Specify Computed Columns in a Table

https://docs.microsoft.com/en-us/sql/relational-databases/tables/specify-computed-columns-in-a-table?view=sql-server-2017

 Before You Begin

Limitations and Restrictions

• A computed column cannot be used as a DEFAULT or FOREIGN KEY constraint definition or with a NOT NULL constraint definition. 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. For example, if the table has integer columns a and b, the computed column a + b may be indexed, but computed column a + DATEPART(dd, GETDATE()) cannot be indexed, because the value might change in subsequent invocations.

• A computed column cannot be the target of an INSERT or UPDATE statement.

Security

Permissions

Requires ALTER permission on the table.

 

 

 Using SQL Server Management Studio

To add a new computed column

1. In Object Explorer, expand the table for which you want to add the new computed column. Right-click Columns and select New Column.

2. Enter the column name and accept the default data type (nchar(10)). The Database Engine determines the data type of the computed column by applying the rules of data type precedence to the expressions specified in the formula. For example, if the formula references a column of type money and a column of type int, the computed column will be of type money because that data type has the higher precedence. For more information, see Data Type Precedence (Transact-SQL).

3. In the Column Properties tab, expand the Computed Column Specification property.

4. In the (Formula) child property, enter the expression for this column in the grid cell to the right. For example, in a SalesTotal column, the formula you enter might be SubTotal+TaxAmt+Freight, which adds the value in these columns for each row in the table.

   Important

   When a formula combines two expressions of different data types, the rules for data type precedence specify that the data type with the lower precedence is converted to the data type with the higher precedence. If the conversion is not a supported implicit conversion, the error "Error validating the formula for column column_name." is returned. Use the CAST or CONVERT function to resolve the data type conflict. For example, if a column of type nvarchar is combined with a column of type int, the integer type must be converted to nvarchar as shown in this formula ('Prod'+CONVERT(nvarchar(23),ProductID)). For more information, see CAST and CONVERT (Transact-SQL).

5. Indicate whether the data is persisted by choosing Yes or No from the drop-down for the Is Persisted child property.

6. On the File menu, click Savetable name.

To add a computed column definition to an existing column

1. In Object Explorer, right-click the table with the column for which you want to change and expand the Columns folder.

2. Right-click the column for which you want to specify a computed column formula and click Delete. Click OK.

3. Add a new column and specify the computed column formula by following the previous procedure to add a new computed column.

 

作者:Chuck Lu    GitHub