Question

Create MySQL Split String Function SPLIT_STR fedecarg.com/…/mysql-split-string-function/

CREATE FUNCTION SPLIT_STR(
  x VARCHAR(255),
  delim VARCHAR(12),
  pos INT
)
RETURNS VARCHAR(255)
RETURN REPLACE(SUBSTRING(SUBSTRING_INDEX(x, delim, pos),
       LENGTH(SUBSTRING_INDEX(x, delim, pos -1)) + 1),
       delim, '');

Run SQL:

SELECT t.en AS `type`, SPLIT_STR(l.en, ',', 1) AS city,
SPLIT_STR(l.en, ',', 2) AS country
FROM table1
JOIN table2
USING ( id )
LEFT JOIN table3 AS t ON table1.type = t.id
/* the next line has failure with SPLIT_STR */
LEFT JOIN table3 AS l ON table1.location = l.id
WHERE language.lang =  'en'
AND language.url =  'europe-countries'
LIMIT 1;

table1

id               | type            | location
-----------------+-----------------+-----------------
6BC45C02         | place           | london,england

table2

id               | url
-----------------+-----------------
6BC45C02         | europe-countries

table3

id               | en
-----------------+-----------------
london           | London
england          | England

Failed result:

type             | city            | country
-----------------+-----------------+----------------
place            | NULL            | NULL

Expected result would be to return city and country:

type             | city            | country
-----------------+-----------------+-----------------
place            | London          | England

---

On checking if SPLIT_STR is working with simple SQL:

SELECT SPLIT_STR(location, ',', 1) AS city, SPLIT_STR(location, ',', 2) AS contry
FROM table1
WHERE id =  '6BC45C02'
LIMIT 1;

it returns fine result:

city             | contry
-----------------+-----------------
london           | england

Answer 1

Maybe this… but performance will be terrible.

SELECT T1.type,
  SPLIT_STR(t.en, ',', 1) AS city,
  SPLIT_STR(l.en, ',', 2) AS country
FROM table1 t1
INNER JOIN table2 t2 
  ON T1.ID = T2.ID
LEFT JOIN table3 t 
  ON t.id = SPLIT_STR(t1.location, ',', 1)
LEFT JOIN table3 l 
  ON l.id = SPLIT_STR(t1.location, ',', 2)
WHERE t2.url = 'europe-countries'
LIMIT 1;

This would be better as it doesn’t require calling the function 4 times: (used coalesce to determine if function isn’t working as expected will return proper case, then lower case, then function broke if it’s not working as expected)

SELECT InTable.Type, 
   coalesce(t.en, inTable.City, 'FunctionBroke') as city, 
   coalesce(l.en, intable.country, 'FunctionBroke2') as country
FROM 
    (SELECT t1.type,
      SPLIT_STR(T1.Location, ',', 1) AS City,
      SPLIT_STR(T1.Location, ',', 2) AS Country
    FROM table1 T1
    INNER JOIN table2 T2 
      ON T1.ID = T2.ID 
        AND t2.url='europe-countries'
    ) InTable
LEFT JOIN table3 t 
  ON InTable.City = t.id
LEFT join table3 l 
  ON InTable.Country = l.id
LIMIT 1;

Still better exists: is table 3’s only purpose to Proper case the city/country name; and is the only purpose of the UDF (user defined function) to split the values?

• EDIT 1 Fixed first should have been t.en instead of l.en on first left
  join.
• EDIT 2 Fixed 2nd should have been t1 (that’s copy and paste for
  you)
• EDIT 3 Relooked at everything in both responses copied several of the errors in the OP