LEFT JOIN gives different result when changing the location of condition

Question

I have this query:

SELECT
    bc.name,
    SUM(amount) AS total
FROM
    budget_categories bc
LEFT JOIN budget_items bi ON
    bc.id = bi.budget_category_id
LEFT JOIN balance_lines bl ON
    bl.budget_item_id = bi.id
WHERE
    bc.id = 25 AND family_id = 97109 AND bl.balance_id = 264553

The result is:

NULL | NULL

But when I put the condition in the LEFT JOIN part it works:

SELECT
    bc.name,
    SUM(amount) AS total
FROM
    budget_categories bc
LEFT JOIN budget_items bi ON
    bc.id = bi.budget_category_id
LEFT JOIN balance_lines bl ON
    bl.budget_item_id = bi.id  AND family_id = 97109 AND bl.balance_id = 264553
WHERE
    bc.id = 25

The result is:

Bannana | NULL

Why is the difference?

See https://stackoverflow.com/a/34455091/5389997 answer in the duplicate question — Shadow, Jul 06 '22 at 13:37
please use the table alias in every column reference - e.g. which table does "amount" come from? — Paul Maxwell, Jul 06 '22 at 13:41

Paul Maxwell · Accepted Answer · 2022-07-06T13:46:31.923

When you use an outer join (e.g. left outer join) you are asking for the query to return rows even if that table does not have an exact match. Hence there may be NULL values from the left outer joined columns.

In the first where clause you ask for EVERY row in the overall result row to have bl.family_id = 97109 AND bl.balance_id = 264553 but here all the returned rows must have NULL bc.name and NULL amount. (btw: This is sometimes known as an implied inner join and in effect is equivalent to the query below:)

SELECT
    bc.name,
    SUM(amount) AS total
FROM
    budget_categories bc
LEFT JOIN budget_items bi ON
    bc.id = bi.budget_category_id
INNER JOIN balance_lines bl ON -- the where clause has this effect
    bl.budget_item_id = bi.id
WHERE
    bc.id = 25 AND bl.family_id = 97109 AND bl.balance_id = 264553

In the second query you allow only the rows of balance_lines to be restricted by the join conditions, so that more rows can be retuned from the other tables - and in those additional rows you get a bc.name of banana.

When using left joins you must take care with any predicates that reference outer joined tables if you are to allow NULLs to be returned e.g.

SELECT
    bc.name,
    SUM(amount) AS total
FROM
    budget_categories bc
LEFT JOIN budget_items bi ON
    bc.id = bi.budget_category_id
LEFT JOIN balance_lines bl ON
    bl.budget_item_id = bi.id
WHERE
    bc.id = 25 
    AND (bl.family_id = 97109 OR bl.family_id IS NULL)
    AND (bl.balance_id = 264553 OR bl.balance_id IS NULL)

This revised where clause will behave just like your second query now.

LEFT JOIN gives different result when changing the location of condition

1 Answers1