Having trouble matching a single character in an SQL table

问题

I need to use the '_' wildcard to find all id that are only one letter which there are a few of. However when I run my query no rows are returned.

Heres my query:

SELECT *
FROM table
WHERE id LIKE '_';

I have a table lets call Table1 that has two columns, id and name. id either has 1 or 2 characters to label a name. I'm trying to only find the names where the id is only one character. Heres an example of the table:

id     name
A      Alfred
AD     Andy
B      Bob
BC     Bridget

I only want to return Alfred and Bob in this example.

I don't want the solution but any advice or ideas would be helpful.

Here is a screenshot of my query: http://i.imgur.com/EWTfoVI.png?1

And here is a small example of my table: http://i.imgur.com/urGRZeK.png?1

So in this example of my table I would ideally like only East Asia... to be returned.

I if I search specifically for the character it works but for some strange reason the '_' wildcard doesn't.

For example:

SELECT *
FROM icao
WHERE prefix_code ='Z';

This works.

回答1:

In MySQL, the underscore is used to represent a wildcard for a single character. You can read more about that Pattern Matching here.

The way you have it written, your query will pull any rows where the id column is just one single character, you don't need to change anything.

Here is an SQL Fiddle example.

EDIT

One trouble shooting tip is to be sure there is no whitespace before/after the prefix code. If there is, and you need to remove it, add TRIM():

SELECT *
FROM myTable
WHERE TRIM(id) LIKE '_';

Here is an example with TRIM.

EDIT 2

A little explanation to your weird behavior, hopefully. In MySQL, if there is trailing white space on a character, it will still match if you say id = 'Z'; as seen by this fiddle now. However, leading white space will not match this, but will still be corrected by TRIM(), because that removes white space on the front and back end of the varchar.

TL;DR You have trailing white space after Z and that's causing the problem.

回答2:

Try using TRIM

Select *
FROM [Table]
where TRIM(ID) LIKE '_';

回答3:

The most likely explanation for the behavior you observe is trailing spaces (or other whitespace) in the value. That is, you see one character

'A'

But the value may actually be stored as two (or more) characters.

'A '

To see what's actually stored, you can use the HEX and LENGTH functions.

SELECT t.foo
     , LENGTH(t.foo)
     , HEX(t.foo)
  FROM mytable t
 WHERE t.foo LIKE 'A%'

The % is a wildcard for the LIKE operator that matches any number of characters (zero, one or more).

You can use the RTRIM() function to remove trailing spaces...

SELECT RTRIM(t.foo)
     , LENGTH(RTRIM(t.foo))
     , HEX(RTRIM(t.foo))
  FROM mytable t
 WHERE t.foo LIKE 'A%'

回答4:

SELECT *
FROM table
WHERE LENGTH(id)=1

回答5:

Strange..., in my case works perfectly (I am using mysql 5.5). Please, try this:

select * from mysql.help_topic where name like '_';

What set you get?

来源：https://stackoverflow.com/questions/29975260/having-trouble-matching-a-single-character-in-an-sql-table

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mysql

sql

wildcard