SQL 中两个表之间的条件更新

Question

下面两张表，对于同校名的学生和家长：我想让它把parent_name赋值给student_name。

CREATE TABLE students2 (
     student_id CHAR(2),
     student_name VARCHAR2(30),
     student_grade NUMBER(2),
     school_name VARCHAR2(50)
)



CREATE TABLE parents (
     student_id CHAR(2),
     parent_name VARCHAR2(30),
     school_name VARCHAR2(50)
)


UPDATE students2
SET student_name = (SELECT parent_name 
                    FROM parents 
                    WHERE parents.school_name = students2.school_name)

当我写这个查询时，更新完成了，但是 student_name 不满足条件的 NULL。他们不应该改变。

Answer 1

您需要说明应该更新哪些行；参见 WHERE EXISTS。

UPDATE students2 a SET 
  a.student_name = (SELECT b.parent_name 
                    FROM parents b
                    WHERE b.school_name = a.school_name)
WHERE EXISTS (SELECT NULL
              FROM parents c
              WHERE c.school_name = a.school_name);

或者，由于您还没有学习 EXISTS：

UPDATE students2 a SET 
  a.student_name = (SELECT b.parent_name 
                    FROM parents b
                    WHERE b.school_name = a.school_name)
WHERE a.school_name IN (select c.school_name
                        FROM parents c);

Answer 2

据我了解，如果要将 parent_name 分配给 student_name，查询不应该是这样的-

UPDATE students2 SET student_name = (SELECT parent_name FROM parents WHERE parents.student_id= students2.student_id)

无论如何，如果我遗漏了什么或者你已经知道了，也许你可以试试这个空值：

UPDATE students2 SET student_name =(SELECT parent_name FROM parents
WHERE
    CASE
        WHEN %s IS NOT NULL
            < statements > 
        ELSE < statements >
        END
    [AND/OR] <other constraints here> )

如有错误请指正

SQL 中两个表之间的条件更新

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