computed columns

How to Create a Computed Column in SQLite

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Generated column support was added to SQLite in version 3.31.0, which was released on 22 January 2020.

Generated columns and computed columns are the same thing. They are columns whose values are a function of other columns in the same row.

In SQLite, generated columns are created using the GENERATED ALWAYS column-constraint when creating or altering the table.

There are two types of generated column; STORED and VIRTUAL. Only VIRTUAL columns can be added when altering a table. Both types can be added when creating a table.

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What is a Generated Column?

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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.

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How to Tell if a Computed Column is Deterministic in SQL Server

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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.

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How to Check if a Computed Column is “Persisted” in SQL Server

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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.

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Create a Computed Column that Uses Data from Another Table in SQL Server

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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.

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Change an Existing Column to a Computed Column in SQL Server (T-SQL Example)

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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.

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