SQL JOIN在何处放置WHERE条件？

Question

I have two following examples.

1. Example (WHERE)

SELECT 1
  FROM table1 t1
  JOIN table2 t2 ON t1.id = t2.id
 WHERE t2.field = true

2. Example (JOIN AND)

SELECT 1
  FROM table1 t1
  JOIN table2 t2 ON t1.id = t2.id AND t2.field = true

What is the faster way in terms of performance? What do you prefer?

Answer 1

If a filter enters in a JOIN condition functionally (ie it is an actual join condition, not just a filter), it must appear in the ON clause of that join.

Worth noting:

• If you place it in the WHERE clause instead, the performances are the same if the join is INNER , otherwise it differs. As mentioned in the comments it does not really matter since anyway the outcome is different.

• Placing the filter in the WHERE clause when it really is an OUTER JOIN condition implicitely cancels the OUTER nature of the condition ("join even when there are no records") as these filters imply there must be existing records in the first place. Example:

... table1 t LEFT JOIN table2 u ON ... AND t2.column = 5 is correct

... table1 t LEFT JOIN table2 u ON ... 
WHERE t2.column = 5 

is incorrect, as t2.column = 5 tells the engine that records from t2 are expected, which goes against the outer join. Exception to this would be an IS NULL filter, such as WHERE t2.column IS (NOT) NULL (which is in fact a convenient way to build conditional outer joins)

• LEFT and RIGHT joins are implicitely OUTER joins.

Hope it helped.

Answer 2

JOIN conditions should normally be independent from filter conditions. You define rules of your join (the how ) with ON . You filter what you want with WHERE . Performance wise, there's no general rule across all engines and designs, so your mileage will vary greatly.

Answer 3

我认为更快的方法是将过滤器放在where子句中，因为它将首先进行过滤，然后是join子句，因此不需要对过滤器进行排列。