Tag: SQL Server

Code, Microsoft, Performance, SQL, SQL Server, T-SQL, Technical

Computed Columns in SQL Server 2005

Posted on by Balaji Ramesh

“Computed Columns” as the name suggests allows you to set the value of a column based on other columns data. Here’s how you could specify the formula in T-SQL: 

--Create Table
CREATE TABLE T1 (
    a INT, 
    b INT, 
    operator CHAR,
    c AS CASE operator
        WHEN '+' THEN a+b
        WHEN '-' THEN a-b
        ELSE a*b
    END
    PERSISTED
) ;

--Insert dummy data into it
INSERT INTO T1 VALUES(1,2,'+')
INSERT INTO T1 VALUES(5,3,'-')
INSERT INTO T1 VALUES(4,4,'')

--View the results
SELECT * FROM T1

Notice the keyword PERSISTED. This will enable the computed column value to be stored physically on to the disk so that it need not be created every time you ask for it. Creating an index on this computed column will improve performance by doing a seek rather than a table scan.

If you want to specify the above computed column in SQL Server management studio directly you can do this:

Modify the table. Go to the computed column specification for the specific column. Expand it and type the following in the “Formula” field:

(CASE [operator] WHEN '+' THEN [a]+[b] WHEN '-' THEN [a]-[b]
 
ELSE [a]*[b] END)

Enter without line breaks. If you want the column values to be persisted in the storage medium, just change the value of “Is Persisted” to “Yes”

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