What's the difference between where clause and on clause when table left join?

问题

SQL1:

select t1.f1,t2.f2 
from t1 
   left join t2 on t1.f1 = t2.f2 and t1.f2=1 and t1.f3=0 

SQL2:

select t1.f1,t2.f2 
from t1 
  left join t2 on t1.f1 = t2.f2 
where t1.f2=1 and t1.f3=0

The difference is where and on clause, is there same return result? and what\'s the difference ? does DBMS run them in same way? thanks.

回答1:

The where clause applies to the whole resultset; the on clause only applies to the join in question.

In the example supplied, all of the additional conditions related to fields on the inner side of the join - so in this example, the two queries are effectively identical.

However, if you had included a condition on a value in the table in the outer side of the join, it would have made a significant difference.

You can get more from this link: http://ask.sqlservercentral.com/questions/80067/sql-data-filter-condition-in-join-vs-where-clause

For example:

select t1.f1,t2.f2 from t1 left join t2 on t1.f1 = t2.f2 and t2.f4=1

select t1.f1,t2.f2 from t1 left join t2 on t1.f1 = t2.f2 where t2.f4=1

- do different things - the former will left join to t2 records where f4 is 1, while the latter has effectively been turned back into an inner join to t2.

回答2:

The first query is quicker than the second one as the join condition is more specific than the second one: it does not makes sense to return records that you will filter with the where clause (it would be better do not return them at all- query1)

Anyway it really depends by the query optimizer.

have a look at the below:

Is a JOIN faster than a WHERE?

回答3:

The relational algebra allows interchangeability of the predicates in the WHERE clause and the INNER JOIN, so even INNER JOIN queries with WHERE clauses can have the predicates rearrranged by the optimizer so that they may already be excluded during the JOIN process.

I recommend you write the queries in the most readble way possible.

Sometimes this includes making the INNER JOIN relatively "incomplete" and putting some of the criteria in the WHERE simply to make the lists of filtering criteria more easily maintainable.

You can get more from this link: http://ask.sqlservercentral.com/questions/80067/sql-data-filter-condition-in-join-vs-where-clause

For example, instead of:

SELECT *
FROM Customers c
INNER JOIN CustomerAccounts ca
    ON ca.CustomerID = c.CustomerID
    AND c.State = 'NY'
INNER JOIN Accounts a
    ON ca.AccountID = a.AccountID
    AND a.Status = 1

Write:

SELECT *
FROM Customers c
INNER JOIN CustomerAccounts ca
    ON ca.CustomerID = c.CustomerID
INNER JOIN Accounts a
    ON ca.AccountID = a.AccountID
WHERE c.State = 'NY'
    AND a.Status = 1

But it depends, of course.

回答4:

1)

SQL1: select t1.f1,t2.f2 from t1 left join t2 on t1.f1 = t2.f2 **and** t1.f2=1 and t1.f3=0 

In this, parser will check each row of t1 with each row of t2 with these 3 conditions. Getting faster result.

2) SQL2: select t1.f1,t2.f2 from t1 left join t2 on t1.f1 = t2.f2 **where** t1.f2=1 and t1.f3=0

In this, join only take 1st condition and then the result got from join is filtered with those 2 conditions. And will take more time than 1st query.

You can get more from this link: http://ask.sqlservercentral.com/questions/80067/sql-data-filter-condition-in-join-vs-where-clause

回答5:

The two queries are NOT identical.

Mark Bannister was right in pointing out that the where clause is applied to the whole result set but the on clause applies to the join.

In your case, for SQL 1 LEFT JOIN conditions filter joins on the right but the left side is always returned before any WHERE filtering. Since there are no WHERE conditions all of t1 is always returned.

In SQL 2, the LEFT JOIN conditions filter some results showing up on the right but again all t1 is returned. But this time the WHERE conditions may filter some records of t1 away.

INSERT INTO `t1` (`f1`,`f2`,`f3`) VALUES (1,1,1); INSERT INTO `t2` (`f3`) VALUES (1);

Since they point to different logic the query must be written based on that and it gives us great power and flexibility.

An INNER JOIN however returns the same result so yes check the optimiser.

来源：https://stackoverflow.com/questions/8311096/whats-the-difference-between-where-clause-and-on-clause-when-table-left-join