When a unique index is created on multiple columns the values can be duplicated in a single column but the combination of column values in each row must be unique?
When you define an UNIQUE index for a column, the column cannot store multiple rows with the same values. If you define a UNIQUE index for two or more columns, the combined values in these columns cannot be duplicated in multiple rows.
How do you change a column to unique?
First we write ALTER TABLE, then we list the name of the table (in our example: product ), and next we add the clause ADD CONSTRAINT with the name of the unique constraint (in our example: UQ_product_name ). This is followed by the UNIQUE keyword with column/columns (in our example it is column: name ) in parentheses.
How to add unique constraint on multiple columns?
Here’s an example adding a multi-column unique constraint to an existing table. Under the table, right click Indexes ->Click/hover New Index ->Click Non-Clustered Index… A default Index name will be given but you may want to change it. Check the Unique checkbox and click Add… button Click OK in each window and you’re done.
Can a unique index be used in multiple columns?
Fortunately it works and hopefully it will stay that way. Multi column unique indexes do not work in MySQL if you have a NULL value in row as MySQL treats NULL as a unique value and at least currently has no logic to work around it in multi-column indexes.
Can a unique constraint contain a null value?
Yes, Oracle allows UNIQUE constraints to contain columns with NULL contents, but PRIMARY KEY constraints cannot contain columns containing NULL values. (Edited: was “… nullable columns…”, but my example below shows that not to be true. Columns in a PK can be defined as nullable, but cannot contain NULL.)
When to use an index on Column a?
An index on (A, B, C) can be used to seek values on column A, for searching values on both A and B or to search values on all three columns A, B and C. However it cannot be used to search values on B or on C alone.