mysql : turn fraction strings into number - mysql

Question

I have strings like... "3/4" and "5/9" and some like... "1/2 km" and "3/4 degree" stored in mysql columns.
I would like to convert them into numbers.
In first case, 3/4 ==> .75.
In more complicated second case, strip off units like "km" and "degree" so
"1/2 km" ==> 0.5.

Answer 1

I think this isn't something you should do in a query, but rather calculate it when it is stored and save the calculated value in the table next to its text value.
But if you want to, you can use some string functions to slice up the value and the do the math yourself:
select
x.Multiplier / x.Divider as Result
from
(select
cast( substr( t.String,
1,
locate('/', t.String) - 1)
as decimal)
as Multiplier,
cast( substr( t.String,
locate('/', t.String) + 1,
locate( ' ',
concat(t.String, ' ')))
as decimal)
as Divider
from
YourTable t) x
Note however, that this may cause trouble if the data is 'invalid'. If it says '0/0 km', it may fail, if it contains 'no data here' it may fail as well.

Answer 2

The above didn't quite work for me so came up with this which does. The query works for '10', '3/4' and '10 3/4'. Obviously you should replace the constructed rows at the bottom with your string or a selected value from a table:
SELECT
IF (
LOCATE(' ',fraction) > 0 OR LOCATE('/',fraction) = 0,
SUBSTRING_INDEX(SUBSTRING_INDEX(fraction,'/','1'),' ','1')
,0
) AS `integer`,
IF (
LOCATE('/',fraction) > 0,
SUBSTRING_INDEX(SUBSTRING_INDEX(fraction,'/','1'),' ','-1'),
0
) AS numerator,
SUBSTRING_INDEX(fraction,'/','-1') AS denominator,
(SELECT `integer`) + ((SELECT numerator) / (SELECT denominator)) AS `decimal`
FROM (
SELECT '10 3/4' AS fraction
UNION SELECT '10'
UNION SELECT '3/4'
) t;

Answer 3

With PHP you could do:
// assuming $vals has the values from the database
$converted = array();
foreach ($vals as $key => $val) {
preg_match("/^(\\d+)\\/(\\d+)/", $val, $matches)
if (count($matches) > 2) {
$numerator = (int) $matches[1];
$denominator = (int) $matches[2];
$converted[$key] = (float) $numerator / $denominator;
}
}