SQL查询比较（ON子句或WHERE子句中的连接条件）

Question

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

Not Not在结果和可读性上都是一样的。

• 在on子句中写入条件时，包含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，将筛选与带有b.status='disabled'的App_Child相关的行的app

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 --