In this example, the Email column is defined as a unique key. This constraint ensures that every email address in the Employees table is unique, but it allows one record to have a NULL email address. Key Differences between Primary Key and Unique Key Uniqueness and NULL Handling Primary Key:...
T-SQL GETDATE() ? ADD and SUBTRACT depending on the condition is CASE STATEMENT ADD COLUMN to variable table? Add prefix in data column Add Time in SQL HH:MM:SS to another HH:MM:SS Adding a column to a large (100 million rows) table with default constraint adding a extra column in...
index_nameindex_descriptionindex_keys PK__student__3213E83F7F60ED59clustered, unique, primary key located on PRIMARYid In the output you can see the only one index. This is the index that was automatically created because of the primary key constraint on the “id” column. ...
Add a Constraint to restrict a generic to numeric types Add a html content to word document in C# (row.Cells[1].Range.Text) Add a trailing back slash if one doesn't exist. Add a user to local admin group from c# Add and listen to event from static class add characters to String ad...
This article will teach you the difference between a primary key and foreign key. This article will also teach you why both of these keys are important when it comes to the maintenance of a relational database structure. All the examples for this lesson are based on Microsoft SQL Server Mana...
constraint, which should be considered when deciding a primary key, is that the selected column of the table should only contain unique values, and it should not contain any NULL values. Some of the primary keys that are commonly used when designing tables are Social Security Number (SSN), ...
UNIQUE KEY Constraint Specifies that values in the column must be unique. NULL values are not allowed.Thanks!Manoj [InfoAxon Technologies Ltd.] 0 Aug, 2006 4 A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are a...
Nevermind the inconsistency between using surrogates and natural keys throughout the model. Michael L Johnwrote: CreateIndexNonclusteredwrote: Maybe you have an example, but I cannot think of a reason why you would need to change a primary key after a table has been creat...
a unique identification number. In case a student has migrated from another country, then the value of Aadhar_ID will be NULL in his/ her case. This is permissible as only one NULL can be included in the unique constraint. This is an essential difference between primary and unique keys. ...
primary key vs unique key in SQL The primary key and unique key are two important concepts in a relational database and are used to uniquely identify a row in a table. Both primary key and unique keys can identify a row uniquely but there is some subtle difference between them which we ...