SQL查询比较(ON子句或WHERE子句中的联接条件)

本文介绍了SQL查询比较(ON子句或WHERE子句中的联接条件)的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

基本上我的疑问是，是否在 LEFT OUTER JOIN 的 ON 子句中指定条件，还是在 WHERE 中指定条件，是否相同?空检查子句?

Basically my doubt is , is it same if we specify a condition in ON clause of LEFT OUTER JOIN or we specify condition in WHERE clause with null check ?

表架构:

Create table App(ID number , STATUS varchar2(10));
Create table App_Child(child_id number ,
                       STATUS varchar2(10),
                       ID number );

查询1

SELECT a.ID AS appID, a.STATUS AS appSTATUS,
b.child_id AS acOWNID,b.STATUS AS acSTATUS, b.id AS acID
 FROM App  a LEFT OUTER JOIN App_Child b ON (a.id=b.id AND b.STATUS <> 'disabled')  WHERE a.ID = ?;

查询2

SELECT a.ID AS appID, a.STATUS AS appSTATUS,
b.child_id AS acOWNID,b.STATUS AS acSTATUS, b.id AS acID
 FROM App  a LEFT OUTER JOIN App_Child b ON (a.id=b.id)  WHERE a.ID = ? AND (b.STATUS IS NULL OR b.STATUS<>'disabled');

推荐答案

结果和可读性都不相同.

在on子句中编写条件时，您将包含App中的所有行.
在where子句中写入条件时，将过滤结果中的行:

When you write condition in on clause you include all rows from App.
When you write condition in where clause you filter rows from result:

在您的情况下，具有与b.STATUS ='disabled'的App_Child相关的行的App将被过滤

In your case an App with row related with a App_Child with b.STATUS='disabled' will be filtered

这里有一个样本:

INSERT INTO App VALUES(1,'active');

INSERT INTO App_Child VALUES(3,'disabled',1);

SELECT a.ID AS appID, a.STATUS AS appSTATUS,
b.child_id AS acOWNID,b.STATUS AS acSTATUS, b.id AS acID
 FROM App  a LEFT OUTER JOIN App_Child b
      ON (a.id=b.id AND b.STATUS <> 'disabled')
WHERE a.ID = 1;

--- has results ---

SELECT a.ID AS appID, a.STATUS AS appSTATUS,
b.child_id AS acOWNID,b.STATUS AS acSTATUS, b.id AS acID
 FROM App  a LEFT OUTER JOIN App_Child b ON (a.id=b.id)
WHERE a.ID = 1 AND (b.STATUS IS NULL OR b.STATUS<>'disabled');

-- don't has results --

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