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You join two tables by creating a relationship in the WHERE clause between at least one column from one table and at least one column from another. The join creates a temporary composite table where each pair of rows (one from each table) that satisfies the join condition is linked to form a single row.
Relational algebra is the most common way of writing a query and also the most natural way to do so. The code is clean, easy to troubleshoot, and unsurprisingly, it is also the most efficient way to join two tables.
“Is there a performance difference between putting the JOIN conditions in the ON clause or the WHERE clause in MySQL?” No, there's no difference. The following queries are algebraically equivalent inside MySQL and will have the same execution plan.
Yes, you can! The longer answer is yes, there are a few ways to combine two tables without a common column, including CROSS JOIN (Cartesian product) and UNION. The latter is technically not a join but can be handy for merging tables in SQL.
I have two tables in SQL and I need to be able to do a join based off of the timestamp in table B that is earlier than or equal to the timestamp in table A.
So, here is some fake data for two tables and the desired output:
Closed Cases (Table A)
| id | resolution | timestamp | ------------------------------------------------ | 1 | solved | 2006-10-05 11:55:44.888153 | | 2 | closed | 2007-10-07 12:34:17.033498 | | 3 | trashed | 2008-10-09 08:19:36.983747 | | 4 | solved | 2010-10-13 04:28:14.348753 |
Classification (Table B)
| id | value | timestamp | ------------------------------------------------- | 1 | freshman | 2006-01-01 12:02:44.888153 | | 2 | sophomore | 2007-01-01 12:01:19.984333 | | 3 | junior | 2008-01-01 12:02:28.746149 |
Desired Results
| id | resolution | timestamp | value | -------------------------------------------------------------- | 1 | solved | 2006-10-05 11:55:44.888153 | freshman | | 2 | closed | 2007-10-07 12:34:17.033498 | sophomore | | 3 | trashed | 2008-10-09 08:19:36.983747 | junior | | 4 | solved | 2010-10-13 04:28:14.348753 | junior |
So, I know the code needs to look like the following, I just can't figure out what to do with the ON portion of the JOIN ($1 and $2 are variables that will be passed in):
SELECT case.id, case.resolution, case.timestamp, class.value
FROM closed_cases AS case
LEFT JOIN classifications AS class ON ???
WHERE case.timestamp BETWEEN $1 AND $2;
I know I could use a sub-select, but this will be operating on at least a few thousand rows, probably more, and I need it to be really fast; so I was hoping for a simple clause that could do it.
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