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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MySQL's position in databases and programming is very important. It is an open source relational database management system that is widely used in various application scenarios. 1) MySQL provides efficient data storage, organization and retrieval functions, supporting Web, mobile and enterprise-level systems. 2) It uses a client-server architecture, supports multiple storage engines and index optimization. 3) Basic usages include creating tables and inserting data, and advanced usages involve multi-table JOINs and complex queries. 4) Frequently asked questions such as SQL syntax errors and performance issues can be debugged through the EXPLAIN command and slow query log. 5) Performance optimization methods include rational use of indexes, optimized query and use of caches. Best practices include using transactions and PreparedStatemen

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