A column expression is a limited form of expr. It is named column_expression in the syntax of expressions.
A column expression can be a simple expression, compound expression, function expression, or expression list, but it can contain only the following forms of expression:
Columns of the target table (the table created, altered, or indexed)
Constants (strings or numbers)
Deterministic functions (either SQL built-in functions or user-defined functions)
Except for the expression forms mentioned earlier, no other forms are valid column expressions. In addition, column expressions do not support compound expressions or aggregate functions that use the PRIOR keyword.
You can use a column expression for the following purposes:
Create a function-based index.
Explicitly or implicitly define a virtual column. When you define a virtual column, the
column_expressionapplies only to the columns of the target table that have been defined in a prior statement.
The components of a column expression must be deterministic. That is, the same input values must return the same output values.