How to find least non-null column in one particular row in SQL?

Question

I am trying to find the lowest number in two columns of a row in the same table, with the caveat that one of the columns may be null in a particular row. If one of the colum

Answer 1

I've created a function which handles any number of dates, by concatenating them with a separator (CONCAT_WS) as first parameter to the function.

CONCAT_WS besides dynamic number of parameters, will remove all NULL dates ;)

The function accepts two parameters:

• delimiter separated string of dates as TEXT
• delimiter as TEXT (same as used on CONCAT_WS !!) - you can remove it if you use only preferred separator on CONCAT_WS.

CREATE FUNCTION `min_date`(`dates` TEXT, `delim` VARCHAR(10)) RETURNS DATE NO SQL DETERMINISTIC
BEGIN

DECLARE `result` DATE DEFAULT NULL;

DECLARE `count` TINYINT DEFAULT 0;
DECLARE `temp` DATE DEFAULT NULL;

IF `delim` IS NULL THEN SET `delim` = ','; END IF;

IF `dates` IS NOT NULL AND CHAR_LENGTH(`dates`) > 0 THEN 
  SET `count` = LENGTH(`dates`) - LENGTH(REPLACE(`dates`, `delim`, SPACE(CHAR_LENGTH(`delim`) - 1)));

  WHILE `count` >= 0 DO
    SET `temp` = SUBSTRING_INDEX(SUBSTRING_INDEX(`dates`, `delim`, `count` + 1), `delim`, -1);

    IF `result` IS NULL OR `result` > `temp` THEN SET `result` = `temp`; END IF;

    SET `count` = `count` - 1;
  END WHILE;
END IF;

RETURN `result`;

END

Then, you can use in any combination of date fields or as static strings (as long as are valid dates or NULL):

SELECT min_date(CONCAT_WS(',', `date_column_1`, NULL, '2019-03-04', `date_column_2`), ',') AS `min_date`