PLS-00452：Subprogram 'string' violates its associated pragma

错误原因：A packaged function cannot be called from SQL statements unless its purity level is asserted by coding a RESTRICT_REFERENCES pragma in the package specification. The pragma, which is used to control side effects, tells the PL/SQL compiler to deny the packaged function read/write access to database tables, public packaged variables, or both. A SQL statement that violates the pragma will cause a compilation error.