Consider we have two tables, Users and Posts. user_id is the foreign key in Posts table and is primary key in Users table.
Whats the difference between these two sql queries?
select user.name, post.title
from users as user, posts as post
where post.user_id = user.user_id;
vs.
select user.name, post.title
from users as user join posts as post using user_id;
Other than syntax, for the small snippet, they work exactly the same. But if at all possible, always write new queries using ANSI-JOINs.
As for semantically, the comma notation is used to produce a CARTESIAN product between two tables, which means produce a matrix of all records from table A with all records from table B, so two tables with 4 and 6 records respectively produces 24 records. Using the WHERE clause, you can then pick the rows you actually want from this cartesian product. However, MySQL doesn't actually follow through and make this huge matrix, but semantically this is what it means.
A JOIN syntax is the ANSI standard that more clearly defines how tables interact. By putting the ON clause next to the JOIN, it makes it clear what links the two tables together.
Functionally, they will perform the same for your two queries. The difference comes in when you start using other [OUTER] JOIN types.
For MySQL specifically, comma-notation does have one difference
STRAIGHT_JOIN is similar to JOIN, except that the left table is always read before the right table. This can be used for those (few) cases for which the join optimizer puts the tables in the wrong order.
However, it would not be wise to bank on this difference.
where post.user_id = user.user_id
Here you are making a conditional statement
from users as user join posts as post using user_id
Here you are joining two tables using the foreign key
At the end is just the same but JOIN is better used for more advanced queries...
In MySQL JOIN syntax, CROSS JOIN, INNER JOIN, and JOIN are all the same. A comma-separated table list is a JOIN.
The MySQL manual on page https://dev.mysql.com/doc/refman/5.5/en/join.html makes this point about the difference between the two approaches:
However, the precedence of the comma operator is less than that of INNER JOIN, CROSS JOIN, LEFT JOIN, and so on. If you mix comma joins with the other join types when there is a join condition, an error of the form Unknown column 'col_name' in 'on clause' may occur.
来源：https://stackoverflow.com/questions/5294311/difference-between-these-two-joining-table-approaches