I should probably know this by now, but what, if any is the difference between the two statements below?
The nested join:
SELECT
t1.*
FROM
table1 t1
INNER JOIN table2 t2
LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
ON t2.table2_ID = t1.table1_ID
The more traditional join:
SELECT
t1.*
FROM
table1 t1
INNER JOIN table2 t2 ON t2.table2_ID = t1.table1_ID
LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
Different Types of SQL JOINs (INNER) JOIN : Returns records that have matching values in both tables. LEFT (OUTER) JOIN : Returns all records from the left table, and the matched records from the right table. RIGHT (OUTER) JOIN : Returns all records from the right table, and the matched records from the left table.
INNER JOIN: returns rows when there is a match in both tables. LEFT JOIN: returns all rows from the left table, even if there are no matches in the right table. RIGHT JOIN: returns all rows from the right table, even if there are no matches in the left table.
There are four main types of joins: inner join, full outer join, left outer join and right outer join. The major difference between inner and outer joins is that inner joins result in the intersection of two tables, whereas outer joins result in the union of two tables.
The LEFT JOIN includes all records from the left side and matched rows from the right table, whereas RIGHT JOIN returns all rows from the right side and unmatched rows from the left table.
Well, it's the order of operations..
SELECT
t1.*
FROM
table1 t1
INNER JOIN table2 t2
LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
ON t2.table2_ID = t1.table1_ID
could be rewritten as:
SELECT
t1.*
FROM
table1 t1 -- inner join t1
INNER JOIN
(table2 t2 LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID) -- with this
ON t2.table2_ID = t1.table1_ID -- on this condition
So basically, first you LEFT JOIN t2 with t3, based on the join condition: table3_ID = table2_ID, then you INNER JOIN t1 with t2 on table2_ID = table1_ID.
In your second example you first INNER JOIN t1 with t2, and then LEFT JOIN the resulting inner join with table t3 on the condition table2_ID = table1_ID.
SELECT
t1.*
FROM
table1 t1
INNER JOIN table2 t2 ON t2.table2_ID = t1.table1_ID
LEFT JOIN table3 t3 ON t3.table3_ID = t2.table2_ID
could be rewritten as:
SELECT
t1.*
FROM
(table1 t1 INNER JOIN table2 t2 ON t2.table2_ID = t1.table1_ID) -- first inner join
LEFT JOIN -- then left join
table3 t3 ON t3.table3_ID = t2.table2_ID -- the result with this
EDIT
I apologize. My first remark was wrong. The two queries will produce the same results but there may be a difference in performance as the first query may perform slower than the second query in some instances ( when table 1 contains only a subset of the elements in table 2) as the LEFT JOIN will be executed first - and only then intersected with table1. As opposed to the second query which allows the query optimizer to do it's job.
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