you can have NULLs in your columns unless the columns are specified NOT NULL. You will be able to store only one instance of NULLs however (no two sets of same columns will be allowed unless all columns are NULL) : SQL> CREATE TABLE t (id1 NUMBER, id2 NUMBER); Table created SQL> ALTER TABLE t … Read more
Add a unique constraint for that column: ALTER TABLE Foo ADD CONSTRAINT UQ_Name UNIQUE (Name) To add it through SQL Management Studio UI: Open SQL Server Management Studio. Expand the Tables folder of the database where you wish to create the constraint. Right-click the table where you wish to add the constraint and click Design. … Read more
This MSDN article comparing the two is for SQL Server 2000: http://msdn.microsoft.com/en-us/library/aa224827(SQL.80).aspx For most purposes, there’s no difference – the constraint is implemented as an index under the covers. And though there’s the ability to disable the constraint, it doesn’t actually work in SQL Server. It only matters if you want to tweak things like … Read more
The IGNORE keyword extension to MySQL seems to have a bug in the InnoDB version on some version of MySQL. You could always, convert to MyISAM, IGNORE-ADD the index and then convert back to InnoDB ALTER TABLE table ENGINE MyISAM; ALTER IGNORE TABLE table ADD UNIQUE INDEX dupidx (field); ALTER TABLE table ENGINE InnoDB; Note, … Read more
In the Alter Table dialog of MySQL Workbench: Go to Indexes tab. Double-click on a blank row to create a new index. Choose ‘UNIQUE’ as the index type. Check the columns that you want to be unique together. There’s some discussion as to whether this is weird, since an index is not the same as … Read more
The unique piece is not where the difference lies. The index and key are not the same thing, and are not comparable. A key is a data column, or several columns, that are forced to be unique with a constraint, either primary key or explicitly defined unique constraint. Whereas an index is a structure for … Read more
userName(unique: [‘countryId’]) You can include as many other properties in the array that make up the other properties that must be considered in the “unique” constraint on the username. So, for example if you wanted to make userName unique within a countryId and provinceId it would look like this: userName(unique: [‘countryId’, ‘provinceId’]
validates_uniqueness_of :user_id, :scope => [:question_id] if you needed to include another column (or more), you can add that to the scope as well. Example: validates_uniqueness_of :user_id, :scope => [:question_id, :some_third_column]
For developers using Room Persistence Library. You can use @Insert(onConflict = OnConflictStrategy.REPLACE) // or OnConflictStrategy.IGNORE in DAO according to Insert Documentation