MYSQL UNION DISTINCT

Question

I have two selects that I\'m currently running as a UNION successfully.

(SELECT a.user_id,
          a.updatecontents AS city,
          b.country
   FROM use         


        

Answer 1

No. You cannot specify which exact field you need to distinct with. It only works with the whole row.

As of your problem - just make your query a subquery and in outer one GROUP BY user_id

SELECT * FROM 
(SELECT a.user_id,a.updatecontents as city,b.country
FROM userprofiletemp AS a
LEFT JOIN userattributes AS b ON a.user_id=b.user_id
WHERE typeofupdate='city')

UNION DISTINCT

(SELECT a.user_id,c.city,c.country
FROM userverify AS a
LEFT JOIN userlogin AS b ON a.user_id=b.user_id
LEFT JOIN userattributes AS c ON a.user_id=c.user_id
WHERE b.active=1 AND a.verifycity=0) x
GROUP BY user_id

Answer 2

MySQL UNION produces distinct rows—however, all column values in the row need to be distinct. If you wish to limit the distinction to a single or a few columns, when other columns are not distinct, you can wrap the UNION in a sub-query and GROUP BY the sub-query by the columns you wish to be unique.

Here I wrap the entire UNION in a sub-query, give it an alias, then GROUP BY the desired unique column:

SELECT * FROM (

SELECT a.user_id,a.updatecontents as city,b.country
FROM userprofiletemp AS a
LEFT JOIN userattributes AS b ON a.user_id=b.user_id
WHERE typeofupdate='city'

UNION

SELECT a.user_id,c.city,c.country
FROM userverify AS a
LEFT JOIN userlogin AS b ON a.user_id=b.user_id
LEFT JOIN userattributes AS c ON a.user_id=c.user_id
WHERE b.active=1 AND a.verifycity=0

) aa GROUP BY user_id;

If you have more than one column you'd like to include in the distinction, list them after the GROUP BY: such as GROUP BY user_id, city.

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SIDE NOTE: since, in this case, UNION does not provide the desired distinction, there is no benefit to simply using UNION, and apparently "UNION ALL is much faster than UNION", therefore you can use UNION ALL to speed up this query.

Answer 3

(SELECT a.user_id,a.updatecontents as city,b.country
FROM userprofiletemp AS a
LEFT JOIN userattributes AS b ON a.user_id=b.user_id
WHERE typeofupdate='city')

UNION ALL

(SELECT a.user_id,c.city,c.country
FROM userverify AS a
LEFT JOIN userlogin AS b ON a.user_id=b.user_id
LEFT JOIN userattributes AS c ON a.user_id=c.user_id
WHERE b.active=1 AND a.verifycity=0
  AND a.user_id NOT IN
      ( SELECT user_id
        FROM userprofiletemp 
        WHERE typeofupdate='city'
      ) 
);