KITFORMA · EDITORIAL ANSWER
Why does filtering a LEFT JOIN remove rows with no match?
KitForma editorial guide. A condition on the right-hand table in WHERE is evaluated after the outer join. NULL-extended unmatched rows can fail that condition.
Step-by-step answer
A condition on the right-hand table in WHERE is evaluated after the outer join. NULL-extended unmatched rows can fail that condition.
If the condition controls which right-side rows may match, place it in ON:
LEFT JOIN payments p ON p.order_id=o.id AND p.status='paid'. Keep a WHERE filter only when removing unmatched orders is intentional. Test orders with no payment, unpaid payment and multiple paid payments. Moving a predicate changes semantics; it is not merely a performance trick. Also check whether multiple matching child rows multiply the parent result.Sources and verification
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