Some database management systems (DBMSs) include a feature called generated columns.
Also known as “computed columns”, generated columns are similar to a normal column, except that a generated column’s value is derived from an expression that computes values from other columns.
In other words, a generated column’s value is computed from other columns.
Continue readingIn SQL Server, you can use the sys.columns system catalog view to return a list of non-computed columns from a table.
By “non-computed”, I simply mean columns that are not computed columns.
When you create a computed column in SQL Server, the expression you use for the column will either be deterministic or nondeterministic. This can have implications, such as whether or not you can use it in an index or flag it as “persisted”.
A deterministic column is one that will return the same value for a specific set of input values and given the same state of the database. A nondeterministic column can return a different value even when given the same input even if the database state remains the same. For example, a function that returns the current date is nondeterministic, because it will return a different value each day.
You can use the COLUMNPROPERTY() function with the IsDeterministic argument to find out whether or not a computed column is deterministic.
When you create a computed column in SQL Server, you have the option of flagging it as “persisted”. A persisted computed column is one that is physically stored in the table. If you don’t specify that it’s persisted, then the column’s value will be calculated each time you run a query against it.
You can query the sys.computed_columns system catalog view to find out whether a computed column is marked as persisted.
A widely known limitation of computed columns in SQL Server is that they can’t access data from other tables. That is, your expression can use columns in the same table, but not from other tables.
But this is only half-true. While you can’t reference another table’s column directly within your expression, you can invoke a user-defined function. And therefore, you could create a user-defined function that performs the calculation you need, then simply call that function as your computed column’s expression.
Here’s an example to demonstrate.
Continue readingIn SQL Server, you can use T-SQL to get the definition for a computed column by querying the sys.computed_columns system catalog view.
In SQL Server, there are a couple of system catalog views that enable you to get a list of computed columns in a database.
One of these views is called sys.computed_columns. The other is sys.columns.
This article presents three ways to use T-SQL to find out if a column is a computed column in SQL Server.
This is for when you know the column name, but you don’t know whether or not it’s a computed column.
This article demonstrates how to use T-SQL to change an existing column to a computed column in SQL Server.
A computed column is a virtual column that uses an expression to calculate its value. The expression will typically use data from other columns. A computed column is not physically stored in the table unless it’s marked PERSISTED.
This article demonstrates how to use T-SQL to add a computed column to an existing table in SQL Server.
A computed column is a virtual column that uses an expression to calculate its value. The expression will typically use data from other columns. A computed column is not physically stored in the table unless it’s marked PERSISTED.