2016'

有什么建议吗？

Answer 1

试试这个查询：

SELECT e.id_employee, '03' AS month_slip, '2016' AS year_slip,
    CASE WHEN p.id_employee IS NOT NULL THEN 'paid' ELSE 'unpaid' END AS status_slip
FROM employee e LEFT JOIN payroll_slip p
    ON e.id_employee = p.id_employee AND p.month_slip = '03' AND p.year_slip = '2016'

这里的技巧是将对 month_slip 和 year_slip 的限制移动到 JOIN 条件中，而不是 WHERE 子句。

按照下面的 link 进行运行演示：

SQLFiddle

Answer 2

尝试交叉连接：

 SELECT X.id_employee, X.month_slip, X.year_slip
    , CASE WHEN Y.id_employee IS NULL THEN 'Unpaid' ELSE 'Paid' END AS status_slip
 FROM (
    SELECT A.id_employee, B.month_slip, B.year_slip
    from employee A
    cross join (
        select '03' AS month_slip, '2016' AS year_slip 
    ) B
 ) X
 LEFT JOIN payroll_slip Y
 ON X.id_employee = Y.id_employee
    AND X.month_slip = Y.month_slip
    AND X.year_slip = Y.year_slip

MySQL：查询UNION，有条件的WHERE JOIN

MySQL: Query UNION, JOIN with conditional WHERE

mysql

sql

union

join