A unique key in SQL is the set of fields or columns of a table that helps us uniquely identify records. The unique key guarantees the uniqueness of the columns in the database. It is similar to the primary key but can accept a null value, unlike it.
In relational database management systems, a unique key is a candidate key. All the candidate keys of a relation can uniquely identify the records of the relation, but only one of them is used as the primary key of the relation.
The primary key is accepted as a unique or sole identifier for every record in the table. In the case of a primary key, we cannot save NULL values. In the case of a unique key, we can save a null value, however, only one NULL value is supported.
Unique Key in SQL. A SQL UNIQUE key constraint (or condition) does not allow duplicate values in a column of a table, i.e. it is used to uniquely identify a record in a table. It prevents two records from having same values in a column.
Unique key is a constraint that is used to uniquely identify a tuple in a table. Multiple unique keys can present in a table. NULL values are allowed in case of a unique key. These can also be used as foreign keys for another table.
A primary key should be unique, but a unique key cannot necessarily be the primary key. The primary key by default is a clustered index where data is physically organized in the sequential index. In contrast, the unique key is a unique non-clustered index.
A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values.
On the Table Designer menu, select Indexes/Keys. In the Indexes/Keys dialog box, select Add. In the grid under General, select Type and choose Unique Key from the drop-down list box to the right of the property, and then select Close.
A foreign key can refer to either a unique or a primary key of the parent table. If the foreign key refers to a non-primary unique key, you must specify the column names of the key explicitly.
Sometimes we want to add a unique key to the column of an existing table; then, this statement is used to add the unique key for that column. Following are the syntax of the ALTER TABLE statement to add a unique key: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE(column_list);
Each table can only have one primary key. Access can automatically create a primary key field for you when you create a table, or you can specify the fields that you want to use as the primary key. This article explains how and why to use primary keys.
If the counts do not match, the primary key is not unique and appears in multiple rows. You will need to choose or create a new dimension as your primary key.
Example of Creating Unique Key
In case, if student is changing the university, in that case, he or she would not have any stud ID. The entry may have a null value as only one null is allowed in the unique key constraint.
Unique keys are used when we want to identify specific items, and do not want any duplicate values. Primary keys are also used to uniquely identify items, but every row must have a primary key, while it is possible for the row to be lacking a unique key.
We can create a unique key on both single and multiple columns.
The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
We can find the UNIQUE key in SQL using the DISTINCT keyword or checking the constraint type with the WHERE clause.
To modify a unique constraint
In the Object Explorer, right-click the table containing the unique constraint and select Design. On the Table Designer menu, click Indexes/Keys.... In the Indexes/Keys dialog box, under Selected Primary/Unique Key or Index, select the constraint you wish to edit.
select count(distinct column_name), count(column_name) from table_name; If the # of unique values is equal to the total # of values, then all values are 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.
When you create a PRIMARY KEY constraint, a unique clustered index on the column or columns is automatically created if a clustered index on the table does not already exist and you do not specify a unique nonclustered index. The primary key column cannot allow NULL values.
Primary key attributes are just as updateable as any other attributes of a table. Stability is often a desirable property of a key but definitely not an absolute requirement.
The data in a specific column is always unique. A database table can only have one primary key. It uniquely identifies a record in a relational database table. The value of a primary key can't be deleted from the tree structure or the parent table.
Both Primary key and Unique key both are used to identify records uniquely in a table. These keys are also used to create relationship between tables. A table can have only one primary key whereas there can be multiple unique keys in a single table or relation.
You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value. This means that it's possible to insert rows that appear to be duplicates if one of the values is NULL .