SQL 在 Where 子句中处理可为空的外键

Question

假设我有一个名为 my_table 的表，其中包含 primary_key table_pk 和三个分别名为 X_fk、Y_fk 和 Z_fk 的可空字段（外键），以及另一个名为 data 的字段。表 X、Y 和 Z 都有一个主键字段和一个名称字段（例如 {X_pk, X_name}, {Y_pk, Y_name}, {Z_pk, Z_name}）。

在给定表 X、Y 和 Z 中的名称的情况下，我想要一个从 my_table 中唯一返回一行的查询。

SELECT table_pk 
FROM my_table 
WHERE 
    X_fk = (SELECT X_pk FROM X WHERE X_name = ?) 
  AND 
    Y_fk = (SELECT Y_pk FROM Y WHERE Y_name = ?) 
  AND 
    Z_fk = (SELECT Z_pk FROM Z WHERE Z_name = ?)

当我想找到行 (X_Name, Y_Name, Z_Name) = ('XXX', 'YYY', NULL) 时，这不起作用，因为 Z_fk = NULL 永远不会导致 TRUE。如何修改上述查询以从表中提取一个唯一条目，以说明某些外键为 NULL 的唯一条目？

Answer 1

试试这个：

SELECT min(table_pk)
FROM my_table 
WHERE 
    ((X_fk = (SELECT X_pk FROM X WHERE X_name = ?)) OR (? IS NULL AND X_fk IS NULL))
AND 
    ((Y_fk = (SELECT Y_pk FROM Y WHERE Y_name = ?)) OR (? IS NULL AND Y_fk IS NULL)) 
AND 
    ((Z_fk = (SELECT Z_pk FROM Z WHERE Z_name = ?)) OR (? IS NULL AND Z_fk IS NULL))

Answer 2

与 LukLed 的答案相同，但输入仅从前端绑定一次

SELECT table_pk 
FROM my_table
cross join (select ? iX, ? iY, ? iZ) i
WHERE 
    ((i.iX is null and X_fk is null) OR X_fk = (SELECT X_pk FROM X WHERE X_name = i.iX))
  AND 
    ((i.iY is null and Y_fk is null) or Y_fk = (SELECT Y_pk FROM Y WHERE Y_name = i.iY))
  AND 
    ((i.iZ is null and Z_fk is null) or Z_fk = (SELECT Z_pk FROM Z WHERE Z_name = i.iZ))

这看起来更短，但对索引没有任何好处（假设 -1 不是有效值）。

SELECT table_pk 
FROM my_table
WHERE 
    Isnull(X_fk,-1) = isnull((SELECT X_pk FROM X WHERE X_name = ?),-1)
  AND 
    Isnull(Y_fk,-1) = Isnull((SELECT Y_pk FROM Y WHERE Y_name = ?),-1)
  AND 
    Isnull(Z_fk,-1) = Isnull((SELECT Z_pk FROM Z WHERE Z_name = ?),-1)