如何在SQL中显示计数值而不是列出多行？

Question

我有三张桌子：考试，目标和考试。

• 考试可以有零个或多个目标
• 考试可以有零次或多次测试

我创建了这个SQL来报告数据：

SELECT name, objectiveId, TestId
FROM exam
LEFT JOIN Test ON Test.ExamId = Exam.ExamId
LEFT JOIN Objective ON Objective.ExamId = Exam.ExamId

我想做的不是显示objectiveId和TestId，而是我愿意 想要显示每次考试有多少目标或考试的数量。某物 像：

Exam    Objectives   Tests
Exam1            1      10
Exam2            2       0

Answer 1

您可以使用：

SELECT name Exam, COALESCE(COUNT(objectiveId), 0) Objectives, COALESCE(COUNT(TestId), 0) Tests
FROM exam
LEFT JOIN Test ON Test.ExamId = Exam.ExamId
LEFT JOIN Objective ON Objective.ExamId = Exam.ExamId
GROUP BY name;