What is the difference between GROUP BY and DISTINCT?

Question

I have the table with the following data

empid   empname deptid   address
--------------------------------
aa76    John     6       34567
aa75    rob      4       23456
aa71    smith    3       12345
aa74    dave     2       12345
a77     blake    2       12345
aa73    andrew   3       12345
aa90    sam      1       12345
aa72    will     6       34567
aa70    rahul    5       34567

I've used the following queries:

select deptid, EMPID ,EMPNAME ,ADDRESS
from mytable
group by 1,2,3,4

Which gives the result:

deptid  empid  empname address
------------------------------
1       aa90   sam      12345
2       aa74   dave     12345
2       aa77   blake    12345
3       aa71   smith    12345
3       aa73   andrew   12345
4       aa75   rob      23456
5       aa70   rahul    34567
6       aa76   John     34567
6       aa72   will     34567

And for the query:

select distinct (deptid),EMPID,EMPNAME,ADDRESS
from mytable

The result set is:

deptid empid empname address   
----------------------------
1      aa90  sam     12345
2      aa74  dave    12345
2      aa77  blake   12345
3      aa71  smith   12345
3      aa73  andrew  12345
4      aa75  rob     23456
5      aa70  rahul   34567
6      aa72  will    34567
6      aa76  John    34567

In the second query though I've given DISTINCT for DEPTID, how come I got the duplicate DEPTID...

Could you explain this?

Answer 1

DISTINCT eliminates repeating rows. GROUP BY groups unique records, and allows you to perform aggregate functions.