MYSQL LEFT JOIN and multiple ORDER BY-CodePudding

I need to join 2 tables.

In the first table (geoname) there is a list of city codes, country codes and area codes. In the second table (alternatename) there is a list of available translations for each city code.

The sqlfiddle example is here: http://sqlfiddle.com/#!9/7df393/1

What I need is: give me an ordered list of cities by name, taking the italian language version if available or the english if it is not available or the NULL language if previous are not available. For each language, take the preferred version if available.

I tried with many queries, but I don't get the expected result. This is the last one:

SELECT g.geonameid, name 
FROM geoname g 
LEFT JOIN alternatename a ON g.geonameid = a.geonameid AND 
  (a.lang = 'it' OR a.lang = 'en' OR a.lang = NULL) 
WHERE name IS NOT NULL and fcode="PPL"
GROUP BY g.geonameid 
ORDER BY isPreferred DESC, name ASC 
LIMIT 0,20

The result is

geonameid   name
78910   Milan
12345   New York City
34542   Tampere

But I expect

geonameid   name
78910   Milano
12345   New York
34542   Tampere
71132   Sanaa

Any help?

Table schema:

CREATE TABLE geoname (
  geonameid INT(11) PRIMARY KEY,
  country CHAR(2),
  fcode VARCHAR(5));
  
CREATE TABLE alternatename (
  id_name INT(11) PRIMARY KEY AUTO_INCREMENT,
  geonameid INT(11),
  lang CHAR(2),
  name VARCHAR(250),
  isPreferred TINYINT(1) NULL); 
  
INSERT INTO geoname (geonameid,country,fcode) VALUES 
("12345","US","PPL"), 
("78910","IT","PPL"), 
("34542","FI","PPL"),
("5","IR","PPL"),
("71132","YE","ADM1");
 
INSERT INTO alternatename (geonameid,lang,name,isPreferred) VALUES 
("12345","en","New York City",NULL),
("12345","en","Big Apple",NULL),
("12345","en","New York",1),
("12345","it","La Grande Mela",NULL),
("12345","it","New York",1),
("12345","ru","New York",1),
("78910","en","Milan",1),
("78910","it","Milano",1),
("34542","en","Tampere",NULL),
("5","fa","Yekāhī",NULL),
("71132","ar","صنعاء",NULL),
("71132","fr","Muhafadat Sanaa",NULL),
("71132",NULL,"Sanaa",NULL);

CodePudding user response：

In MySQL 8 you can use a lateral join that gets the top name record for each location according to your sort order.

SELECT g.geonameid,
       a.name 
       FROM geoname g 
            JOIN LATERAL (SELECT a.name
                                 FROM alternatename a
                                 WHERE g.geonameid = a.geonameid
                                       AND a.lang IN ('it',
                                                      'en')
                                        OR a.lang IS NULL
                                 ORDER BY a.lang DESC,
                                          coalesce(a.ispreferred, 0) DESC
                                 LIMIT 1) a
       WHERE g.fcode = 'PPL'
       LIMIT 0,
             20;

Side note: Don't get used to use double quotes for string or date literals. Yes, sadly MySQL accepts that, but in SQL double quotes are usually for identifiers, such as column names. Should you ever use another DBMS (or future MySQL versions become more sane about this) you'll likely get an "invalid object name" error. Always use single quotes for string or date literals.

Also note that = NULL is never true as NULL isn't equal to anything, even NULL. Use IS NULL to check if a value is `NULL.

CodePudding user response：

One solution using conditional aggregation :

SELECT
  g.geonameid,
  COALESCE(
    MAX(CASE WHEN a.lang = 'it' THEN name END),
    MAX(CASE WHEN a.lang = 'en' THEN name END),
    MAX(CASE WHEN a.lang IS NULL THEN name END)
  ) as name
FROM
  geoname g
  LEFT JOIN alternatename a ON g.geonameid = a.geonameid AND (a.lang = 'it' OR a.lang = 'en' OR a.lang IS NULL)
WHERE
  a.name IS NOT NULL
  AND g.fcode = "PPL"
GROUP BY
  g.geonameid 
ORDER BY
  a.isPreferred DESC,
  a.name ASC 
LIMIT 0,20

Output :

geonameid	name
78910	Milano
12345	New York
71132	Sanaa
34542	Tampere

Another solution would be to use subqueries, or to join multiple times on the alternatename table (for each language you want to consider in the order by), then still use coalesce to get first non null retrieved values.

CodePudding user response：

An option for MySQL 8 is using ROW_NUMBER(), in your sample scripts, there is no index on alternatename.geonameid, better add one.

DEMO

WITH selection AS (
  SELECT 
    g.geonameid, 
    a.name, 
    ROW_NUMBER() OVER(ORDER BY a.lang DESC, COALESCE(a.isPreferred, 0) DESC) name_order
  FROM geoname g
  LEFT JOIN alternatename a ON g.geonameid = a.geonameid AND ( a.lang IN ( 'it', 'en' ) OR a.lang IS NULL )
  WHERE g.fcode = 'PPL' AND g.geonameid = 12345
  LIMIT 0, 20
)
SELECT geonameid, name
FROM selection
WHERE name_order = 1

Edit

WITH selection AS (
  SELECT 
    g.geonameid, 
    a.name, 
    ROW_NUMBER() OVER(PARTITION BY g.geonameid ORDER BY a.lang DESC, COALESCE(a.isPreferred, 0) DESC) name_order
  FROM geoname g
  LEFT JOIN alternatename a ON g.geonameid = a.geonameid AND ( a.lang IN ( 'it', 'en' ) OR a.lang IS NULL )
  WHERE g.fcode = 'PPL'
)
SELECT geonameid, name
FROM selection
WHERE name_order = 1
LIMIT 0, 20
;