Mar 5, 2012 · Unique Key (UK): It's a column or a group of columns that can identify a uniqueness in a row. Primary Key (PK): It's also a column or group of columns that can identify a uniqueness in a row. So the Primary key is just another name for unique key, but the default implementation in SQL Server is different for Primary and Unique Key. By Default:

Apr 25, 2015 · Primary key can be related with another table's as a Foreign Key. We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value. Unique Key. Unique Constraint may have a NULL value. Each table can have more than one Unique Constraint. By default, Unique key is a unique non-clustered index.

Sep 25, 2010 · In complex tables, a unique key can be combinations of several columns and it will be inefficient to use unique key for identifying records for transactions. Hence primary key is quick way of identifying a particular record in the table, while unique key guarantees that no two records have same key attributes.)

Jul 5, 2010 · The above query will find all UNIQUE key constraints. However, it will not find PRIMARY KEY constraints, or UNIQUE indexes that were created outside a UNIQUE key constraint. To find the primary key, replace the last line with: and TC.CONSTRAINT_TYPE = 'PRIMARY KEY'

So, basically, it means that every row returned in the result will be unique in terms of the combination of the select query columns. In OP's question, the below two result rows are already distinct, as they have different values for column1 and column 3. 1 item1 data1 2 item1 data2

Nov 7, 2009 · Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values. (In oracle one null is not equal to another null). KEY or INDEX refers to a normal non-unique index. Non-distinct values for the index are allowed, so the index may contain rows with identical values in all columns of the index.

Primary keys are essentially combination of (unique +not null). also when referencing a foreign key the rdbms requires Primary key. Unique key just imposes uniqueness of the column.A value of field can be NULL in case of uniqe key. Also it cannot be used to reference a foreign key, that is quite obvious as u can have null values in it

Jul 5, 2011 · Here is the syntax for creating a unique CONSTRAINT as opposed to a unique INDEX. ALTER TABLE publishers ADD CONSTRAINT uqc_pub_name UNIQUE (pub_name) It is important to note that there are subtle differences dependent on which method you use to enfore the uniqueness of a column.

Jul 11, 2014 · I was making composite key of (ItemName,ItemSize) to uniquely identify item. And now after reading some answers on stackoverflow suggesting the use of UNIQUE i revised it as . Item(ItemID*, ItemName, ItemSize, Price, Notes) But How to apply UNIQUE constraint on ItemName and ItemSize. please correct if there is something wrong in question

Apr 21, 2016 · A primary key is the identifying column or set of columns of a table. Can be surrogate key or any other unique combination of columns (for example a compound key). MUST be unique for any row and cannot be NULL. Example code: CREATE TABLE Example ( PrimaryKey INT PRIMARY KEY -- A primary key is just an unique identifier )