Question

I have two tables with one to many relationship. I want to write a query which outputs all records from table with one record and only one record from the table having many records.

So the table having many records with first show the most occurring record. If there are equal occurrences then it will Order by ascending and show the first record.

Table1

Col1 Col2 Col3
a1     1   4
a2     2   5
a3     3   6

Table2

Col1 Col4
a1     10
a1     11
a1     22
a1     11
a2     10
a2     11
a3     19
a3     22
a3     22
a3     23

Query output:

Col1 Col2 Col3 Col4
a1     1   4    11
a2     2   5    10
a3     3   6    22

Hope I made it clear.

Answer 1

First you need to use a group by along with a min() to get the smallest number from table2, then you join to table1 to get the columns you need. I’ve used a left join as I’m assuming there may not be a match in table2 but you can change it to an INNER JOIN if there are always 1 or more corresponding records in table2.

SELECT  a.col1, a.col2, a.col3, b.col4
FROM    table1 a
        LEFT JOIN (
                    SELECT  col1, col4 = MIN(col4)
                    FROM    table2
                    GROUP BY col1
                    ) b
            ON a.col1 = b.col1