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Oracle concept 学习 6---foreign key, primary, unique key, check
unique key: allow null ,but primary key do not allow.You can only add a column with a NOT NULL constraint if the table does not contain any rows or if you specify a default value.primary key creates a unique index and a not null constraint.1.In some cases, as when y...
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Oracle concept 学习 4---index clustering factor（集群因子）
index clustering factor 是衡量数据在数据块中存放的好坏与整齐有序。 这个值越高，证明查询相应的值造成的I/O冲突越多。从all_index 里查询相关的数据：select index_name ,clustering_factor from all_indexes;
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Oracle concept 学习 3---Index full scan 和 Index fast full scan
首先这个设计到B-tree索引的一些知识,每个叶子block之间有相互的关 ，请参照concept文档介绍。 先简述下区别：1.index fast full scan 是无...
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Oracle concept 学习 2--DATE类型的RR和YY
原文：RR is similar to YY (the last two digits of the year), but the century of the return value varies according to the specified two-digit year and the last two digits of the current year. Assume that in 1999 the database displays 01-JAN-11. If the date format uses RR, then 11 specifies 2011, w...
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Oracle concept 学习 1---精度和 维度
原文：PrecisionThe precision specifies the total number of digits. If a precision is not specified, then the column stores the values exactly as provided by the application without any rounding.ScaleThe scale specifies the number of digits from the decimal point to the least significant digit....
