SQL查询比较（ON子句或WHERE子句中的联接条件）

基本上我的疑问是，如果我们在左外连接的ON子句中指定一个条件，或者在WHERE子句中使用null检查指定一个条件，这是相同的吗

表架构：

问题1

问题2

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结果和可读性都不一样

当您在on子句中写入条件时，将包括应用程序中的所有行。 在where子句中写入条件时，从结果中筛选行： 在您的情况下，将筛选与b.STATUS='disabled'的App_子级相关行的App

以下是一个示例：

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 --

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 = ?;

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');

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 --