选择MySQL没有'_'的值

Question

I was looking for a way to exclude values with a '_' in the results set from a mysql database.

Why would the following sql statement return no results?

select questionKey 
from labels 
where set_id = 674 
and questionKey like 'Class%' 
and questionKey not like '%_%' ;

which was the first sql I tried where as

select questionKey 
from labels 
where set_id = 674 
and questionKey like 'Class%' 
and locate('_',questionKey) = 0 ;

returns

questionKey
ClassA
ClassB
ClassC
ClassD
ClassE
ClassF
ClassG
ClassNPS
ClassDis

which is the result I wanted. Both SQL statements appear to me to be logically equivalent though they are not.

Answer 1

In the LIKE context _ takes on special meaning and represents any single character . It's the only one other than % that means something here.

Your LOCATE() version is probably the best here, though it's worth noting that doing table scans like this can get cripplingly slow on large amounts of data. If underscore represents something important you might want to have a flag field you can set and index.

You could also use a regular expression to try and match records with a single condition:

REGEXP '^Class[^_]+'

Answer 2

As tadman and PM77 already pointed out, it's a special character. If you want to use the first query, try to escape it like this (note the backslash):

select questionKey 
from labels 
where set_id = 674 
and questionKey like 'Class%' 
and questionKey not like '%\_%' ;