In SQL / MySQL, what is the difference between “ON” and “WHERE” in a join statement?

Question

The following statements give the same result (one is using on, and the other using where):

mysql> select * from gifts INNER JOI         


        

Answer 1

• ON is applied to the set used for creating the permutations of each record as a part of the JOIN operation
• WHERE specifies the filter applied after the JOIN operation

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In effect, ON replaces each field that does not satisfy its condition with a NULL. Given the example by @Quassnoi

gifts

1   Teddy bear
2   Flowers

sentgifts

1   Alice
1   Bob

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SELECT  *
FROM    gifts g
LEFT JOIN
        sentgifts sg
ON      g.giftID = sg.giftID

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The LEFT JOIN permutations would have been calculated for the following collections if there was no ON condition:

{ 'Teddy bear': {'ALICE', 'Bob'}, 'Flowers': {'ALICE', 'Bob'} }

with the g.giftID = sg.giftID ON condition, this is the collections that will be used for creating the permutations:

{ 'Teddy bear': {'ALICE', 'Bob'}, 'Flowers': {NULL, NULL} }

which in effect is:

{ 'Teddy bear': {'ALICE', 'Bob'}, 'Flowers': {NULL} }

and so results in the LEFT JOIN of:

Teddy bear Alice
Teddy bear Bob
Flowers    NULL

and for a FULL OUTER JOIN you would have:

{ 'Teddy bear': {'ALICE', 'Bob'}, 'Flowers': {NULL} } for LEFT JOIN and { 'ALICE': {'Teddy bear', NULL}, 'Flowers': {'Teddy bear', NULL} } for RIGHT JOIN:

Teddy bear Alice
Teddy bear Bob
Flowers    NULL

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If you also had a condition such as ON g.giftID = 1 it would be

{ NULL: {'ALICE', 'Bob'}, 'Flowers': {NULL} }

which for LEFT JOIN would result in

Flowers NULL

and for a FULL OUTER JOIN would result in { NULL: {'ALICE', 'Bob'}, 'Flowers': {NULL} } for LEFT JOIN and { 'ALICE': {NULL, NULL}, 'Flowers': {NULL, NULL} } for RIGHT JOIN

NULL    Alice
NULL    Bob
Flowers NULL

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Note MySQL does not have a FULL OUTER JOIN and you need to apply UNION to LEFT JOIN and RIGHT JOIN