是否可以使用包含的 SQL 子查询中的值作为其包含查询的表名

Question

Might be a bit of a strange request, but I'm trying to create a query in SQL that retrieves a value from one of three possible tables whose name, the choice of which is dynamically derived from the value of a field taken from the parent query.

So in theory

SELECT doc.[DOC ID], doc.[PO Number], doc.[Supplier Coder], doc.[APPROVER DEPT],
(SELECT [Approver] FROM {dat} WHERE {dat}.[DOC ID] == doc.[DOC ID])
FROM documents doc

where the table name {dat} would be a concatenation of the the string "DEPT" and the value of doc.[APPROVER DEPT]. For example if the record returned 1 for the APPROVER DEPT field, I would want to retreive the value of the Approver field held in the DEPT1 where the doc IDs match.

Is this possible to achieve within a single SQL statement? I can incorporate the query into the existing C# code and provide a solution that way, however, the desired SQL statements are going to be used for views and may be subjected to editing from third parties, so I'd prefer to provide an SQL only solution if possible.

Answer 1

While you shouldn't have three separate tables storing the same data, I'd recommend using a union if you can't change the schema to be normalized.

SELECT doc.[DOC ID], doc.[PO Number], doc.[Supplier Coder], doc.[APPROVER DEPT],
(select [Approver] from (
SELECT [Approver], 'DEPT1' as dept, [DOC ID] FROM DEPT1
union all
SELECT [Approver], 'DEPT2' as dept, [DOC ID] FROM DEPT2
union all
SELECT [Approver], 'DEPT3' as dept, [DOC ID] FROM DEPT3
) s where s.[DOC ID] = doc.[DOC ID] and s.dept = doc.[APPROVER DEPT]
)
    FROM documents doc