Using MySQL "NOT IN" but allowing for substrings - mysql

Question

I am trying to query for rows that are not in another set of rows. However, the other set of rows may contain strings that include strings from the first table.
I'm confusing myself trying to explain so I'll use the following example tables:
mysql> DESCRIBE tablea;
+------------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+----------+------+-----+---------+-------+
| name | char(40) | NO | PRI | | |
+------------+----------+------+-----+---------+-------+
mysql> DESCRIBE tableb;
+------------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+----------+------+-----+---------+-------+
| nametag | char(40) | NO | PRI | | |
+------------+----------+------+-----+---------+-------+
mysql> SELECT name FROM tablea;
+------------+
| name |
+------------+
| cat |
| dog |
| cow |
+------------+
mysql> SELECT nametag FROM tableb;
+------------+
| nametag |
+------------+
| wolf |
| dog |
| browncow |
+------------+
I am trying to find a method similar to the NOT IN operation, however because cow is "in" browncow, I also want to exclude this value.
mysql> SELECT name FROM tablea WHERE name NOT IN ( SELECT nametag FROM tableb );
+------------+
| name |
+------------+
| cat |
| cow |
+------------+
# I am looking for something that would only return "cat" for this example.
Is there any operation where I can search for rows that aren't contained in another set with additional modifiers?

Answer 1

You could use an anti-join pattern, with a LIKE predicate to do the matching. (The anti-join is an outer join, to return all the rows from one table, along with matches from another table, and then a predicate to exclude the rows that had a match
SELECT a.name
FROM tablea a
JOIN tableb b
ON b.nametag LIKE CONCAT('%',a.name,'%')
WHERE b.nametag IS NULL
(Any rows from a that had a matching row from b... the row from b will have a non-NULL value. Or, to put it another way... rows from a that didn't have a matching row in b will have a NULL value for the columns from b.)
If there's a row in a that has name='cow', and a row from b that has nametag='browncow', those rows will match.
The row from a with name='cat' will only be returned if the string 'cat' doesn't appear in any values of b.nametag.
NOTE: The percent and underscore characters are wildcards in the LIKE predicate. If you want to do matching on those characters, you'd need to "escape" those with a backslash. There's similar issues using a REGEXP match, but a lot more possible mischievous characters.
There are other query patterns that will return an equivalent result.
For example:
SELECT a.name
FROM tablea a
WHERE NOT EXISTS
( SELECT 1
FROM tableb b
WHERE b.nametag LIKE CONCAT('%',a.name,'%')
)
Personally, I prefer the anti-join pattern.

Answer 2

Use "NOT EXISTS" along with INSTR
select *
from tablea a
where not exists(select 1 from tableb b where INSTR(a.name, b.nametag) > 0)
;
To exclude empty strings:
select *
from tablea a
where not exists(select 1 from tableb b where INSTR(a.name, b.nametag) > 0)
and length(a.name) > 0
;