将 IF EXISTS 移至 WHERE 子句

Question

在此示例查询中（针对设计不佳的供应商数据库）：

DECLARE @OrderNo AS CHAR(8) = LEFT(@FullOrderNo,8)

IF EXISTS (SELECT 1 FROM Foo WHERE FullOrderNumber = @FullOrderNo)

    SELECT  Stuff
    FROM    Foo
    WHERE   X = 'Y'
            AND FullOrderNumber = @FullOrderNo

  ELSE

    SELECT  Stuff
    FROM    Foo
    WHERE   X = 'Y'
            AND OrderNumber = @OrderNo

由于 SELECT 和 WHERE 子句的第一部分是相同的，有没有办法安全地组合查询同时确保首先检查 FullOrderNumber 匹配项？我看到有no guarantees for WHERE clause evaluation order。

Answer 1

你可以这样写：

SELECT  Stuff
FROM    Foo
WHERE   X = 'Y' AND
        (FullOrderNumber = @FullOrderNo OR
         (NOT EXISTS (SELECT 1 FROM Foo WHERE FullOrderNumber = @FullOrderNo) and OrderNumber = @OrderNo) )

如果您只寻找一行，您可以使用order by 进行优先排序：

SELECT  TOP (1) Stuff
FROM    Foo
WHERE   X = 'Y' AND
        (FullOrderNumber = @FullOrderNo OR OrderNumber = @OrderNo)
ORDER BY (CASE WHEN FullOrderNumber = @FullOrderNo THEN 1 ELSE 2 END)

其实就算有重复，你也可以这样使用with ties：

SELECT  TOP (1) WITH TIES Stuff
FROM    Foo
WHERE   X = 'Y' AND
        (FullOrderNumber = @FullOrderNo OR OrderNumber = @OrderNo)
ORDER BY (CASE WHEN FullOrderNumber = @FullOrderNo THEN 1 ELSE 2 END)