MySQL: Inner join vs Where [duplicate]

Answer 1

the second query is just another notation for an inner join, so if there is a difference in porformance it's only because one query can be parsed faster than the other one - and that difference, if it exists, will be so tiny that you won't notice it.

for more information you could try to take a look at this question (and use the search on SO next time before asking a question that already is answered)

Answer 2

The first query is easier to understand for MySQL so it is likely that the execution plan will be better and that the query will run faster.

The second query without the where clause, is a cross join. If MySQL is able to understand the where clause good enough, it will do its best to avoid cross joining all the rows, but nothing guarantee that.

In a case as simple as yours, the performance will be strictly identical.

Performance wise, the first query will always be better or identical to the second one. And from my point of view it is also a lot easier to understand when rereading.

Answer 3

Actually they are virtually the same, The JOIN / ON is newer ANSI syntac, the WHERE is older ANSI syntax. Both are recognized by query engines

Answer 4

I would think so because the first example explicitly tells mysql which columns to join and how to join them where the second one mysql has to try and figure out where you want to join.

Answer 5

The comma in a FROM clause is a CROSS JOIN. We can imagine that SQL server has a select query execution procedure which somehow should look like that: 1. iterate through every table 2. find rows that meet join predicate and put it into result table. 3. from the result table, get only those rows that meets the where condition.

If it really looks like that, then using a CROSS JOIN on a table that has a few thousands rows could allocate a lot of memory, when every row is combined with each other before the where condition is examined. Your SQL server could be quite busy then.

Answer 6

They are the same. This can be seen by running the EXPLAIN command:

mysql> explain Select * from Table1 T1 
    -> Inner Join Table2 T2 On T1.ID = T2.ID;
+----+-------------+-------+-------+---------------+---------+---------+------+------+---------------------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                                       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+---------------------------------------------+
|  1 | SIMPLE      | T1    | index | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using index                                 |
|  1 | SIMPLE      | T2    | index | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using where; Using index; Using join buffer |
+----+-------------+-------+-------+---------------+---------+---------+------+------+---------------------------------------------+
2 rows in set (0.00 sec)

mysql> explain Select * from Table1 T1, Table2 T2 
    -> Where T1.ID = T2.ID;
+----+-------------+-------+-------+---------------+---------+---------+------+------+---------------------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                                       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+---------------------------------------------+
|  1 | SIMPLE      | T1    | index | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using index                                 |
|  1 | SIMPLE      | T2    | index | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using where; Using index; Using join buffer |
+----+-------------+-------+-------+---------------+---------+---------+------+------+---------------------------------------------+
2 rows in set (0.00 sec)