SQL TRIM特定字段（和变量集）中的所有值

Question

Excuse me if I ask the obvious, but I am quite mySQL illiterate .

I am referring to a standard wordpress DB install, although this is not a wordpress specific question, but more mySQL general knowledge ..

If I want to change all the values of a certain field across all posts , I usually do :

UPDATE `wp_postmeta` SET `meta_value` = replace(meta_value, 'old_value', 'new_value') WHERE `meta_key` LIKE 'my_meta_key'

The problem is as follows :

In that specific DB , the INITIAL values were inserted with a white space prefix and suffix , e.g. :

"city name" 

was actually inserted as

" city name "

of course , being a PHP person, I immediately thought of the trim() command - which I found out exists also in mySQL.

SO now I thought I could just do

UPDATE `wp_postmeta` SET `meta_value` = TRIM('meta_value') WHERE `meta_key` LIKE 'my_meta_key'

But It is not working ..

What Am I doing wrong ? I know that mySQL is "ticks sensetive" ( so to speak ) But I tried both 'meta_value' and `meta_value' ( sorry, markdown limitations seems to truncate the backticks)

Doing replace(meta_value, ' ', '') is dengaurous because some cities havea spaces in their name, and LTRIM RTRIM is somewhat not working for me as well

I also know there is a way to set a variable in sql ( @my_var ) but what is the correct syntax of doing that in my case ?

SET @my_var = `meta_value`

and then

UPDATE `wp_postmeta` TRIM(@my_var) WHERE `meta_key` LIKE 'my_meta_key'

I of course know how to fix it in a php loop (with trim() ) but I would like to learn what is the best way to do it in mySQL..

Answer 1

To refer to the column, you need backticks:

UPDATE `wp_postmeta`
    SET `meta_value` = TRIM(`meta_value`)
    WHERE `meta_key` LIKE 'my_meta_key';

Your expression TRIM('meta_value') is trimming the string 'meta_value', not the value in the column.