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Specify computed columns in a table

  • Статия

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

A computed column is a virtual column that isn't 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. You can specify an expression for a computed column in SQL Server by using SQL Server Management Studio (SSMS) or Transact-SQL (T-SQL).

Limitations

  • A computed column can't 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, a computed column defined as a + b might be indexed, but computed column defined as a + DATEPART(dd, GETDATE()) can't be indexed, because the value might change in subsequent invocations.

  • A computed column can't be the target of an INSERT or UPDATE statement.

  • SET QUOTED_IDENTIFIER must be ON when you're creating or changing indexes on computed columns or indexed views. For more information, see SET QUOTED_IDENTIFIER (Transact-SQL).

Permissions

Requires ALTER permission on the table.

Use SQL Server Management Studio

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 dropdown for the Is Persisted child property.

  6. On the File menu, select Save table name.

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

Use Transact-SQL

Add a computed column when creating a table

The following example creates a table with a computed column that multiplies the value in the QtyAvailable column times the value in the UnitPrice column.

CREATE TABLE dbo.Products
   (
      ProductID int IDENTITY (1,1) NOT NULL
      , QtyAvailable smallint
      , UnitPrice money
      , InventoryValue AS QtyAvailable * UnitPrice
    );

-- Insert values into the table.
INSERT INTO dbo.Products (QtyAvailable, UnitPrice)
   VALUES (25, 2.00), (10, 1.5);

-- Display the rows in the table.
SELECT ProductID, QtyAvailable, UnitPrice, InventoryValue
FROM dbo.Products;

-- Update values in the table.
UPDATE dbo.Products 
SET UnitPrice = 2.5
WHERE ProductID = 1;

-- Display the rows in the table, and the new values for UnitPrice and InventoryValue.
SELECT ProductID, QtyAvailable, UnitPrice, InventoryValue
FROM dbo.Products;

Add a new computed column to an existing table

The following example adds a new column to the table created in the previous example.

ALTER TABLE dbo.Products ADD RetailValue AS (QtyAvailable * UnitPrice * 1.5);

Optionally, add the PERSISTED argument to physically store the computed values in the table:

ALTER TABLE dbo.Products ADD RetailValue AS (QtyAvailable * UnitPrice * 1.5) PERSISTED;

Change an existing column to a computed column

The following example modifies the column added in the previous example.

ALTER TABLE dbo.Products DROP COLUMN RetailValue;
GO

ALTER TABLE dbo.Products ADD RetailValue AS (QtyAvailable * UnitPrice * 1.5);
GO

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