UPDATE project_file set file = null where id = 7 結果爲:空(null) 不是字符串'null'
UPDATE project_file set file = 'null' where id = 7 結果爲:'null' 是字符串
UPDATE project_file set file is null where id = 7 報錯
UPDATE project_file set file = null where id = 7 結果爲:空(null) 不是字符串'null'
UPDATE project_file set file = 'null' where id = 7 結果爲:'null' 是字符串
UPDATE project_file set file is null where id = 7 報錯