How are Computed Columns used in SQL Server?
Fortunately, SQL Server provides us with a feature called computed columns that allows you to add a new column to a table with the value derived from the values of other columns in the same table. For example, you can add the full_name column to the persons table by using the ALTER TABLE ADD column as follows:
How to add a computed column to an existing column?
To add a computed column definition to an existing column. In Object Explorer, right-click the table with the column for which you want to change and expand the Columns folder. Right-click the column for which you want to specify a computed column formula and click Delete. Click OK.
Which is an example of a computed column?
A computed column is a virtual column that is not physically stored in the table, unless the column is marked PERSISTED. The column is computed from an expression that uses other columns in the same table. For example, a computed column can have the definition: cost AS price * qty. The expression can be a noncomputed column name, constant,…
Can a computed column be used in an index?
However, if the computed column value is defined by a deterministic expression and the data type of the result is allowed in index columns, a computed column can be used as a key column in an index or as part of any PRIMARY KEY or UNIQUE constraint.
In this case, you can utilize computed columns in SQL Server. Similarly, in another example of [Employee] table, we have a DOB for each employee. We need a computed column to show [Age] of the employee. A computed column in SQL Server is a virtual column that computes its values from an expression.
How to mark a computed column as persisted?
If we mark a computed column as persisted, we can define constraints such as Check, Not NULL, or Foreign key. It also allows you to define an index for the persisted computed column. Let’s create a new table using the SSMS table designer wizard. Expand Databases -> right-click on Tables and navigate to New -> Table :
Do you have to persist Computed Columns in SQL?
There are three important caveats to this: The computed column must be persisted. Even if indexed, the column must also be persisted. Match back from general query expressions to persisted computed columns is disabled. The documentation does not describe the function of the trace flag, and does not prescribe it for any other use.
What is the definition of a computed column?
A computed column is a virtual column that is not physically stored in the table, unless the column is marked PERSISTED. A computed column expression can use data from other columns to calculate a value for the column to which it belongs.