Question

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

Answer 1

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.