Informix的更新加入

Question

下面的查詢似乎不工作，並給語法錯誤：

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 

如果我轉換成一個子查詢具體步驟如下，它的工作原理，但更新的更多數比要求的記錄：

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來自表school_category_type。）

Answer 1

通常需要一個語句級WHERE子句，以限制記錄數量更新爲具有匹配條目的記錄數量。我認爲，在這個例子中，它可以採取以下形式：

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) 

^{未經測試的SQL}

這限制了行更新那些其中有一個匹配的記錄。在缺少語句級WHERE子句的情況下，表中的所有行（const_acad_record）都會更新，而那些沒有與SELECT匹配的條目的行將被設置爲NULL。

如果我理解表更好，我可能會使用IN子句中的語句級查詢，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) 

^{未經測試的SQL}

除其他問題，不清楚列education_level是來自表school_category_type還是來自表sch_rec;這可能會改變什麼是合適的。