将distinct关键字限制为一列

Question

我如何在mysql中应用distinct关键字,以便它只检查一个列字段是唯一的,同时仍然从我的表返回其他列？

Answer 1

为了能够做到这一点,mysql必须知道如何处理其他列.您是GROUP BY应该是唯一的列,并使用一个函数来告诉它如何处理其他(一个所谓的聚合函数).MAX()并且COUNT()是常见的例子:

SELECT studentId, COUNT(courseId) AS AmountEnrolledCourses
FROM student_enrollment
GROUP BY studentId

SELECT athlete, MAX(distance) AS PersonalRecord
FROM longjump
GROUP BY athlete