使用多连接从其他表更新 informix

Question

以下查询似乎不起作用并给出语法错误：

update const_acad_record
set const_acad_record.education_level = school_category_type.education_level
FROM school_category_type, sch_rec sch_rec
where const_acad_record.organization_ID = sch_rec.id and
sch_rec.ctgry = school_category_type.code

如果我将其转换为子查询，如下所示，它可以工作，但更新的记录数量超过了要求：

update const_acad_record
set education_level = (SELECT education_level
FROM school_category_type sct, sch_rec sr
where const_acad_record.organization_ID = sr.id and
sr.ctgry = sct.code)

（列education_level来自表school_category_type。）

Answer 1

您通常还需要语句级别的WHERE 子句，以将更新的记录数限制为具有匹配条目的记录数。我认为在这个例子中，它可以采取以下形式：

UPDATE const_acad_record
   SET education_level = (SELECT education_level
                            FROM school_category_type sct
                            JOIN sch_rec sr
                              ON sr.ctgry = sct.code
                           WHERE const_acad_record.organization_ID = sr.id)
 WHERE EXISTS(SELECT education_level
                FROM school_category_type sct
                JOIN sch_rec sr
                  ON sr.ctgry = sct.code
               WHERE const_acad_record.organization_ID = sr.id)

^{未经测试的 SQL}

这会将行更新限制为有匹配记录的行。在没有语句级 WHERE 子句的情况下，表 (const_acad_record) 中的所有行都会更新，而那些与 SELECT 中没有匹配条目的行将设置为 NULL。

如果我更好地理解这些表，我可能会在语句级查询中使用 IN 子句，例如：

UPDATE const_acad_record
   SET Education_Level = (SELECT education_level
                            FROM school_category_type sct
                            JOIN sch_rec sr
                              ON sr.ctgry = sct.code
                           WHERE const_acad_record.organization_ID = sr.id)
 WHERE Organization_ID IN (SELECT sr.id
                             FROM school_category_type sct
                             JOIN sch_rec sr
                               ON sr.ctgry = sct.code)

^{未经测试的 SQL}

除其他问题外，尚不清楚列education_level是来自表school_category_type还是表sch_rec；这可能会改变什么是合适的。