MySQL varchar to number

Question

I am trying a simple conversion - a mysql varchar column which contains longitude values (eg "-1.234" to -1.234) to enhance optimisation.

I have read and read on this and nothing works, last tried was:

UPDATE latlng_data SET lng_copy = CAST(lng AS DECIMAL(6,4));

met with:

Error Code: 1366. Incorrect decimal value: '' for column '' at row -1

Target column, created to recover from errors is:

FLOAT(6,4)

and null allowed.

Why is mysql error messages so useless.

Following 'dognose' advice (below) I have used:

UPDATE latlng_data SET lng='999.9999' where lng='';-- to give invalid lng in this case

then increase the copy data field (with the aim to rename it orig) and then:

UPDATE latlng_data SET lng_copy = CAST(lng AS DECIMAL(7,4));

this appears to be required in mysql cmd as Workbench timesout but using:

show full processlist;

still shows it as running - so here the best monitor is cmd.

Answer 1

Checked this with:

SELECT * FROM t31data_happyenergy.latlng_data where lng_copy <> CAST(lng AS DECIMAL(7,4));

which now returns no results and can be better optimised as all are numerical, with bad data fully valued for a longitude.

I also recommend using a mysql command window as most apps timeout when extensive queries are used with this large data.

So basically the process went [ALWAYS CHECK BETWEEN EACH QUERY AS I GOT LAZY AND HAD TO START AGAIN AS ONE QUERY WAS WRONG):

-- convert all mostly read only tables to myisam:
-- dropped old lat and lng after checking same data by

SELECT count(*) FROM latlng_postcode where CAST(lat AS DECIMAL(7,4))!=latcopy;
SELECT count(*) FROM latlng_postcode where CAST(lng AS DECIMAL(7,4))!=lngcopy;

ALTER TABLE `latlng_postcode`
CHANGE COLUMN `latcopy` `lat` FLOAT(7,4) NULL DEFAULT NULL,
CHANGE COLUMN `lngcopy` `lng` FLOAT(7,4) NULL DEFAULT NULL;

-- then index (are these the best settings ??
ALTER TABLE `latlng_postcode` 
DROP INDEX `pcode` ,
ADD INDEX `pcode` USING BTREE (`postcode`(5) ASC),
ADD INDEX `lat` USING BTREE (`lat`(4) ASC),
ADD INDEX `lng` USING BTREE (`lng`(4) ASC);

Answer 2

I think the easiest way to convert numbers in MySQL is to use silent conversion. That is, use the string in an arithmetic operation and any conversion errors are ignored. A string with no numbers is interpreted as 0:

UPDATE latlng_data
    SET lng_copy = lng + 0.0;

Here is some relevant documentation.

EDIT:

If you are concerned about matching only correct values, then check the format of the string

WHERE lng rlike '[0-9]+[.]?[0-9]*'