来自Query的SQL空结果

Question

我正在努力解决的问题是当下面的查询推断结果仍为空时：

SELECT teacher.name, teacher.surname, assignment.code
FROM teacher
JOIN assignment
ON assignment.code = teacher.bsnt
WHERE assignment.working_on_assignment = FALSE

我错了什么？

Answer 1

如果没有教师有任何匹配的作业记录，那么内部联接会产生空结果集。您可以尝试执行左（外）连接，并将WHERE条件移动到ON子句。

SELECT
    teacher.name,
    teacher.surname,
    COALESCE(assignment.code, 'NA') AS code
FROM teacher
LEFT JOIN assignment
    ON assignment.code = teacher.bsnt AND
       assignment.working_on_assignment = FALSE

如果每位教师都有NA代码，那么您就会知道没有匹配。