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Home / Tricky Select Queries / Question No: 63

How to retrieve one child row against each parent id?

How to retrieve only one child row against each parent id even when the child table has multiple matching rows. Multiple rows are like below:
department_id    employee_id
1 1
1 2
2 3
2 4
3 5
3 6
4 7
4 8
4 9
4 10

Answer No: 63

For this MySQL provides GROUP BY which also works even when the SELECT does not specify aggregate function. So the GROUP BY can also be used as follow:
SELECT d.dept_id department_id, e.emp_id AS employee_id  
FROM departments d INNER JOIN employees e
ON e.emp_dept_id = d.dept_id GROUP BY d.dept_id

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