SQL JOIN - WHERE clause vs. ON clause

Question

After reading it, this is not a duplicate of Explicit vs Implicit SQL Joins. The answer may be related (or even the same) but the question is diffe

Answer 1

Let's consider those tables :

A

id | SomeData

B

id | id_A | SomeOtherData

id_A being a foreign key to table A

Writting this query :

SELECT *
FROM A
LEFT JOIN B
ON A.id = B.id_A;

Will provide this result :

/ : part of the result
                                       B
                      +---------------------------------+
            A         |                                 |
+---------------------+-------+                         |
|/////////////////////|///////|                         |
|/////////////////////|///////|                         |
|/////////////////////|///////|                         |
|/////////////////////|///////|                         |
|/////////////////////+-------+-------------------------+
|/////////////////////////////|
+-----------------------------+

What is in A but not in B means that there is null values for B.

---

Now, let's consider a specific part in B.id_A, and highlight it from the previous result :

/ : part of the result
* : part of the result with the specific B.id_A
                                       B
                      +---------------------------------+
            A         |                                 |
+---------------------+-------+                         |
|/////////////////////|///////|                         |
|/////////////////////|///////|                         |
|/////////////////////+---+///|                         |
|/////////////////////|***|///|                         |
|/////////////////////+---+---+-------------------------+
|/////////////////////////////|
+-----------------------------+

---

Writting this query :

SELECT *
FROM A
LEFT JOIN B
ON A.id = B.id_A
AND B.id_A = SpecificPart;

Will provide this result :

/ : part of the result
* : part of the result with the specific B.id_A
                                       B
                      +---------------------------------+
            A         |                                 |
+---------------------+-------+                         |
|/////////////////////|       |                         |
|/////////////////////|       |                         |
|/////////////////////+---+   |                         |
|/////////////////////|***|   |                         |
|/////////////////////+---+---+-------------------------+
|/////////////////////////////|
+-----------------------------+

Because this removes in the inner join the values that aren't in B.id_A = SpecificPart

---

Now, let's change the query to this :

SELECT *
FROM A
LEFT JOIN B
ON A.id = B.id_A
WHERE B.id_A = SpecificPart;

The result is now :

/ : part of the result
* : part of the result with the specific B.id_A
                                       B
                      +---------------------------------+
            A         |                                 |
+---------------------+-------+                         |
|                     |       |                         |
|                     |       |                         |
|                     +---+   |                         |
|                     |***|   |                         |
|                     +---+---+-------------------------+
|                             |
+-----------------------------+

Because the whole result is filtered against B.id_A = SpecificPart removing the parts B.id_A IS NULL, that are in the A that aren't in B