![]() ![]() NULL is returned when the value is NULL, or the non- NULL value is not found in the list and the list contains at least one NULL value.FALSE is returned when the non- NULL value is not found in the list and the list does not contain NULL values.TRUE is returned when the non- NULL value in question is found in the list.IN predicate (unlike EXISTS) is trivalent, i. Note that NULL values do not safisfy the equality conditions, so both LEFT JOIN / IS NULL and NOT EXISTS will always return rows from t_left that have value set to NULL, even is there are rows with value IS NULL in t_right. NOT EXISTS, therefore, will return TRUE only if no row satisfying the equality condition is found in t_right (same as for LEFT JOIN / IS NULL). EXISTS always returns TRUE or FALSE and it will return TRUE as soon as it finds only a single matching row in t_right, or FALSE, if it find none. Since it's a predicate, not a JOIN condition, the rows from t_left can only be returned at most once too. This means that LEFT JOIN / IS NULL is guaranteed to return at most one row from t_left, and these row's value is not equal to one of those in t_right. Since NULL values can never satisfy an equality JOIN condition, the NULL values returned by the query are guaranteed to be substituted by the LEFT JOIN, not fetched out of the actual t_right's row. ![]() If for some row in t_left there is no corresponding row in t_right (which means no row with that exact value is present in t_right), the row from t_left will be returned once, and the NULL values will be substituted instead of t_right's actual values. LEFT JOIN is guaranteed to return every row from t_left, and then filtering is applied to the values returned from t_right. These method differ in how they handle NULL values in t_right LEFT JOIN / IS NULL) is best to select values present in one table but missing in another one?įirst of all, LEFT JOIN / IS NULL and NOT EXISTS are semantically equivalent, while NOT IN is not. This series of articles is inspired by multiple questions asked by the site visitors and Stack Overflow users, including Tony, Philip, Rexem and others. ![]()
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