Populate column with number of substrings in another column - mysql

Question

I have two tables "A" and "B". Table "A" has two columns "Body" and "Number." The column "Number" is empty, the purpose is to populate it.
Table A: Body / Number
ABABCDEF /
IJKLMNOP /
QRSTUVWKYZ /
Table "B" only has one column:
Table B: Values
AB
CD
QR
Here is what I am looking for as a result:
ABABCDEF / 3
IJKLMNOP / 0
QRSTUVWKYZ / 1
In other words, I want to create a query that looks up, for each string in the "Body" column, how many times the substrings in the "Values" column appear.
How would you advise me to do that?

Answer 1

Here's the finished query; explanation will follow:
SELECT
Body,
SUM(
CASE WHEN Value IS NULL THEN 0
ELSE (LENGTH(Body) - LENGTH(REPLACE(Body, Value, ''))) / LENGTH(Value)
END
) AS Val
FROM (
SELECT TableA.Body, TableB.Value
FROM TableA
LEFT JOIN TableB ON INSTR(TableA.Body, TableB.Value) > 0
) CharMatch
GROUP BY Body
There's a SQL Fiddle here.
Now for the explanation...
The inner query matches TableA strings with TableB substrings:
SELECT TableA.Body, TableB.Value
FROM TableA
LEFT JOIN TableB ON INSTR(TableA.Body, TableB.Value) > 0
Its results are:
BODY VALUE
-------------------- -----
ABABCDEF AB
ABABCDEF CD
IJKLMNOP
QRSTUVWKYZ QR
If you just count these you'll only get a value of 2 for the ABABCDEF string because it just looks for the existence of the substrings and doesn't take into consideration that AB occurs twice.
MySQL doesn't appear to have an OCCURS type function, so to count the occurrences I used the workaround of comparing the length of the string to its length with the target string removed, divided by the length of the target string. Here's an explanation:
REPLACE('ABABCDEF', 'AB', '') ==> 'CDEF'
LENGTH('ABABCDEF') ==> 8
LENGTH('CDEF') ==> 4
So the length of the string with all AB occurrences removed is 8 - 4, or 4. Divide the 4 by 2 (LENGTH('AB')) to get the number of AB occurrences: 2
String IJKLMNOP will mess this up. It doesn't have any of the target values so there's a divide by zero risk. The CASE inside the SUM protects against this.

Answer 2

You want an update query:
update A
set cnt = (select sum((length(a.body) - length(replace(a.body, b.value, '')) / length(b.value))
from b
)
This uses a little trick for counting the number of occurrence of b.value in a given string. It replaces each occurrence with an empty string and counts the difference in length of the strings. This is divided by the length of the string being replaced.
If you just wanted the number of matches (so the first value would be "2" instead of "3"):
update A
set cnt = (select count(*)
from b
where a.body like concat('%', b.value, '%')
)