我在Oracle中加入联接的更新语句有什么问题？

Question

我正在使用 Oracle 10g数据库。

我有以下两个表：

T_DEBTOR :
    - ID_DEBTOR
    - HEADER
T_ELEMENT :
    - ID_ELEMENT
    - ID_DEBTOR
    - INSURER

使用ID_DEBTOR字段将这两个表连接在一起。

仅当HEADER不为null时，我才想使用关联的T_DEBTOR.HEADER更新T_ELEMENT.INSURER值。换句话说：

If T_DEBTOR.HEADER != null
    Then T_ELEMENT.INSURER = T_DEBTOR.HEADER
    Else T_ELEMENT.INSURER is not modified!

我尝试使用以下SQL查询：

update
    T_ELEMENT elt
    set elt.INSURER = (
        select HEADER
            from T_DEBTOR debtor
            where
                debtor.HEADER is not null
                and debtor.ID_DEBTOR = elt.ID_DEBTOR);

该查询适用于与HEADER不为null的债务人链接的所有元素。但是，当T_DEBTOR.HEADER为null时，此查询会将T_ELEMENT.INSURER设置为null，这是不正确的。

IE：

If T_DEBTOR.HEADER != null
    Then T_ELEMENT.INSURER = T_DEBTOR.HEADER   --> This part is OK
    Else T_ELEMENT.INSURER is set to null      --> This part is NOT OK

我的查询出了什么问题？

编辑，关于Brian Storrar的答案：

我想做的是这样的：

update
    T_ELEMENT elt
    set elt.INSURER = (
        select HEADER
            from T_DEBTOR debtor
            where
                debtor.HEADER is not null
                and debtor.ID_DEBTOR = elt.ID_DEBTOR)
    where debtor.HEADER is not null;

Answer 1

好问题。

为了模拟您的情况，我创建了示例表：

SQL> create table t_debtor(id_debtor,header)
  2  as
  3  select 1, 'Header 1' from dual union all
  4  select 2, null from dual union all
  5  select 3, 'Header 3' from dual
  6  /

Tabel is aangemaakt.

SQL> create table t_element (id_element,id_debtor,insurer)
  2  as
  3  select 1, 1, 'to be updated' from dual union all
  4  select 2, 1, 'to be updated' from dual union all
  5  select 3, 2, 'not to be updated' from dual union all
  6  select 4, 2, 'not to be updated' from dual union all
  7  select 5, 3, 'to be updated' from dual
  8  /

Tabel is aangemaakt.

并且使用您当前的update语句，问题变得很清楚：“不更新”值设置为NULL：

SQL> update
  2      T_ELEMENT elt
  3      set elt.INSURER = (
  4          select HEADER
  5              from T_DEBTOR debtor
  6              where
  7                  debtor.HEADER is not null
  8                  and debtor.ID_DEBTOR = elt.ID_DEBTOR)
  9  /

5 rijen zijn bijgewerkt.

SQL> select * from t_element
  2  /

ID_ELEMENT  ID_DEBTOR INSURER
---------- ---------- -----------------
         1          1 Header 1
         2          1 Header 1
         3          2
         4          2
         5          3 Header 3

5 rijen zijn geselecteerd.

进行此更新的最佳方法是更新两个表的联接。但是有一些限制：

SQL> rollback
  2  /

Rollback is voltooid.

SQL> update ( select elt.insurer
  2                , dtr.header
  3             from t_element elt
  4                , t_debtor dtr
  5            where elt.id_debtor = dtr.id_debtor
  6              and dtr.header is not null
  7         )
  8     set insurer = header
  9  /
   set insurer = header
       *
FOUT in regel 8:
.ORA-01779: cannot modify a column which maps to a non key-preserved table

通过绕过ujvc提示，我们可以规避此限制。但是不建议这样做，除非您真的确定t_debtor.id_debtor是唯一的。

SQL> update /*+ bypass_ujvc */
  2         ( select elt.insurer
  3                , dtr.header
  4             from t_element elt
  5                , t_debtor dtr
  6            where elt.id_debtor = dtr.id_debtor
  7              and dtr.header is not null
  8         )
  9     set insurer = header
 10  /

3 rijen zijn bijgewerkt.

SQL> select * from t_element
  2  /

ID_ELEMENT  ID_DEBTOR INSURER
---------- ---------- -----------------
         1          1 Header 1
         2          1 Header 1
         3          2 not to be updated
         4          2 not to be updated
         5          3 Header 3

5 rijen zijn geselecteerd.

最好只添加一个主键。您可能已经安装了一个：

SQL> rollback
  2  /

Rollback is voltooid.

SQL> alter table t_debtor add primary key (id_debtor)
  2  /

Tabel is gewijzigd.

SQL> update ( select elt.insurer
  2                , dtr.header
  3             from t_element elt
  4                , t_debtor dtr
  5            where elt.id_debtor = dtr.id_debtor
  6              and dtr.header is not null
  7         )
  8     set insurer = header
  9  /

3 rijen zijn bijgewerkt.

SQL> select * from t_element
  2  /

ID_ELEMENT  ID_DEBTOR INSURER
---------- ---------- -----------------
         1          1 Header 1
         2          1 Header 1
         3          2 not to be updated
         4          2 not to be updated
         5          3 Header 3

5 rijen zijn geselecteerd.

问候，罗布。