Question

基本上我的疑问是，如果我们在 LEFT OUTER JOIN 的 ON 子句中指定条件，或者我们在 WHERE 中指定条件，是否相同带有空检查的子句？

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');

Answer 1

结果和可读性都不一样。

当您在on子句中写入条件时，您将包含App中的所有行。
当您在where子句中写入条件时，您会从结果中过滤行：

如果您的App行与b_STATUS ='已禁用'的App_Child相关，则会过滤

这是一个示例：

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 --

SQL查询比较（ON子句或WHERE子句中的连接条件）

1 个答案: