MySQL：查询联合，使用条件WHERE连接

假设我们有以下表格、员工和工资单：

表：雇员

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

表：工资单

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，其中

月结单='03'

和

年结单='2016'

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

我尝试了以下查询：

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'

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'

对此有何建议？

尝试以下查询：

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'

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'

---

这里的技巧是在

JOIN

条件中，而不是在

WHERE

子句中，移动对

month\u slip

和

year\u slip

的限制

按照下面的链接进行运行演示：

---

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

---

id\u employee=2

的

month\u slip

和

year\u slip

将为

null

。很好的捕获…没有看到id\u employee 2在您按书面方式运行查询时会显示出来。如果您混淆了

左联接

并使用了

内部联接

或只是

联接

，则ie_employee 2将不会显示。