CODEWITHSUNDEEP
mysqlsqljoinunion
Taufik Arief Widodo
Taufik Arief Widodo

Reputation: 58

MySQL: Query UNION, JOIN with conditional WHERE

Say we have the following tables, employee and payroll_slip:

Table: employee

 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      |

And we want to get the following result where 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

I tried this query:

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'

Any suggestion for this?

Upvotes: 1

Views: 69

Answers (2)

Kim
Kim

Reputation: 825

Try having a cross join:

 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

Upvotes: 1

Tim Biegeleisen
Tim Biegeleisen

Reputation: 521249

Try this query:

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'

The trick here is to move the restrictions on the month_slip and year_slip inside the JOIN condition, rather than in the WHERE clause.

Follow the link below for a running demo:

SQLFiddle

Upvotes: 1

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