Either OR non-null constraints in MySQL

Question

What\'s the best way to create a non-NULL constraint in MySQL such that fieldA and fieldB can\'t both be NULL. I don\'t care if either one is NULL by itself, just as long as the

Answer 1

I accomplished this using a GENERATED ALWAYS column with COALESCE ... NOT NULL:

DROP TABLE IF EXISTS `error`;

CREATE TABLE IF NOT EXISTS `error` (
    id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
    left_id BIGINT UNSIGNED NULL,
    right_id BIGINT UNSIGNED NULL,
    left_or_right_id BIGINT UNSIGNED GENERATED ALWAYS AS (COALESCE(left_id, right_id)) NOT NULL,
    when_occurred TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    message_text LONGTEXT NOT NULL,
    INDEX id_index (id),
    INDEX when_occurred_index (when_occurred),
    INDEX left_id_index (left_id),
    INDEX right_id_index (right_id)
);

INSERT INTO `error` (left_id, right_id, message_text) VALUES (1, 1, 'Some random text.');  -- Ok.
INSERT INTO `error` (left_id, right_id, message_text) VALUES (null, 1, 'Some random text.'); -- Ok.
INSERT INTO `error` (left_id, right_id, message_text) VALUES (1, null, 'Some random text.'); -- Ok.
INSERT INTO `error` (left_id, right_id, message_text) VALUES (null, null, 'Some random text.'); -- ER_BAD_NULL_ERROR: Column 'left_or_right_id' cannot be null

on MySQL version 8.0.22