在 JPQL 中，在左连接的情况下，当我们在 ON 子句或 where 子句中给出特定条件时有什么区别吗？

Question

Is there any difference in these two queries in terms of processing and data obtained:

SELECT * from Table A Inner JOIN Table B ON A.id = B.id 
LEFT JOIN Table B ON A.id = C.id AND (C.status IS NULL OR C.status <>3) 
WHERE A.status = 1 and B.status = 1

OR

SELECT * from Table A Inner JOIN Table B ON A.id = B.id 
LEFT JOIN Table B ON A.id = C.id 
WHERE A.status = 1 and B.status = 1 AND (C.status IS NULL OR C.status <>3)

Answer 1

After a deep dive into left joins and different clauses, Here is what I have came up:

1) When to use where and on clauses in case of joins:

In case of inner joins, it really does not matter where you give your conditions in where clause or on clause. Giving conditions in where clause will be used to filtering the data and giving the conditions in on clause will be to used to join the table. For readability purpose it is good to provide the filtering conditions in where clause only just to make sure you convince your intent through your query. But here is the catch, the query plan will change depending on where you pass your conditions and can heavily affect you. In case of outer joins(left, right, full) passing conditions blindly in where and on clause can give you incorrect data.

2) When Left Join(or any outer join) can start behaving like inner joins Be careful when you specify filtering conditions for null values in where clause. Suppose you have two tables: Users and orders and you want irrespective of whether they have placed order or not, so we chose left join. Further we have condition to filter condition like noOfOrders > 3, but since it was a left join values where null in products table. All null values will be filtered out, since null<0, hence left join will behave like an inner join.