SELECT w.*
FROM WIDGET w
WHERE w.widget_id NOT IN (SELECT c.widget
FROM CHOSEN c
WHERE c.user_id = $user_id)
SELECT w.*
FROM WIDGET w
WHERE NOT EXISTS (SELECT NULL
FROM CHOSEN c
WHERE c.widget_id = w.widget_id
AND c.user_id = $user_id)
SELECT w.*
FROM WIDGET w
LEFT JOIN CHOSEN c ON c.widget_id = w.widget
AND c.user_id = $user_id
WHERE w.widget IS NULL
如果比较的列（任一表中的widget_id）不可为空，则LEFT JOIN / IS NULL在MySQL上表现最佳。如果列可为空（值可以为NULL），则NOT IN或NOT EXISTS的性能更好。
602392714
清零编程群