group by sql

1GROUP BY: is used to collaborate
2with the SELECT statement to arrange 
3matching data into groups.
4
5ORDER BY: is for sorting result
6either in descending or ascending order.

1--- GROUP FUNCTION | MULTI ROW FUNCTION | AGGREGATE FUNCTION 
2--- COUNT , MAX , MIN , SUM , AVG

1 SELECT column_name(s)
2  FROM table_name
3  WHERE condition
4  GROUP BY column_name(s)
5  HAVING condition
6  ORDER BY column_name(s); 

1SELECT `gender` FROM `members` GROUP BY `gender`;

1/*Group by clause is used to group a selected  set of rows into a set of summary 
2rows by the values of one or more column or expression. It is always used in
3Conjunction with one or more aggregate function.*/
4SELECT AGGREGATE_FUNCTION(Column_Name) FROM Table_Name
5/*Group by exmaple*/
6SELECT city, sum(Salary) as TotalSalary FROM tblEmployee GROUP BY City

1class groupby(object):
2    # [k for k, g in groupby('AAAABBBCCDAABBB')] --> A B C D A B
3    # [list(g) for k, g in groupby('AAAABBBCCD')] --> AAAA BBB CC D
4    def __init__(self, iterable, key=None):
5        if key is None:
6            key = lambda x: x
7        self.keyfunc = key
8        self.it = iter(iterable)
9        self.tgtkey = self.currkey = self.currvalue = object()
10    def __iter__(self):
11        return self
12    def next(self):
13        while self.currkey == self.tgtkey:
14            self.currvalue = next(self.it)    # Exit on StopIteration
15            self.currkey = self.keyfunc(self.currvalue)
16        self.tgtkey = self.currkey
17        return (self.currkey, self._grouper(self.tgtkey))
18    def _grouper(self, tgtkey):
19        while self.currkey == tgtkey:
20            yield self.currvalue
21            self.currvalue = next(self.it)    # Exit on StopIteration
22            self.currkey = self.keyfunc(self.currvalue)
23