如何在WHERE CLAUSE中使用CASE别名？

Question

我正在尝试将“val”放入where子句，但它返回错误：

    Select ff.FormID, ff.FieldID, ff.FieldName, ff.Title, 
    ff.DefaultValue, fv.Value, 
    val = case fv.Value when null then cast(ff.DefaultValue as nvarchar) else fv.Value end,
    ff.DataType from
    (SELECT FormID, FieldID, FieldName, Title, DataType, DefaultValue FROM FormFields where FormID = '766A38D8-8058-42C6-AC46-A18C00D3C1DC' and DEL = 0) as ff
    left join
    (select FormID, FieldID, Value from FormValues where FormID = '766A38D8-8058-42C6-AC46-A18C00D3C1DC' and ItemID = 'FD63CCA2-C95F-4AB4-B84B-A220017027E7' and DEL = 0) as fv
    on ff.FormID = fv.FormID and ff.FieldID = fv.FieldID
where val is not null


Msg 207, Level 16, State 1, Line 9
Invalid column name 'val'.

感谢任何帮助：）

Answer 1

不允许在where子句（在sql server中）使用别名，因为查询的逻辑执行顺序如下：

1. FROM
2. ON
3. JOIN
4. WHERE
5. GROUP BY
6. WITH CUBE或WITH ROLLUP
7. HAVING
8. 选择
9. DISTINCT
10. ORDER BY
11. TOP

正如您所见WHERE子句正在SELECT之前执行，这就是为什么您不能引用SELECT子句中的别名

尝试：

Select ff.FormID, ff.FieldID, ff.FieldName, ff.Title, 
ff.DefaultValue, fv.Value, 
val = case fv.Value when null then cast(ff.DefaultValue as nvarchar) else fv.Value end,
ff.DataType from
(SELECT FormID, FieldID, FieldName, Title, DataType, DefaultValue FROM FormFields where FormID = '766A38D8-8058-42C6-AC46-A18C00D3C1DC' and DEL = 0) as ff
left join
(select FormID, FieldID, Value from FormValues where FormID = '766A38D8-8058-42C6-AC46-A18C00D3C1DC' and ItemID = 'FD63CCA2-C95F-4AB4-B84B-A220017027E7' and DEL = 0) as fv
on ff.FormID = fv.FormID and ff.FieldID = fv.FieldID
where case fv.Value when null then cast(ff.DefaultValue as nvarchar) else fv.Value end is not null