Best way to do a weighted search over multiple fields in mysql?

There is a native and clean way to do this using MySQL's CASE function (https://dev.mysql.com/doc/refman/5.7/en/case.html).

Example (untested):

SELECT * FROM `myTable` 
WHERE (`name` LIKE "%searchterm%" OR `description` LIKE %searchterm%" OR `url` LIKE "%searchterm%")
ORDER BY CASE
WHEN `name`        LIKE "searchterm%"  THEN 20
WHEN `name`        LIKE "%searchterm%" THEN 10
WHEN `description` LIKE "%searchterm%" THEN 5
WHEN `url`         LIKE "%searchterm%" THEN 1
ELSE 0
END
LIMIT 20

Have used this for many weighted searches of my own and works an absolute treat!

you can add multiple mysql MATCH() values together, first multiplying each one by their weight.

simplified of course...

'(MATCH(column1) AGAINST(\''.$_GET['search_string'].'\') * '.$column1_weight.')
 + (MATCH(column2) AGAINST(\''.$_GET['search_string'].'\') * '.$column2_weight.')
 + (MATCH(column3) AGAINST(\''.$_GET['search_string'].'\') * '.$column3_weight.')
 AS relevance'

then

'ORDER BY relevance'

Probably this approach of doing a weighted search / results is suitable for you:

SELECT *,
    IF(
            `name` LIKE "searchterm%",  20, 
         IF(`name` LIKE "%searchterm%", 10, 0)
      )
      + IF(`description` LIKE "%searchterm%", 5,  0)
      + IF(`url`         LIKE "%searchterm%", 1,  0)
    AS `weight`
FROM `myTable`
WHERE (
    `name` LIKE "%searchterm%" 
    OR `description` LIKE "%searchterm%"
    OR `url`         LIKE "%searchterm%"
)
ORDER BY `weight` DESC
LIMIT 20

It uses a select subquery to provide the weight for ordering the results. In this case three fields searched over, you can specify a weight per field. It's probably less expensive than unions and probably one of the faster ways in plain MySQL only.

If you've got more data and need results faster, you can consider using something like Sphinx or Lucene.