{"id":9854,"date":"2019-02-26T07:27:19","date_gmt":"2019-02-26T07:27:19","guid":{"rendered":"https:\/\/dbtut.com\/?p=9854"},"modified":"2019-02-26T07:34:27","modified_gmt":"2019-02-26T07:34:27","slug":"computed-column-in-sql-server","status":"publish","type":"post","link":"https:\/\/dbtut.com\/index.php\/2019\/02\/26\/computed-column-in-sql-server\/","title":{"rendered":"Computed Column in SQL Server"},"content":{"rendered":"

\u00a0<\/p>\n

Computed Column is a column generated by a calculation using one or more columns in a table. For example, you have columns A, B and C in the table. If you want the sum of your A and B columns to be in your table as a D column, you must create a computed column in this table.<\/p>\n

Example of Computed Column:<\/strong><\/p>\n

CREATE TABLE dbo.ComputedColumnTest   \n(  \n    A int NOT NULL  \n  , B int NOT NULL  \n  , C int NOT NULL  \n  , D AS A + B  \n);  \n<\/pre>\n
Add a computed column to an existing table:<\/strong><\/p>\n
ALTER TABLE dbo.ComputedColumnTest\u00a0\u00a0 ADD D AS (A + B);<\/pre>\n
By default, Computed Columns are not stored on the disk unless they are marked as PERSISTED. So they’re actually virtual. Data is automatically calculated when the data is requested.<\/p>\n
If it is marked persisted, the calculation and to write to disk operations are performed on each insert operation or when column associated with “computed column” is updated.<\/p>\n
In other words, marking as persisted decreases the performance of Insert, Update, and delete. Select performance will increase slightly. You must determine this process according to the needs of your system.<\/p>\n
PERSISTED Computed Column:<\/strong><\/p>\n
CREATE TABLE dbo.ComputedColumnTest   \n(  \n    A int NOT NULL  \n  , B int NOT NULL  \n  , C int NOT NULL  \n  , D AS A + B  PERSISTED\n);<\/pre>\n
You cannot insert or update these columns. If we want to insert and update these columns as below, we will get an error.<\/p>\n
The error we received when we wanted to do insert;<\/p>\n
INSERT INTO dbo.ComputedColumnTest(A,B,C,D) VALUES(1,1,1,1)<\/pre>\n
Msg 271, Level 16, State 1, Line 9<\/em><\/span><\/p>\n
The column “D” cannot be modified because it is either a computed column or is the result of a UNION operator.<\/em><\/span><\/p>\n
\"\"<\/p>\n
If you want to add default constraint to the computed column, the below error will be occured.<\/p>\n
ALTER TABLE dbo.ComputedColumnTest ADD CONSTRAINT dc DEFAULT 1 FOR D<\/pre>\n
Msg 1752, Level 16, State 0, Line 9<\/em><\/span><\/p>\n
Column ‘D’ in table ‘ComputedColumnTest2’ is invalid for creating a default constraint.<\/em><\/span><\/p>\n
Msg 1750, Level 16, State 0, Line 9<\/em><\/span><\/p>\n
Could not create constraint or index. See previous errors.<\/em><\/span><\/p>\n
\"\"<\/p>\n
Below you can find the results of the tests I have done with Computed Column;<\/strong><\/p>\n
\u2022 If the Computed Column is defined as PERSISTED, the primary key and the foreign key can be defined.<\/p>\n
\u2022 You can create index and unique constraint.<\/p>\n
\u2022 You can create the Computed Column using non-deterministic functions(generating different values each time it is called) such as GETDATE (). But you can’t set PERSISTED, and you can’t create index.<\/p>\n
\u2022 You cannot insert or update to the computed column.<\/p>\n
\u00a0<\/p>\n
You can read the following articles about the concepts such as primary key, foreign key, unique constraint, default constraint.<\/p>\n
“What is Primary Key and Foreign Key<\/a>“,<\/p>\n
“What is Unique Constraint<\/a>“,<\/p>\n
“Differences Between Primary Key and Unique Constaint<\/a>“,<\/p>\n
“How To Create Default Constraint On SQL Server<\/a>“<\/p>\n\n\n
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<\/div>","protected":false},"excerpt":{"rendered":"
\u00a0 Computed Column is a column generated by a calculation using one or more columns in a table. For example, you have columns A, B and C in the table. If you want the sum of your A and B columns to be in your table as a D column, you must create a computed …<\/p>\n
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<\/div>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_uf_show_specific_survey":0,"_uf_disable_surveys":false,"footnotes":""},"categories":[3],"tags":[2156,2148,2152,2149,2154,2153,2150,2155,2151],"class_list":["post-9854","post","type-post","status-publish","format-standard","","category-mssql","tag-column-in-table-is-invalid-for-creating-a-default-constraint","tag-computed-column","tag-create-index-on-computed-column","tag-persisted","tag-set-computed-column-as-foreign-key","tag-set-computed-column-as-primary-key","tag-set-computed-column-persisted","tag-the-column-cannot-be-modified-because-it-is-either-a-computed-column-or-is-the-result-of-a-union-operator","tag-update-computed-column"],"aioseo_notices":[],"yoast_head":"\nComputed Column in SQL Server - 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