Using group by on multiple columns

Question

I understand the point of GROUP BY x  But how does GROUP BY x  y work  and what does it mean

User · Answer

In simple English from GROUP BY with two parameters what we are doing is looking for similar value pairs and get the count to a 3rd column  Look at the following example for reference  Here I m using International football results from 1872 to 2020  ---------- ---------------- -------- --- --- -------- --------- ------------------- -----           c0               c1       c2  c3  c4       c5        c6                  c7    c8   ---------- ---------------- -------- --- --- -------- --------- ------------------- -----   1872-11-30         Scotland  England   0   0 Friendly   Glasgow            Scotland FALSE   1873-03-08          England Scotland   4   2 Friendly    London             England FALSE   1874-03-07         Scotland  England   2   1 Friendly   Glasgow            Scotland FALSE   1875-03-06          England Scotland   2   2 Friendly    London             England FALSE   1876-03-04         Scotland  England   3   0 Friendly   Glasgow            Scotland FALSE   1876-03-25         Scotland    Wales   4   0 Friendly   Glasgow            Scotland FALSE   1877-03-03          England Scotland   1   3 Friendly    London             England FALSE   1877-03-05            Wales Scotland   0   2 Friendly   Wrexham               Wales FALSE   1878-03-02         Scotland  England   7   2 Friendly   Glasgow            Scotland FALSE   1878-03-23         Scotland    Wales   9   0 Friendly   Glasgow            Scotland FALSE   1879-01-18          England    Wales   2   1 Friendly    London             England FALSE   1879-04-05          England Scotland   5   4 Friendly    London             England FALSE   1879-04-07            Wales Scotland   0   3 Friendly   Wrexham               Wales FALSE   1880-03-13         Scotland  England   5   4 Friendly   Glasgow            Scotland FALSE   1880-03-15            Wales  England   2   3 Friendly   Wrexham               Wales FALSE   1880-03-27         Scotland    Wales   5   1 Friendly   Glasgow            Scotland FALSE   1881-02-26          England    Wales   0   1 Friendly Blackburn             England FALSE   1881-03-12          England Scotland   1   6 Friendly    London             England FALSE   1881-03-14            Wales Scotland   1   5 Friendly   Wrexham               Wales FALSE   1882-02-18 Northern Ireland  England   0  13 Friendly   Belfast Republic of Ireland FALSE   ---------- ---------------- -------- --- --- -------- --------- ------------------- -----   And now I m going to group by similar country column  c7  and tournament  c5  value pairs by GROUP BY operation  SELECT   c5    c7  count     FROM res GROUP BY   c5    c7    -------------------- ------------------- --------                     c5                  c7 count 1    -------------------- ------------------- --------               Friendly   Southern Rhodesia       11               Friendly             Ecuador       68   African Cup of Na               Ethiopia       41   Gold Cup qualific    Trinidad and Tobago        9   AFC Asian Cup qua                 Bhutan        7   African Nations C                  Gabon        2               Friendly            China PR      170   FIFA World Cup qu                 Israel       59   FIFA World Cup qu                  Japan       61   UEFA Euro qualifi                Romania       62   AFC Asian Cup qua                  Macau        9               Friendly         South Sudan        1   CONCACAF Nations                Suriname        3            Copa Newton           Argentina       12               Friendly         Philippines       38   FIFA World Cup qu                  Chile       68   African Cup of Na             Madagascar       29   FIFA World Cup qu           Burkina Faso       30    UEFA Nations League             Denmark        4           Atlantic Cup            Paraguay        2   -------------------- ------------------- --------   Explanation  The meaning of the first row is there were 11 Friendly tournaments held on Southern Rhodesia in total  Note  Here it s mandatory to use a counter column in this case

User · Answer

Here I am going to explain not only the GROUP clause use  but also the Aggregate functions use  The GROUP BY clause is used in conjunction with the aggregate functions to group the result-set by one or more columns  e g   -- GROUP BY with one parameter  SELECT column name  AGGREGATE FUNCTION column name  FROM table name WHERE column name operator value GROUP BY column name   -- GROUP BY with two parameters  SELECT     column name1      column name2      AGGREGATE FUNCTION column name3  FROM     table name GROUP BY     column name1      column name2   Remember this order    SELECT  is used to select data from a database   FROM  clause is used to list the tables   WHERE  clause is used to filter records   GROUP BY  clause can be used in a SELECT statement to collect data across multiple records and group the results by one or more columns   HAVING  clause is used in combination with the GROUP BY clause to restrict the groups of returned rows to only those whose the condition is TRUE   ORDER BY  keyword is used to sort the result-set     You can use all of these if you are using aggregate functions  and this is the order that they must be set  otherwise you can get an error  Aggregate Functions are   MIN              returns the smallest value in a given column MAX   returns the maximum value in a given column  SUM      returns the sum of the numeric values in a given column AVG              returns the average value of a given column COUNT        returns the total number of values in a given column COUNT     returns the number of rows in a table  SQL script examples about using aggregate functions  Let s say we need to find the sale orders whose total sale is greater than  950  We combine the HAVING clause and the GROUP BY clause to accomplish this  SELECT      orderId  SUM unitPrice   qty  Total FROM     OrderDetails GROUP BY orderId HAVING Total  gt  950   Counting all orders and grouping them customerID and sorting the result ascendant  We combine the COUNT function and the GROUP BY  ORDER BY clauses and ASC  SELECT      customerId  COUNT    FROM     Orders GROUP BY customerId ORDER BY COUNT    ASC   Retrieve the category that has an average Unit Price greater than  10  using AVG function combine with GROUP BY and HAVING clauses  SELECT      categoryName  AVG unitPrice  FROM     Products p INNER JOIN     Categories c ON c categoryId   p categoryId GROUP BY categoryName HAVING AVG unitPrice   gt  10   Getting the less expensive product by each category  using the MIN function in a subquery  SELECT categoryId         productId         productName         unitPrice FROM Products p1 WHERE unitPrice                     SELECT MIN unitPrice                  FROM Products p2                 WHERE p2 categoryId   p1 categoryId   The following statement groups rows with the same values in both categoryId and productId columns  SELECT      categoryId  categoryName  productId  SUM unitPrice  FROM     Products p INNER JOIN     Categories c ON c categoryId   p categoryId GROUP BY categoryId  productId

User · Answer

Group By X means put all those with the same value for X in the one group  Group By X  Y means put all those with the same values for both X and Y in the one group  To illustrate using an example  let s say we have the following table  to do with who is attending what subject at a university  Table  Subject Selection   --------- ---------- ----------    Subject   Semester   Attendee    --------- ---------- ----------    ITB001           1   John         ITB001           1   Bob          ITB001           1   Mickey       ITB001           2   Jenny        ITB001           2   James        MKB114           1   John         MKB114           1   Erica       --------- ---------- ----------   When you use a group by on the subject column only  say  select Subject  Count    from Subject Selection group by Subject  You will get something like   --------- -------    Subject   Count    --------- -------    ITB001        5     MKB114        2    --------- -------      because there are 5 entries for ITB001  and 2 for MKB114 If we were to group by two columns  select Subject  Semester  Count    from Subject Selection group by Subject  Semester  we would get this   --------- ---------- -------    Subject   Semester   Count    --------- ---------- -------    ITB001           1       3     ITB001           2       2     MKB114           1       2    --------- ---------- -------   This is because  when we group by two columns  it is saying  quot Group them so that all of those with the same Subject and Semester are in the same group  and then calculate all the aggregate functions  Count  Sum  Average  etc   for each of those groups quot   In this example  this is demonstrated by the fact that  when we count them  there are three people doing ITB001 in semester 1  and two doing it in semester 2  Both of the people doing MKB114 are in semester 1  so there is no row for semester 2  no data fits into the group  quot MKB114  Semester 2 quot   Hopefully that makes sense

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