distinct這個(gè)關(guān)鍵字來(lái)過(guò)濾掉多余的重復(fù)記錄只保留一條，但往往只用 它來(lái)返回不重復(fù)記錄的條數(shù)，而不是用它來(lái)返回不重記錄的所有值。其原因是distinct只有用二重循環(huán)查詢(xún)來(lái)解決，而這樣對(duì)于一個(gè)數(shù)據(jù)量非常大的站來(lái)說(shuō)，無(wú)疑是會(huì)直接影響到效率的。
    SQL> -- create demo table
    SQL> create table Employee(
    2 ID VARCHAR2(4 BYTE) NOT NULL,
    3 First_Name VARCHAR2(10 BYTE),
    4 Last_Name VARCHAR2(10 BYTE),
    5 Start_Date DATE,
    6 End_Date DATE,
    7 Salary Number(8,2),
    8 City VARCHAR2(10 BYTE),
    9 Description VARCHAR2(15 BYTE)
    10 )
    11 /
    Table created.
    SQL>
    SQL> -- prepare data
    SQL> insert into Employee(ID, First_Name, Last_Name, Start_Date, End_Date, Salary, City, Description)
    2 values ('01','Jason', 'Martin', to_date('19960725','YYYYMMDD'), to_date('20060725','YYYYMMDD'), 1234.56, 'Toronto', 'Programmer')
    3 /
    1 row created.
    SQL> insert into Employee(ID, First_Name, Last_Name, Start_Date, End_Date, Salary, City, Description)
    2 values('02','Alison', 'Mathews', to_date('19760321','YYYYMMDD'), to_date('19860221','YYYYMMDD'), 2334.78, 'Vancouver','Tester')
    3 /
    1 row created.
    SQL> insert into Employee(ID, First_Name, Last_Name, Start_Date, End_Date, Salary, City, Description)
    2 values('03','James', 'Smith', to_date('19781212','YYYYMMDD'), to_date('19900315','YYYYMMDD'), 2334.78, 'Vancouver','Tester')
    3 /
    1 row created.
    SQL> insert into Employee(ID, First_Name, Last_Name, Start_Date, End_Date, Salary, City, Description)
    2 values('04','Celia', 'Rice', to_date('19821024','YYYYMMDD'), to_date('19990421','YYYYMMDD'), 2334.78, 'Vancouver','Manager')
    3 /
    1 row created.
    SQL> insert into Employee(ID, First_Name, Last_Name, Start_Date, End_Date, Salary, City, Description)
    2 values('05','Robert', 'Black', to_date('19840115','YYYYMMDD'), to_date('19980808','YYYYMMDD'), 2334.78, 'Vancouver','Tester')
    3 /
    1 row created.
    SQL> insert into Employee(ID, First_Name, Last_Name, Start_Date, End_Date, Salary, City, Description)
    2 values('06','Linda', 'Green', to_date('19870730','YYYYMMDD'), to_date('19960104','YYYYMMDD'), 2334.78,'New York', 'Tester')
    3 /
    1 row created.
    SQL> insert into Employee(ID, First_Name, Last_Name, Start_Date, End_Date, Salary, City, Description)
    2 values('07','David', 'Larry', to_date('19901231','YYYYMMDD'), to_date('19980212','YYYYMMDD'), 2334.78,'New York', 'Manager')
    3 /
    1 row created.
    SQL> insert into Employee(ID, First_Name, Last_Name, Start_Date, End_Date, Salary, City, Description)
    2 values('08','James', 'Cat', to_date('19960917','YYYYMMDD'), to_date('20020415','YYYYMMDD'), 2334.78,'Vancouver', 'Tester')
    3 /
    1 row created.
    SQL>
    SQL>
    SQL>
    SQL> -- display data in the table
    SQL> select * from Employee
    2 /
    ID FIRST_NAME LAST_NAME START_DAT END_DATE SALARY CITY DESCRIPTION
    ---- ---------- ---------- --------- --------- ---------- ---------- ---------------
    01 Jason Martin 25-JUL-96 25-JUL-06 1234.56 Toronto Programmer
    02 Alison Mathews 21-MAR-76 21-FEB-86 2334.78 Vancouver Tester
    03 James Smith 12-DEC-78 15-MAR-90 2334.78 Vancouver Tester
    04 Celia Rice 24-OCT-82 21-APR-99 2334.78 Vancouver Manager
    05 Robert Black 15-JAN-84 08-AUG-98 2334.78 Vancouver Tester
    06 Linda Green 30-JUL-87 04-JAN-96 2334.78 New York Tester
    07 David Larry 31-DEC-90 12-FEB-98 2334.78 New York Manager
    08 James Cat 17-SEP-96 15-APR-02 2334.78 Vancouver Tester
    8 rows selected.
    SQL>
    SQL>
    SQL>
    SQL>
    SQL>
    SQL> -- Remember that the DISTINCT operator applies to the entire select list.
    SQL>
    SQL> SELECT DISTINCT City, Description FROM Employee;
    CITY DESCRIPTION
    ---------- ---------------
    New York Manager
    Vancouver Tester
    Toronto Programmer
    Vancouver Manager
    New York Tester
    同時(shí)與groupy count 使用的用法
    SQL> select Coder
    2 , count(distinct course)
    3 , count(*)
    4 from offerings
    5 group by Coder;
    CODER COUNT(DISTINCTCOURSE) COUNT(*)
    ---------- --------------------- ----------
    1 2 3
    4 2 2
    8 2 2
    11 1 1
    13 2 2
    3 3
    6 rows selected.