Question

我正在使用MYSQL。我的数据库中有两个表。

表1：'courseTable'有两个列：

courseId
courseName

表2：'prereqTable'有两个列：

courseId
prereqCourseId

表2中的两列对应于表1中的courseId。我正在尝试运行一个查询，该查询将生成一个包含courseId的courseName和prereqCourseId的courseName的表。

我被困在这里：

SELECT `course`.courseName, `prereq`.prereqCourseId FROM `course`
LEFT OUTER JOIN `prereq`
ON `course`.courseId = `prereq`.courseId

Answer 1

您需要两次加入课程表，并参考prereqTable表

SELECT a.courseName,c.courseName
FROM courseTable AS a
LEFT OUTER JOIN prereqTable AS b ON a.courseId=b.courseId
LEFT OUTER JOIN courseTable AS c ON b.prereqCourseId=c.courseId

Answer 2

如果要两次加入同一个表，则必须使用别名。

SELECT p.name AS parent, c.name AS child
  FROM relations AS r
  LEFT JOIN nodes AS p
    ON(p.id=r.parent)
  LEFT JOIN nodes AS c
    ON(c.id=r.child )

AS是可选的，但建议使用，因为它提高了可读性，并表明您不会忘记逗号。比较：

SELECT  a b, c d, e, f, g, h i, ...

SELECT a AS b, c AS d, e, f, g, h AS i, ...

Answer 3

可以连接两次相同的表，您只需在每个实例中为表分配一个不同的别名，以便您可以从表的相应实例中选择列。像这样：

SELECT
    `course_main`.courseName,
    `course_prereq`.courseName AS `prereqCourseName`

FROM `course` AS `course_main`

LEFT OUTER JOIN `prereq`
ON `course_main`.courseId = `prereq`.courseId

LEFT OUTER JOIN `course` AS `course_prereq`
ON `course_prereq`.courseId = `prereq`.prereqCourseId

如何将SQL表中的2列连接到另一个SQL表中的一列？

3 个答案: