另: UNIQUE Constraints vs UNIQUE Indexes Many database administrators ask about the difference between a UNIQUE constraint and a UNIQUE index. While you may use different Transact-SQL commands to create them (AL
Unless a NOT NULLconstraint is also defined, a null always satisfies a unique key constraint. Thus,columns with both unique key constraints and NOT NULL constraints are typical.This combination forces the user to enter values in the unique key andeliminates the possibility that new row data confl...
Theterm key refers only to the columns defined in the integrity constraint. Because the database enforces a unique constraint byimplicitly creating or reusing anindexon the key columns, the term uniquekey is sometimes incorrectly used as a synonym for unique key constraint orunique index. --数据...
public void setIndexID(DBObjectIDid) Set the Index that implements this UniqueConstraint Parameters: id- the ID of the Index isUnique public static boolean isUnique(Columncol) Returns true if the given table is constrained by a UniqueConstraint (including PKConstraint) in its parent table. T...
using CREATE UNIQUE INDEX. Creating unique indexes through a primary key or unique constraint is not guaranteed to create a new index, and the index they create is not guaranteed to be a unique index.It is just that in a unique index, the rowid is not considered "part of the key" and...
Therefore, in our little example, we can’t simply create a single concatenated unique index on both the ID and NAME columns and use it to police a unique constraint on just the ID column. We must either use a unique index on the ID column or use a non-unique index on both the ID...
unique indexes, 10-24 UNIQUE key constraints, 25-8 composite keys, 25-9, 25-11 constraint checking, 25-23 indexes used to enforce, 25-10 maximum number of columns, 25-10 NOT NULL constraints and, 25-11 nulls and, 25-11 size limit of, 25-10 unique keys, 1-57, 1-58...
Rails42 add unique index creates unique constraint [#617] Modify remove_column to add cascade constraint to avoid ORA-12991 [#617] Add null: true to avoid DEPRECATION WARNING [#489, #499] Rails 4.2 Add connection.supports_views? [#496] text? has been removed from Column class [#48...
UNIQUE约束 以下ALTER TABLE语句包含约束,如果在GaussDB(DWS)直接调用会报错:Cannot create index whose evaluation cannot be enforced to remote nodes. 该约束迁移和PRIMARY KEY类似。如果已有PRIMARY KEY/UNIQUE约束,无需迁移,保持原样。 输入 1 2 3 4 5 6 7 8 CREATE TABLE GCC_PLAN.T1033 ( ROLLOUT_PLAN...
When you create a primary key or unique constraint, Oracle Database will automatically create a unique index for you (assuming there isn't an index already available). In most cases you'll add the constraint to the table and let the database build the index for you....