Database Management Systems
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.
In SQL Server, a computed column is a virtual column that uses an expression to calculate its value.
A computed column will typically use data from other columns in its calculation. If data in any of those columns is updated, the computed column will automatically reflect the update in its calculation.
Continue readingThis article demonstrates how to use T-SQL to create 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.
In SQL Server, the @@MAX_CONNECTIONS configuration function returns the maximum number of simultaneous user connections allowed on an instance of SQL Server. The number returned is not necessarily the number currently configured.
No argument is required. You can simply use it in a SELECT statement to return the maximum number of simultaneous user connections allowed on the current server.