Oracle中试图对一个子查询进行更新时可能会出现ORA-01779错误。该错误的内容为: ORA-01779: cannot modify a column which maps
Oracle中试图对一个子查询进行更新时可能会出现ORA-01779错误。该错误的内容为:
ORA-01779: cannot modify a column which maps to a non-key-preserved table例如,使用以下的更新查询就会出现该错误。
这个错误的意思是,子查询的结果中,更新数据源(test2)的内容不唯一,导致被更新对象(test1)中的一行可能对应数据源(test2)中的多行。 本例中,,test2表的id不唯一,因此test2表中可能存在id相同但是num不相同的数据,这种数据是无法用来更新 test1 的。
解决方法就是保证数据源的唯一性,例如本例中可以为test2.id创建一个唯一索引:
CREATE UNIQUE INDEX test2_idx_001 ON test2 (id);
之后上面的更新就可以执行了。
另外也可以强制 Oracle 执行,方法是加上 BYPASS_UJVC 注释。
BYPASS_UJVC的作用是跳过Oracle的键检查。 这样虽然能够执行了,但是如果test2中存在不唯一的数据,test1就会被更新多次而导致意想不到的结果。

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