在JOIN和WHERE中过滤查询之间的区别? [英] Difference between filtering queries in JOIN and WHERE?

问题描述

在SQL中，我试图根据ID过滤结果，并想知道两者之间是否存在逻辑差异

In SQL I am trying to filter results based on an ID and wondering if there is any logical difference between

SELECT value 
FROM table1 
JOIN table2 ON table1.id = table2.id 
WHERE table1.id = 1

和

SELECT value 
FROM table1 
JOIN table2 ON table1.id = table2.id AND table1.id = 1

在我看来，逻辑似乎是不同的，尽管您总是会得到相同的结果集，但是我想知道在任何条件下您将获得两个不同的结果集(还是它们总是返回完全相同的结果)两个结果集)

To me, it seems as if the logic is different though you will always get the same set of results but I wondered if there were any conditions under which you would get two different result sets (or would they always return the exact same two result sets)

推荐答案

答案是否不同，但是:

我将始终喜欢执行以下操作.

I will always prefer to do the following.

• 始终将加入条件保留在ON子句中
• 始终将过滤器的放在where子句中

• Always keep the Join Conditions in ON clause
• Always put the filter's in where clause

这使查询更具可读性.

所以我将使用以下查询:

So I will use this query:

SELECT value
FROM table1
INNER JOIN table2
        ON table1.id = table2.id
WHERE table1.id = 1

但是，当您使用OUTER JOIN'S时，将过滤器保持在ON条件和Where条件中会有很大的不同.

However when you are using OUTER JOIN'S there is a big difference in keeping the filter in the ON condition and Where condition.

逻辑查询处理

以下列表包含查询的一般形式，以及根据逻辑上处理不同子句的顺序分配的步骤号.

The following list contains a general form of a query, along with step numbers assigned according to the order in which the different clauses are logically processed.

(5) SELECT (5-2) DISTINCT (5-3) TOP(<top_specification>) (5-1) <select_list>
(1) FROM (1-J) <left_table> <join_type> JOIN <right_table> ON <on_predicate>
| (1-A) <left_table> <apply_type> APPLY <right_table_expression> AS <alias>
| (1-P) <left_table> PIVOT(<pivot_specification>) AS <alias>
| (1-U) <left_table> UNPIVOT(<unpivot_specification>) AS <alias>
(2) WHERE <where_predicate>
(3) GROUP BY <group_by_specification>
(4) HAVING <having_predicate>
(6) ORDER BY <order_by_list>;

流程图逻辑查询处理

• (1)FROM:FROM阶段标识查询的源表， 进程表运算符.每个表运算符都应用一系列 子阶段.例如，联接中涉及的阶段是(1-J1) 笛卡尔乘积，(1-J2)ON过滤器，(1-J3)添加外部行.来自 阶段生成虚拟表VT1.

• (1) FROM: The FROM phase identifies the query’s source tables and processes table operators. Each table operator applies a series of sub phases. For example, the phases involved in a join are (1-J1) Cartesian product, (1-J2) ON Filter, (1-J3) Add Outer Rows. The FROM phase generates virtual table VT1.

(1-J1)笛卡尔积:此阶段执行笛卡尔积 (交叉连接)表运算符中涉及的两个表之间， 生成VT1-J1.

(1-J1) Cartesian Product: This phase performs a Cartesian product (cross join) between the two tables involved in the table operator, generating VT1-J1.

它是从 此出色链接 .

It is referred from this excellent link.

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