Count distinct column value : COUNT « Aggregate Functions « Oracle PL/SQL Tutorial

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Oracle PL/SQL Tutorial » Aggregate Functions » COUNT 
12. 3. 7. Count distinct column value
SQL>
SQL> create table employees(
  2    empno      NUMBER(4)
  3  , ename      VARCHAR2(8)
  4  , init       VARCHAR2(5)
  5  , job        VARCHAR2(8)
  6  , mgr        NUMBER(4)
  7  , bdate      DATE
  8  , msal       NUMBER(6,2)
  9  , comm       NUMBER(6,2)
 10  , deptno     NUMBER(2) ) ;

Table created.

SQL>
SQL>
SQL> insert into employees values(1,'Jason',  'N',  'TRAINER', 2,   date '1965-12-18',  800 , NULL,  10);

row created.

SQL> insert into employees values(2,'Jerry',  'J',  'SALESREP',3,   date '1966-11-19',  1600300,   10);

row created.

SQL> insert into employees values(3,'Jord',   'T' 'SALESREP',4,   date '1967-10-21',  1700500,   20);

row created.

SQL> insert into employees values(4,'Mary',   'J',  'MANAGER', 5,   date '1968-09-22',  1800, NULL,  20);

row created.

SQL> insert into employees values(5,'Joe',    'P',  'SALESREP',6,   date '1969-08-23',  19001400,  30);

row created.

SQL> insert into employees values(6,'Black',  'R',  'MANAGER', 7,   date '1970-07-24',  2000, NULL,  30);

row created.

SQL> insert into employees values(7,'Red',    'A',  'MANAGER', 8,   date '1971-06-25',  2100, NULL,  40);

row created.

SQL> insert into employees values(8,'White',  'S',  'TRAINER', 9,   date '1972-05-26',  2200, NULL,  40);

row created.

SQL> insert into employees values(9,'Yellow', 'C',  'DIRECTOR',10,  date '1973-04-27',  2300, NULL,  20);

row created.

SQL> insert into employees values(10,'Pink',  'J',  'SALESREP',null,date '1974-03-28',  24000,     30);

row created.

SQL>
SQL>
SQL>
SQL> select count(deptno), count(distinct deptno)
  2  ,      avg(comm),     avg(nvl(comm,0))
  3  from   employees;

COUNT(DEPTNOCOUNT(DISTINCTDEPTNO)  AVG(COMMAVG(NVL(COMM,0))
------------- --------------------- ---------- ----------------
           10                     4        550              220

SQL>
SQL>
SQL> drop table employees;

Table dropped.

SQL>
SQL>
12. 3. COUNT
12. 3. 1. COUNT(x) gets the number of rows returned by a query.
12. 3. 2. Passes ROWID to COUNT() and gets the number of rows
12. 3. 3. COUNT() with null
12. 3. 4. Using an aggregate with the GROUP BY clause to count by city
12. 3. 5. COUNT(1) from a table
12. 3. 6. Count column with table alias
12. 3. 7. Count distinct column value
12. 3. 8. Compare the difference between count(*) and count(distinct course)
12. 3. 9. Count employees in the same department
12. 3. 10. JOIN with AND and aggregate function
12. 3. 11. count(*) - count(onhand)
12. 3. 12. count(*) vs count(column name)
12. 3. 13. Count all employees by even/odd employee id
12. 3. 14. Count date field value, and calculation
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