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分组函数Group functions avg 求平均count 求数量max 求值可针对数字,日期,字符min 求值可针对数字,日期,字符sum 求总和listaggstddevvariance
Backup and Recovery Concepts 备份和恢复的概念 you should be able to:Identify the types of failure that c
集合操作 UNION 及 UNION ALLUNION :列名,数据类型要一致,结果会默认ASC自动排序UNION ALL :结果不会去重,不排序,其他和UNION一样用类似to_char(null)