SQL> select index_name, index_type, UNIQUENESS, BLEVEL, LEAF_BLOCKS, DISTINCT_KEYS from dba_indexes where index_name in ('IDX_T_NORMALID','IDX_T_UNIID'); INDEX_NAMEINDEX_TYPEUNIQUENESSBLEVEL LEAF_BLOCKS DISTINCT_KEYS --- --- --- --- --- --- IDX_T_UNIIDNORMALUNIQUE110651330 IDX...
Following is the syntax for creating a UNIQUE INDEX in SQL −CREATE UNIQUE INDEX index_name ON table_name (column1, column2, ..., columnN); Here,index_name is the name of the index that you want to create. table_name is the name of the table on which you want to create the ...
一般来说,在WHERE和JOIN中出现的列需要建立索引,但也不完全如此,因为MySQL只对 <,<=,=,>,>=,BETWEEN,IN,以及某些时候的LIKE(后面有说明)才会使用索引。 SELECT t.vc_Name FROM testIndex t LEFT JOIN myIndex m ON t.vc_Name=m.vc_Name WHERE m.i_Age=20 AND m.vc_City='郑州' 时,有对myIndex...
唯一索引(unique index,一般写成unique key)、 普通索引(index,只有这一种才是纯粹的index)等,也是基于是不是把index看作了key。 比如create table t(id int,unique indexinx_tx_id (id));--index当作了key使用 (2)最重要的也就是,不管如何描述,需要理解index是纯粹的index(普通的key,或者普通索引index),还...
而Index则处于实现层面,比如可以对表的任意列建立索引,那么当建立索引的列处于SQL语句中的Where条件中时,就可以得到快速的数据定位,从而快速检索。 至于Unique Index,则只是属于Index中的一种而已,建立了Unique Index表示此列数据不可重复,猜想MySQL对Unique Index类型的索引可以做进一步特殊优化吧。
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This topic describes how to create a unique index on a table in SQL Server by using SQL Server Management Studio or Transact-SQL. A unique index guarantees that the index key contains no duplicate values and therefore every row in the table is in some way unique. There are no significant ...
对于唯一索引(unique index),每个索引值对应着唯一的一个 rowid。对于非唯一索引(nonunique index),每个索引值对应 … blog.csdn.net|基于414个网页 2. 唯一性索引 唯一性索引(Unique Index)与普通索引(Normal Index)差异(上) (realkid4, 2011-6-16)AWR集中管理 (redhouser, 2011-6-… ...
nulls in SQL are not values - they are the lack thereof. Since they are not values, they don't affect the uniqueness, and a unique column can have as many nulls as you like. In JPA, you can specify this with the unique attribute of the @Column annotation: @Column(un...
You can create a unique constraint in SQL Server by using SQL Server Management Studio or Transact-SQL to ensure no duplicate values are entered in specific columns that don't participate in a primary key. Creating a unique constraint automatically creates a corresponding unique index....