Using CAST with an insert into ... select - mysql

Question

I am trying to cast some bad data into proper data using CAST(column AS DECIMAL(7,2)), which works when just SELECTing, but once combined with an INSERT statement, the statement fails:
/* setup */
drop table if exists dst;
create table dst (
id int not null auto_increment
, columnA decimal(7,2)
, primary key (id)
);
drop table if exists src;
create table src (
id int not null auto_increment
, columnA varchar(255)
, primary key (id)
);
insert into src (columnA) values ('');
/* test */
/* This works as I would like it to*/
select CAST(columnA AS decimal(7,2)) from src; /* returns 0.00 */
/* This fails */
insert into dst (columnA)
select CAST(columnA AS decimal(7,2)) from src;
/*0 19 21:01:56 insert into dst (columnA)
select cast(columnA as decimal(7,2)) from src Error Code: 1366. Incorrect decimal value: '' for column '' at row -1 0.000 sec*/
Edit: this is a pared down reproduction example - the data I am
working with is much more varied than just an empty string - it might
also be a non-numeric string value or a numeric value that is outside
of the bounds of decimal(7,2) - in which the CAST statement works
the way I would like it to when only in a SELECT but fails when
trying to use them as part of the INSERT.
Am I doing something wrong, or is there another way to achieve what I am looking for?
I am using MYSQL 5.6.11 on WIN 7 x64

Answer 1

Set explicitly the value for the empty string with CASE or IF:
SELECT CASE WHEN columnA = '' THEN 0 ELSE CAST(columnA AS decimal(7,2)) END
FROM src;
SELECT IF(columnA <> '', CAST(columnA AS decimal(7,2)), 0)
FROM src;
Or use 2 SELECTs with UNION ALL :
SELECT CAST(columnA AS decimal(7,2))
FROM src
WHERE columnA <> ''
UNION ALL
SELECT 0
FROM src
WHERE columnA = '';

Answer 2

You can try using REGEXP to filter out any irregularities in source data:
INSERT INTO dst (columnA)
SELECT CAST(CASE
WHEN columnA REGEXP '^[0-9]{0,7}(\.[0-9]{0,2})$' THEN columnA
ELSE 0
END AS decimal(7,2))
FROM src;
SQL Fiddle Demo here