Both the CAST and CONVERT forms seem to be correct.
SELECT CAST((SUBSTRING(t.position,1,LOCATE(',',t.position))) AS DECIMAL(17,15)) AS lat_
, CONVERT(SUBSTRING(t.position,LOCATE(', ',t.position)+1),DECIMAL(18,15)) AS long_
FROM (SELECT '45.390746926938185, -122.75535710155964' AS `position`) t
lat_ long_
------------------ ----------------------
45.390746926938185 -122.755357101559640
I think position is a reserved word, but I don't think that matters in this case. But it wouldn't hurt to assign a table alias and qualify all column references
UPDATE cameras c
SET c.latitude = CAST((SUBSTRING(c.position,1,LOCATE(',',c.position))) AS DECIMAL(17,15))
, c.longitude = CAST((SUBSTRING(c.position,LOCATE(',',c.position)+1)) AS DECIMAL(18,15))
But I suspect that won't resolve the problem.
One thing to check is for a before or after update trigger defined on the table, which is rounding/modifying the values assigned to the latitude and longitude columns?
I suggest you try running just a query.
SELECT CAST((SUBSTRING(c.position,1,LOCATE(',',c.position))) AS DECIMAL(17,15)) AS lat_
, CAST((SUBSTRING(c.position,LOCATE(',',c.position)+1)) AS DECIMAL(18,15)) AS lon_
FROM cameras c
and verify that produces the decimal values you expect.
A dot character should be recognized as a decimal point. Does the position column contain some other special characters, like a space or something?
From what you posted, it looks like the CAST and CONVERT working on the integer portion up to the decimal point. (There shouldn't be an implicit convert to signed integer in there, so it's not clear why the characters following the decimal point aren't being included.)
If you can figure out what character(s) are being used to represent the decimal point, then you could use a MySQL REPLACE()
function to replace those with a simple dot character.