内部连接条件或在哪里使用？

Question

i have this query

SELECT t1.col1
FROM table1 t1
INNER JOIN table2 t2 ON t2.col1 = t1.col1
WHERE t2.col IN (1, 2, 3)

and this query

SELECT t1.col1
FROM table1 t1
INNER JOIN table2 t2 ON t2.col1 = t1.col1 AND t2.col IN (1, 2, 3)

both game me the same execution plan and told me

Using where;

does that mean that my 2nd query is converted to the first form from the optimizer and that i should follow the first form?

and is there a way to check the optimizer new query?

Answer 1

SQL is not a procedural language. It is a descriptive language. A query describes the result set that you want to produce.

With an inner join, the two queries in your question are identical -- they produce the same result set under all circumstances. Which to prefer is a stylistic preference. MySQL should treat the two the same way from an optimization perspective.

One preference is that filters on a single table are more appropriate for WHERE and ON .

With an outer join, the two queries are not the same, and you should use the one that expresses your intent.

Answer 2

The first one filters after joining the two tables; while the second one joins using the filtering of the values as a condition of joining the two tables. You may check this thread:

SQL join: where clause vs. on clause

Answer 3

您可以检查执行两个查询需要花费多少，但是无论如何，您都可以用两种方式编写查询，无论是第一个查询还是第二个查询，优化器都会尝试优化查询。

Answer 4

I like this rule:

• ON ... says how the tables are related .
• WHERE ... filters the results.

This becomes important with LEFT JOIN because ON does not act as a filter.

This shows you that the Optimizer treats them the same:

EXPLAIN EXTENDED SELECT ...
SHOW WARNINGS;