Having the following SQL
SELECT
u.user_id AS user_id,
u.user_name AS user_name,
d.user_data AS user_data
FROM
tbl_users u,
tbl_data d
WHERE
d.user_id = u.user_id
Does this still produces a JOIN (as there's no JOIN keyword in this sql)? How does MySQL handle this query?
And is there any difference if the WHERE clause would be u.user_id = d.user_id instead of d.user_id = u.user_id ?
Thanks.
What you're writing is the old style SQL syntax before ANSI standardized the JOIN keywords back in 1992.
With a simple query like what you're writing above, most RDBMS's will internally convert it to the same lookup method as an inner join.
As for the order of the columns in the where clause, the only time it matters is when you have a derived column, or a function. Generally you would want to keep the non-derived or non-functional column first, in that case, so that the engine can figure out the correct index to use.
yes this is an alternate (older) syntax for joining.
It is still perfectly valid. The order of the AND conditions does not matter.
Your query gives the same result as one where you did an INNER JOIN.
And there is no difference in order for the WHERE clause.
Yes, this implicitly tells MySQL to do an inner join, it's equivalent to:
SELECT
u.user_id AS user_id,
u.user_name AS user_name,
d.user_data AS user_data
FROM
tbl_users u
INNER JOIN
tbl_data d
ON
d.user_id = u.user_id
FROMclause, and are relating them in theWHEREclause - it's a JOIN in disguise.