在MySQL中将文本列转换为整数

Question

I'm trying to create a cleaned-up table that replaces a varchar(50) field with an integer. The original field has occasional text values that I'd like to convert to 0 or null values. I can do this in a select statement just fine, but get an error when trying to create my table.

Below is a minimal example that uses strings:

/* This works */
DROP TABLE IF EXISTS mytest;
CREATE TABLE mytest
AS
   SELECT convert("123", unsigned) AS mynum;

/* This works, returning a row of 0 */
SELECT convert("TEST", unsigned) AS mynum;

/* But this fails, with:  Truncated incorrect INTEGER value: 'TEST'*/
DROP TABLE IF EXISTS mytest;
CREATE TABLE mytest
AS
   SELECT convert("TEST", unsigned) AS mynum;`

What is wrong with the above, and is there a better way to accomplish what I want? Thanks for the help.

Answer 1

I don't have an explanation for why that error occurs, but I found a workaround using a subquery:

DROP TABLE IF EXISTS mytest;
CREATE TABLE mytest
AS
    SELECT mynum
    FROM (SELECT convert("TEST"), unsigned) AS mynum) t;

Demo: http://www.sqlfiddle.com/#!2/4909a/1

Answer 2

I'd use a case statement for it and also switch over to using CAST instead of convert so you'll get failures instead of implicit conversions.

CREATE TABLE mytest(myVarchar varchar(10));
INSERT mytest VALUES ('123'),('TEST');

SELECT CASE
    WHEN  myVarchar REGEXP '^[0-9]+$' THEN CAST(myVarchar,unsigned)
    ELSE 0
END As mynum
FROM mytest;

I don't have a mysql instance handy to test this so hopefully I didn't goof any syntax errors.

Answer 3

Rather than convert, you could update the data and then alter the table:

UPDATE mytest SET mynum=mynum + 0;
ALTER TABLE mytest CHANGE COLUMN mynum mynum INT UNSIGNED;

Answer 4

It looks like you are implicitly defining the column name by the select statement in the create. This may be assuming the "TEST" string is the datatype. It would help to see the error message you get, but my assumption would be to explicitly define the column as:

CREATE TABLE mytest(mynum int);

Then

Insert into mytest(mynum)
select convert("TEST",unsigned) as mynum;

This is also why the answer with the subquery may work, by the time it gets to the outer query it is implicitly defined as an int.