sql - 不能在索引或统计中使用或用作分区键

Question

我有一个 Varchar 列，其日期类似于 01/02/2020 以及“N?A” 我想使用以下表达式创建一个名为 DateValue 的计算列:

(case when isdate([Value])=(1) then CONVERT(smalldatetime, [Value]) else null  end)

然后我想在 DateValue 上放置一个非聚集索引，但是我得到了

Column 'DateValue' in table 'dbo.tblTrkCustomDate' cannot be used in an index or statistics or as a partition key because it is non-deterministic.

对此有什么想法吗？

Answer 1

这是由于创建索引时的一个细节造成的。要在索引中使用，表达式必须是确定性的。作为documentation描述:

Expressions are deterministic if they always return the same result for a specified set of inputs. The IsDeterministic property of the COLUMNPROPERTY function reports whether a computed_column_expression is deterministic.

转换为 smalldatetime 为 non-deterministic under these conditions :

Source or target type is datetime or smalldatetime, the other source or target type is a character string, and a nondeterministic style is specified. To be deterministic, the style parameter must be a constant. Additionally, styles less than or equal to 100 are nondeterministic, except for styles 20 and 21. Styles greater than 100 are deterministic, except for styles 106, 107, 109 and 113.

因此，如果您可以选择一种确定性样式，那么这应该可行。