在PHP中一次从两个MySQL表中选择数据

Question

What I have tried to achieve;

Counting the number of rows in two tables (adding them together) grouping them by the country code in the Country column in the table. I will then output the results by the number of rows and country eg

• GB => 200 views
• US => 500 views
• CH => 119 views

MySQL code I have so far;

SELECT COUNT(id) as TotalClicks, Country FROM adverts_analytics_user_clicks WHERE AdvertID =:AdvertID GROUP BY Country

This works well, however I need it to select the same data from another table at the same time and combine the COUNT() values according to the Country field.

Any help would be appreciated!

Answer 1

Repeat your first query against the second table, such that the output coumns have the same name and datatype.

Then union those results together.

Finally Sum the total clicks by country.

SELECT Country, sum(TotalClicks) as TotalClicks
FROM 
(
SELECT COUNT(id) as TotalClicks, Country FROM adverts_analytics_user_clicks WHERE AdvertID =:AdvertID GROUP BY Country
UNION ALL
SELECT COUNT(id) as TotalClicks, Country FROM table 2 WHERE AdvertID =:AdvertID GROUP BY Country
)
GROUP BY Country

Answer 2

One way to do that is to append another query to your query with a UNION ALL operator. (That query can reference a different table name; the important part is that it returns the same number of columns with the same datatypes.) Then wrap that in a set of parens, and assign it an alias, and then use that in place of a tablename in an outer query:

For example:

SELECT SUM(t.TotalClicks) AS TotalClicks, t.Country
  FROM (
         /* original query*/

         SELECT COUNT(id) as TotalClicks, Country
           FROM adverts_analytics_user_clicks
          WHERE AdvertID =:AdvertID
          GROUP BY Country

          /* append results from query that returns same columns */
          UNION ALL
          SELECT COUNT(id) as TotalClicks, Country
            FROM some_other_table
           WHERE AdvertID =:AdvertID2
           GROUP BY Country
      ) t
  GROUP BY t.Country

NOTE: we've encountered issues when the same bind variable name is used twice in the same statement. We resorted to separate bind variable names, and binding the same value twice.

Because of the way that MySQL handles inline views (materializing it as a temporary table) it's almost always more efficient to get the aggregates from each table, and then concatenating the aggregates together, than it is to concatenate the non-aggregated tables.

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FOLLOWUP

simon funiic said... the statement is working, however it is not returning the right number for TotalClicks (there are two rows for Country GB - one in each table) but only 1 is returned.

response

Given the GROUP BY on the outer query (in my answer above), we would expect only a single row returned for a Country value of 'GB' . If you run just the query in the inline view (aliased as t in the query above) you would get both rows.

The outermost query is combining those two rows (with the GROUP BY t.Country), and adding the values from TotalClicks together ( SUM(t.TotalClicks) )

If you want the rows returned separately, there's no need for that outer query at all.

If you want to include a "tag" on each row to identify which query the row comes from, it's easy to add a literal in the SELECT list of each query:

     SELECT COUNT(id) AS TotalClicks
          , Country
          , 'aauc' AS `source`
       FROM adverts_analytics_user_clicks
      WHERE AdvertID =:AdvertID
      GROUP BY Country
      UNION ALL
     SELECT COUNT(id) AS TotalClicks
          , Country
          , 'sot' AS `source`
       FROM some_other_table
      WHERE AdvertID =:AdvertID2
      GROUP BY Country
      ORDER BY 2,3