使用 JOIN 时的 WHERE 子句与 ON
假设我有以下 T-SQL 代码:
Assuming that I have the following T-SQL code:
SELECT * FROM Foo f
INNER JOIN Bar b ON b.BarId = f.BarId;
WHERE b.IsApproved = 1;
下面的也返回相同的一组行:
The following one also returns the same set of rows:
SELECT * FROM Foo f
INNER JOIN Bar b ON (b.IsApproved = 1) AND (b.BarId = f.BarId);
这可能不是最好的例子，但这两者之间有什么性能差异吗?
This might not be the best case sample here but is there any performance difference between these two?
推荐答案
不，查询优化器足够聪明，可以为两个示例选择相同的执行计划.
No, the query optimizer is smart enough to choose the same execution plan for both examples.
您可以使用SHOWPLAN来检查执行计划.
You can use SHOWPLAN to check the execution plan.
尽管如此，您应该将所有连接连接放在 ON 子句上，并将所有限制放在 WHERE 子句上.
Nevertheless, you should put all join connection on the ON clause and all the restrictions on the WHERE clause.
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